
Why a Typical Home Solar Setup Does Not Work with the Grid Down - cesarb
https://syonyk.blogspot.com/2018/05/why-typical-home-solar-setup-does-not-work-off-grid.html
======
dublin
A huge amount of the info on solar PV power on the net is absolute garbage. (I
spent 5 years working on the most advanced solar performance optimization and
monitoring system developed to date.) I haven't read all of this article, but
so far, it looks dead on (you have no idea how rare that is!) Solar is not the
panacea many people wish it was, simply due to the way God built the universe
- and there's not a thing we can do about that. (And yes, the panels are now
only a small part of the cost of a solar system, especially one capable of
operating off-grid...)

~~~
rebootthesystem
Looks like I great article. I'll have to defer reading it. During a quick scan
I saw a mention of the SMA technology that allows for dedicated outlets to be
powered-up in case of grid failure.

I built a 13 kW ground-mount system feeding a pair of SMA inverters. I have
tested this feature by disconnecting from the grid and enabling the outlets
(one per inverter). I didn't quite get to the 2,000 W rating SMA claims but
got close. Which means that with this size of an array and two inverters I get
somewhere between 3kW and 4kW of power to run various devices while the sun is
up.

Considering that we might have a couple of power outages a year on average (if
that), I felt this was a reasonable investment. Going with batteries is just
too expensive and not justifiable at all given the reliability of the grid.
One way to think about this is that the grid is your battery. A stretch, I
know.

Funny that there's a picture of a gasoline generator towards the end of the
article. My guess is that I am likely to invest in a 5 kW to 6 kW generator
before I ever add batteries to this system. Again, it's a matter of ROI. Also,
I would not go with a gasoline powered generator at all. Gasoline degrades
with time and could be a nightmare to maintain the system with sporadic use. I
think a propane fueled generator might be a better idea. The fuel does not
degrade. So long as you don't have leaks it'll be there ready to go when you
need it.

I know way too many people who have been mercilessly duped by these solar
companies who come in, hook them on some kind of a lease, install inadequate
systems and move on to the next victim. Lack of understanding on the side of
consumers has created a situation where solar is equivalent to magic and
unscrupulous actors can take advantage of them. That part is sad.

------
taneq
> Backfeeding the power grid, according to some lineworkers I've talked to, is
> really not a big concern for two reasons. First, lineworkers assume lines
> are live until proven otherwise. And, second, no residential system is going
> to successfully backfeed a large dead section of grid. [...] it's really not
> that big a concern from a technical/safety perspective.

This is wrong and dangerous. Suddenly energising a section of grid which has
been isolated from all known power sources could easily kill someone working
on it. Yes, lineworkers will isolate and test lines before touching them. No,
this won't protect them because your inverter could start outputting after
they've tested. Yes, your inverter will _probably_ overload and trip out
before putting _too_ much voltage into your local section of the grid,
depending on where it's isolated. No, it's not OK to bet a random stranger's
life on it.

~~~
alkonaut
When I work on electrical wires at home, I test if they are hot after flipping
what I believe is the correct fuse. If I’m right I know it won’t become hot
while I work on it because I control the switches and fuses.

If I worked on wires where I was _not_ in control of incoming power, then I
can _not_ be sure the wire isn’t hot just because I measured _once_. This is
for example the case when working with stuff with big capacitors.

So my point is: can line workers really assume a wire isn’t hot just because
they tested it? They don’t control both ends, since as you pointed out, anyone
could have a) a power source or b) a faulty connection, whether it’s allowed
or not. So as a line worker I’d basically just not touch anything even in
supposedly dead grid sections.

~~~
zaarn
I recommend shorting to ground. Even if it's your own fuse. Shorting means
that if you picked the wrong fuse you notice. And when your friend comes by
and flips the fuse back on you don't get a spark shower (something like this
has happened to me, don't underestimate people willing to flip fuses for no
reason)

~~~
wlesieutre
A common solution for this is lockout-tagout, where you physically lock the
breaker open using a lock with your name on it, and nobody can flip it until
you come back and take the lock off.

Used on electrical circuits as well as dangerous machines if you need to climb
inside a compactor for maintenance or something.

[https://www.osha.gov/Publications/3120.html](https://www.osha.gov/Publications/3120.html)

[http://www.deenergize.com/?target=Universal_Circuit_Breaker_...](http://www.deenergize.com/?target=Universal_Circuit_Breaker_Lockout_Device_\(PSL-
CB\))

~~~
zaarn
That works too, though IIRC the common procedure in the industry in my country
is to use a warning label/shield and hang it on the breaker box during
repairs, locking the breaker box itself, then short out the circuit to ground
and measure to be sure.

Locks on the breaker itself are more rare (usually for low voltage repairs
where the common person might frequent the area)

------
ghubbard
Great article, lots of technical details. A simple answer to the 'Why' can be
found in the middle under the 'Off Grid Without Batteries' headline.

> If you have a typical grid tied system (microinverters or normal string
> inverters, so easily 95+% of installed rooftop solar), the system is
> technically incapable of running off grid (without additional hardware).
> There's no waveform to sync with,

~~~
raverbashing
"there's no waveform to sync with" is technically correct and _a very poor
excuse_

A residential no-break has no waveform to sync with as well.

Something capable of syncing to the grid and then more or less keeping pace
even if the main grid goes down should cost very little today. (And when the
grid goes back it shouldn't have drifted too much unless something
ridiculously big happened)

~~~
zaarn
The grid isn't a fixed frequency. While on average the grid doesn't change
phase, the reality is that the grid may be out of phase by several seconds
most of the time.

When the grid comes back from a blackout, chances are that it browned out
beforehand so your sync is to a low frequency and coming back it'll be high
frequency because the grid wants to compensate for loads jumping back on.

Additionally the components to generate your own waveform are cheap, yes, but
not _that_ cheap, adding them to the microinverter would increase cost quite a
bit.

And you'd still need a transfer switch because if you happen to be 180 degrees
out of phase, which CAN HAPPEN then your panel will behave like a dead short
at double grid voltage. The current flow will definitely exceed the maximum
tolerances and the magic smoke goes out.

You will absolutely need a transfer switch just so you don't fry all your
devices the moment the grid comes back. Even then, syncing to the grid is a
rather delicate maneuver since the grid will be constantly changing phase and
it'll be simpler to shut down all inverters, connect back and have it all run
back up on the grid itself.

~~~
raverbashing
> When the grid comes back from a blackout, chances are that it browned out
> beforehand

Not really, but depends on the state of the infrastructure, if it was really
because of a power overload, yes, but most likely "your circuit" (which could
be your street or your neighborhood) got shut off, in this case there
shouldn't be much difference

Yes, a phase difference of 180 can definitely happen but I guess most
electronics can survive a 1/60s (I'd say even 1/10) switching time, which is
probably enough to have the grid take over.

(Or of course you could have the grid and solar charging batteries then your
own high power inverter for your house but that of course would mean $$$$$$)

~~~
zaarn
A 1/10th switching time would be fine though if you change phase too much and
suddenly some equipment might not like it.

And you'd still have to sync the inverter, having the inverter simply continue
to run until it's back in sync with the grid, then just reconnect (as a
previous comment wanted) is likely not an option for most consumers.

For that it would likely be cheaper to have a full DC stage as you mentioned.

~~~
loa-in-backup
It's knowledge that people in question are lacking

------
driverdan
As this article touches on the solar panels are the cheap part of a solar
setup. If you want to run off grid the costs skyrocket.

I live in an off grid bus. My whole electrical system cost in the US$9-10000
range with only 810W of solar. The other components cost a lot more:

* ~$900 for 3x 270W Renogy solar panels

* $1500 for 3.6kWh of LiFePO4 batteries (12 100A 3.6V cells)

* $1600 for a Victron 3kW inverter / charger

* $2000 for a Honda EU3000is generator to run my AC

The rest was the Victron solar controller, color control panel (that runs
Linux!), breakers, relays, transfer switch, fuses, cable, and all the other
stuff you need to hook it together.

This isn't at house scale either. The cost would go up significantly to
support a house scale system.

~~~
VectorLock
The solar controller, control panel and miscellaneous cost you $3-4k?

~~~
driverdan
The controller was more than $200, the control panel is around $550.

I didn't itemize the rest because I don't have the time to list it all. The
roof rack and hardware cost around $1000. There are a lot of individual items
that cost around $100, (eg starter battery isolator relay, breaker panel +
breakers). Heavy gauge cables are necessary and expensive. All the small
things add up.

------
jefftk
_> The only real way to get off grid power without batteries is to go with an
inverter that has an emergency outlet. Some of the SMA inverters support this
(they call it Secure Power Supply) - you feed the whole rooftop array into
them, and they can, if the sun is shining, provide 1.5kW or so to a dedicated
outlet - assuming there's enough solar power. So, from an 8-10kW array, on a
sunny day, you can get 1.5kW by operating well below the peak power point. If
the array can't keep up with current demand (a cloud goes over), the outlet
shuts down. It's better than nothing, but this is just about the only way you
can get battery-free off grid power. To get any sort of stable battery-free
power, you have to run the panels well, well below peak power (30-50% of peak
is as high as you can really run), and even then, you have a horrifically
unstable system. If the array power briefly drops below demand (perhaps an
airplane has flown over), you shut down the entire output for a while.
Hopefully your devices can handle intermittent power like this. If the array
can source 1300W at the moment and a compressor tries to draw 1301W while
starting, you collapse the array voltage and shut down the outlet. That's
really hard on compressors (and everything else attached to the outlet)._

It's not ideal, but this is so much better than what most installs are built
with (nothing).

~~~
test6554
I thought there were shutoff devices that could detect a grid outage and
instantly disconnect your home from the grid allowing your inverter to power
your house until grid power is restored. I assumed something like this would
be $10K but well worth it, as basically the whole point of battery-backed
solar is energy independence.

~~~
ianburrell
From the article, the problem isn't the transfer switch but that the
microinverters used on most systems don't work without the grid. If the grid
goes down or is disconnected, the microinverters stop providing power.

It is possible to get inverter that will work independently, and batteries to
buffer the load, but the off-grid system is a lot more expensive than normal
rooftop solar system.

------
gnicholas
Super useful. Pacific Gas & Electric (which powers much of CA) recently
announced that they may pre-emptively shut off power to certain parts of
Silicon Valley out of concern for potential fires. In the article I read, they
mentioned that even home solar wouldn't work, which seemed crazy. I can now
see why this is the case, and what folks should do (get a transfer switch and
generator) to be able to power their home with their solar panels.

~~~
tlrobinson
> Pacific Gas & Electric (which powers much of CA) recently announced that
> they may pre-emptively shut off power to certain parts of Silicon Valley out
> of concern for potential fires

Huh, where can I read about this?

~~~
mcpherrinm
TLDR: Power lines are a cause of wildfires, and PG&E may shut down power on
some transmission lines in times of extreme fire danger.

[https://www.sfchronicle.com/business/article/For-PG-E-
cuttin...](https://www.sfchronicle.com/business/article/For-PG-E-cutting-
power-could-prevent-fires-save-13019005.php)

[https://sacramento.cbslocal.com/2018/06/25/pge-cutting-
power...](https://sacramento.cbslocal.com/2018/06/25/pge-cutting-power/)

~~~
joering2
Interesting fact: years ago it was designed to build electric infrastructure
above the ground because air friction addsup about 20% of electricty, free. It
wouldnt be possible underground. But then, you know, whole State of amedium
size country like California, would have no power interruptions in heat
spikes.

------
SigmundA
My dad has a 7kw system with the SMA inverter with emergency power outlet for
1.5kw. Works ok, as said the main issue without batteries to buffer is
stability.

I have a Magnum 3000w Hybrid inverter in my RV with 320w of solar with a MPPT
solar controller. The solar controller feeds the batteries and the inverter
draws from batteries or charges them with UPS grade change over. Magnum
hybrids can do interesting things like if the sun is out and I am running the
air conditioner off generator or shore power it will pull batteries down to
float voltage and invert excess power to lower draw, I usually get about 1 amp
reduction with 320w of panels. You can also dial in your shore amps and if a
load needs more the inverter will step in and provide the overage from the
batteries. Nice if you driveway surfing and only have a 15 amp outlet but want
to run the microwave or whatever. The RV is basically a giant UPS with
generator.

~~~
mikestew
Having _just_ finished a 450W solar system on our RV, along with new big-ass
200ah battery, I was shocked to find out that house systems have no battery
backing. Because I have had to explain to everyone that asks “can it run...?”
that you can run anything you want if the battery can handle it, the solar is
just a battery charger and doesn’t directly run anything.

But as one who is considering residential solar, I guess there’s _more_
research to do as home solar apparently doesn’t work that way at all (though
it probably will when I do it).

Side note: thanks for the inverter mention. That’s next on the list, and the
Magnum sounds to be worth a look.

~~~
rbritton
I went the Victron route on my setup, primarily because they make whole
systems that can talk with each other. I had originally been looking at the
Magnum, though.

~~~
mikestew
I guess in all of my searching when buying, I never discovered that Victron
makes inverters, despite having one of their charge controllers (seems like
Xantrex gets all the mentions for high-end inverters). I'll check them out,
thanks.

~~~
SigmundA
I almost went Victron, they have a nice selection of equipment with some nice
features. I was however put off with their documentation, very lacking, was
not 100% sure what I needed to put the system together, they have like 3
different kinds of communications buses.

Magnum on the other hand has excellent documentation and a somewhat simpler
product line that integrates well together, remote, inverter, battery monitor.
Their comm bus is just RS-485 and documentation can be found, I wrote a node
server to display info in realtime along with my Morningstar Tristar solar
controller which uses modes over TCP.

My understanding is Victrons load support feature does not go down as far as
Magnum, 10 amps vs 5 if thats important .They have a very good reputation as
does Magnum, different league than cheap inverters.

Here is a video I did of the Magnums search watt function as some wanted to
see, also shows how it deals with large loads and the web interface I wrote
for it:
[https://www.youtube.com/watch?v=l_jqzY1wNDU](https://www.youtube.com/watch?v=l_jqzY1wNDU)

------
tlrobinson
> To get any sort of stable battery-free power, you have to run the panels
> well, well below peak power (30-50% of peak is as high as you can really
> run), and even then, you have a horrifically unstable system. If the array
> power briefly drops below demand (perhaps an airplane has flown over), you
> shut down the entire output for a while

It doesn't seem like a battery or capacitor to handle blips like an airplane
flying overhead would need to be very large?

~~~
derefr
No, but using a small battery means using a battery that get heavily
charged/discharged by such events, and therefore has a short lifespan (less
than a year, probably, if you used the kind of battery that's in a UPS.) The
reason the battery banks for solar are so large isn't just to capture the
entire daytime output for night-time use, but also so that they can have a
lifespan closer to that of the solar array itself.

~~~
foota
That's interesting! Is it like an SSD where you plan for a certain amount of
cells to fail or does it reduce load on the cells so they last long
individually?

~~~
derefr
The latter.

Whenever your battery gets super discharged, picture this happening inside it:
[https://www.youtube.com/watch?v=r-YbQN_twpw](https://www.youtube.com/watch?v=r-YbQN_twpw)

Notice how running the current the other way will _break up_ the formed
crystals, but it won't actually _re-dissolve_ them. That's what it means for a
battery to wear out. (Battery electrolytes are chosen specifically to be
resistant to this, so the effect is mainly visible as a thin "rust" of
deposited crystals on the battery's anode, rather than the full-scale crystals
seen here.)

A similar but distinct process occurs when overcharging a battery, where,
instead of splitting the electrolyte molecules apart, you're reacting and
bonding them together to form new molecules or molecular complexes, with the
reaction usually being one-way rather than an equilibrium reaction. (For
lithium batteries this process is exothermic and catalyzed by the presence of
the product—thus lithium battery explosions.)

It's much better for the life of the battery if it basically just hovers
around 40-60% charge for its whole useful life, since then you're just
generating tiny seed crystals (on discharge) and then re-dissolving them (on
charge), where those crystals are small enough that they can be fully re-
absorbed.

And this is true even in battery-cell technologies that require a "deep charge
cycle" to erase their "battery memory." Battery memory is basically the
electrolyte causing enough crystalline rust _specifically on the anode_ to
increase its resistance. A deep discharge can capture and erase this rust—but
it _still_ shortens the battery's lifespan, because you're still producing
non-reabsorbable large crystals within the electrolyte.

~~~
foota
Thank a lot for sharing! That was interesting. So the desire to hover around
50% requires 2X larger batteries as would otherwise be necessary?

~~~
mikestew
Yup. Though I've heard (as it concerns deep-cycle batteries) not to let them
drop below 50% charge rather than keep them in a certain range. So, for
example, when I purchased a new house battery for our RV, I got a 200ah
battery knowing I can only make use of 100ah before recharging. Or put another
way, figure out how much you'll use between charges (or in the case of solar,
how much to tide you over until the sun comes up), and double it.

------
rdl
Maybe this applies in normal markets, but in Puerto Rico the grid is both
shit-tier reliability and horribly expensive. Rooftop solar and lithium
batteries in an off-grid format are both far more reliable and far cheaper
than the grid, assuming a 5% cost of capital. I might throw in some LP or
diesel generation capacity too, but solar plus battery is an easy choice.

~~~
nine_k
Are lithium batteries the best choice here? They have superior energy density,
but IIRC there are choices that are heavier but cheaper (per kWh installed),
like ready-made car acid-lead batteries, or industrial Ni-Cd batteries.

What do people actually use?

~~~
vorpalhex
Not sure for Solar, but most HAMs using field radio setups still use deep
cycle marine batteries as they withstand abuse and are much cheaper.

~~~
tjohns
The big benefit lead acid batteries have is that it's easy to find surplus SLA
batteries from businesses, and even new cells aren't that expensive.

That said, I've started seeing lithium batteries slowly catch on in amateur
radio as the price on the LiFePo4 cells go down. Especially for portable
setups.

------
CaliforniaKarl
I've read the detailed reports for outages in the West and the North-East, and
they touch on alot of the same concepts that were in this (awesome) article.

Does anyone know, what's a good resource for learning more about power
generation, grid regulation, etc.? The relationship between frequency/voltage
and available vs. demanded current, frequency sync, grid maintenance, etc.…

~~~
throwawaycanada
If you have a reasonable electrical background I would suggest this textbook:

Electrical Machines, Drives and Power Systems by Theordore Wildi

------
iamleppert
This is exactly why people need to explore alternative energy options other
than solar. The best off-grid setup I’ve seen is a bunch of conductive pipes
lay over the ground that heated water from the sun, that in turn was used to
generate enough energy to pump water to a storage tank during the day up a
large hill. At night the tank was drained and water came back down and the
resulting pressure was used to generate power, supply back to the grid. It
used minimal components (& simple ones), no batteries, nothing fancy but was
enough energy to supply a community of several single family homes in a
completely off grid system.

~~~
dragonwriter
> This is exactly why people need to explore alternative energy options other
> than solar. The best off-grid setup I’ve seen is a bunch of conductive pipes
> lay over the ground that heated water from the sun, that in turn was used to
> generate enough energy to pump water to a storage tank during the day up a
> large hill. At night the tank was drained and water came back down and the
> resulting pressure was used to generate power, supply back to the grid.

So, your example of energy options other than solar is solar with hydro
storage? Or did you mean “other than photovoltaic” when you said “othet than
solar”.

~~~
iamleppert
Yeah, meant to say conventional PV solar.

------
Johnny555
_During 2017, I saw a lot of news articles talking about how the Evil Power
Companies were being Meanie McMean by not letting people with solar panels use
them when the grid was down. The implication (in many news articles) was that
these powerless people with solar panels could use them to power their home
while the grid was down, if only the evil power company didn 't require that
solar not work if the grid was down._

I don't remember seeing any articles that blame the power company for home
solar not working off-grid... I do remember seeing articles saying that most
home solar installations won't work if the power grid is down, but nothing
that implied that the power companies were behind it.

~~~
0xCMP
In Florida it was popular around hurricane season after we lost power.

I'm glad I finally understand why because all that I'd seen reported was the
solar systems installed must be turned off. I'd always assumed all these
systems (many of which are meant to be used in a storm + lower electricity
costs) had a transfer switch.

------
modeless
So much complexity to deal with synchronizing AC frequency. I wonder if DC
wouldn't be a better choice for a modern power grid?

~~~
rollulus
High-voltage DC [1] is being used to transport power over longer distances
because it can be cheaper and there's less power loss.

Like a sibling commented here, the costs would be enormous to convert an AC
grid into a DC grid. AFAIK are there two reasons that the grid is AC:
generators produce AC, and AC is so easy to transform up and down. That
transforming is essential since you want to transport energy at high voltages
[2] but you don't want 380kV in someone's house.

[1]: [https://en.wikipedia.org/wiki/High-
voltage_direct_current](https://en.wikipedia.org/wiki/High-
voltage_direct_current) [2]:
[https://en.wikipedia.org/wiki/Electric_power_transmission#Ad...](https://en.wikipedia.org/wiki/Electric_power_transmission#Advantage_of_high-
voltage_power_transmission)

~~~
gmueckl
But, apart form installation costs, a low voltage DC power network inside
homes might make sense (e.g. 12V or 24V). There's a lot of modern electronics
that doesn't depend on the regular powerline voltage anymore. Unifying their
power supplies into a central unit should have some benefits.

~~~
ars
> a low voltage DC power network inside homes might make sense (e.g. 12V or
> 24V).

It wouldn't work. The needed wires would be like garden hoses.

~~~
thinkythought
Yup, there's a reason even POE is 48 volt in a lot of applications

------
alkonaut
My cheap and simple 15-minute UPS device has waveform generation (?) and a
battery. Surely with solar you often have to have _huge_ batteries?

In hot climates you have some power use when the sun is up (cooling, pool
heating,) but where I live I’d need my power most when the sun is absent in
night and winter (power use is probably 80% heating). Any amount of solar
power would be nearly useless if only available when the sun is shining.

~~~
mcny
> In hot climates you have some power use when the sun is up (cooling, pool
> heating,) but where I live I’d need my power most when the sun is absent in
> night and winter (power use is probably 80% heating). Any amount of solar
> power would be nearly useless if only available when the sun is shining.

Not a subject matter expert by any means but depending on how much sun you
get, there are probably better options than solar for you (solar might not be
the right choice for you).

~~~
alkonaut
Solar is increasingly popular around here though (latitudes 60-65) which I
only assumed also meant that storage was also handled with batteries. Seasonal
storage is probably not, but at least generation during the day and use during
the night must be.

I learned from the article that lower temps helps efficiency, so perhaps our
climate with short days and subzero temps for 5-7 months is still viable
because of higher efficiency?

~~~
namibj
Keep in mind that thermal storage is cheap compared to electrical, just not
efficiently reversible. E.g. you heat up a block of paraffin in a tub of water
and it takes a lot of energy without large temperature changes due to the
phase change. The output temperature would require rather large radiators or
floor/wall heating.

At this temperature you could even dump waste heat of a computer (well, CPU
and GPU, maybe DRAM, not the rest) into this thermal storage.

------
WalterBright
I'm curious why a large capacitor or inductor cannot be used to provide
momentary power for things like electric motor starts, and to smooth out
events like a plane briefly occluding the sun.

I built an audio amplifier in college that had a couple of huge capacitors on
the power supply (about the size of a can of soda). It would keep the amp
running a few seconds after I shut off the power.

~~~
mindslight
Fundamentally, the size involved. Your amplifier's capacitors were extremely
large for the power the amplifier was using, as their purpose was to reduce
the ripple voltage from the power supply.

One of those "1 Farad" car stereo caps (not the classiest example) can store
72 joules (watt-seconds). So to handle a 1kW excursion for 5 seconds, would
take over 70 of them. Possible, yes. But probably not worthwhile.

The frustrating thing about this article is that it keeps conflating
fundamental physical problems (like needing to store energy to deal with
peaks), with the limitations of currently-productized circuit topologies. It's
not hard to imagine an inverter that would do something similar to MPPT
without _needing_ to pull/dump the maximum power possible, would manage a
small bit of local storage (a combination of caps and abused batteries), and
could intelligently load shed (with "kind" brownouts) on multiple outlets.

It's also not hard to imagine an inverter (brushless motor) air conditioner
that would be a lot kinder of a load, and could even operate at variable
power.

But of course with the current state of homebuilding, these are bespoke ideas
better filed as "off the grid". Which is the entire larger point - an "off the
grid" system fundamentally costs more in materials and also presently
application engineering time, than the on-grid systems being sold to
homeowners.

(Although I do have to wonder about adding a piece to the current puzzle that
would generate/manage a local grid frequency, and shed excess power using off-
the-shelf electric heaters.)

~~~
WalterBright
> over 70

This doesn't sound that bad to me. Size isn't much of an issue, houses have
room for them. Prices for them seem all over the place:

[https://www.alibaba.com/showroom/1-farad-
capacitor.html](https://www.alibaba.com/showroom/1-farad-capacitor.html)
[https://www.amazon.com/dp/B00381UKD0/ref=psdc_2230642011_t2_...](https://www.amazon.com/dp/B00381UKD0/ref=psdc_2230642011_t2_B000701BA6)

The idea with a capacitor is they shouldn't degrade like batteries do.

The same with an inductor, which is just a coil of wire.

~~~
mindslight
$30 x 35, assuming a 50% discount for wholesale [0], is still $500. This may
be worthwhile, but it's certainly not negligible.

Electrolytics _do_ wear out. Lifetime hours versus temperature/ripple current
is a key design parameter.

[0] And assuming stuff designed for the car stereo market actually meets its
advertised specs. I just went to Youtube to verify that said devices weren't
just a can with other components inside of them. A video with a freehanded
jigsaw set to hip hop confirmed.

~~~
WalterBright
Just to be clear, the caps were bought from the Caltech EE supply desk. They
weren't for car stereos. It was before car stereos became a thing.

~~~
mindslight
I was just using car stereo caps as an easy-to-reference bound. (I'm not in
tune with current cost of bulk energy storage caps)

As I was saying before, the main difference from your amplifier was the amount
of power involved. In general, audio amplifiers have large capacitors on the
power rails to smooth out the ripple voltage, and will coast for quite some
time as they don't actually _need_ that much power for typical music and
volumes.

Here's a similar example - high current USB charging bricks, with a power
indicator LED. If you have no devices connected and unplug it from the wall,
the LED will stay on for quite some time. The LED just doesn't draw that much
current (say 10mA) compared to the capacitance, which has been designed for
smoothing the full charging current of 10A. Also the LED will happily ride the
voltage down from 5V to its junction voltage of 2.5V or whatever, using even
less current. And due to the logarithmic nature of your eyes (ears), you don't
fully perceive that dimming.

~~~
WalterBright
The caps were way, way excessive for the application, I was just having fun
putting them in. And they didn't just smooth out the power, they were capable
of running the amplifier for a few seconds. (It was a 125 watt Universal Tiger
amp.)

I do know that caps decay over time, but have no idea how that compares with
batteries. I expect, though, that long life caps could be built.

I also wonder about using inductors instead. They are, after all, just a coil
of wire.

~~~
mindslight
I feel like you're asking the equivalent of why we use DRAM instead of SRAM,
or spinning rust instead of flash. And I don't want to shout that down
because, as in the latter, constraints do change. But the current state of
things is fundamentally based on a cost tradeoff.

[https://electronics.stackexchange.com/questions/42084/can-
yo...](https://electronics.stackexchange.com/questions/42084/can-you-store-
energy-in-an-inductor-and-use-it-later)

[https://electronics.stackexchange.com/questions/326177/energ...](https://electronics.stackexchange.com/questions/326177/energy-
density-comparison-between-inductors-and-capacitors)

Perhaps in the future we'll have supercooled inductors providing power
storage, and not requiring periodic changing the way say batteries do. Alas,
it's just not right now.

As an aside, it's amazing how little power it actually takes to produce
worthwhile sound out of modern speakers. I once wanted to test some tower
speakers without a proper receiver around, so I wired up a breadboard with
some stupid opamps around poorly-heatsunk transistors, powered by a lab
supply. The speakers turned out to be disappointing, but not because of the
amplifier!

~~~
WalterBright
Thanks, that does answer the question.

------
segmondy
Of course, if you want to run off the grid, the first thing is to minimize
your electrical appliance. Get a laptop, the laptop will serve as your TV,
computer, radio, etc.

No TV, no Washer/Dryer (you will hand wash), No regular fridge, get a small
12v fridge. No microwave, use stove to heat up food. No A/C, design the space
to not get hot, high windows, under the shade, facing away from the sun. Maybe
a small fan. The biggest optimization begins with your appliances.

------
ggm
We found that even a well made Honda petrol genset was highly unstable feeding
volts into a UPS for a machineroom. They are good at lights. They are good at
a cooker. They suck at anything which expects regulated, non-noisy volts.

Lots of homebrew 'you can plug this magick cable I made' methods to supply
volts across the fence to neighbours houses when the power goes down are
really bad: behind the meter is just as lethal as in front of the meter, if
you get it wrong.

TL;DR if you need to read a website like this for clue how to avoid losing
power, you are putting your life at risk if you don't follow code, and don't
know what you're doing.

~~~
driverdan
Inverter generators like the popular Hondas are not good for surges. If you
run them at a steady draw they'll give you nice, clean power. If you your
demand fluctuates significantly they will stall or cut out and the voltage
during or after the stall may be dirty.

You have to read the specs carefully to make sure they are a good match for
your use.

~~~
ggm
Fridge. Surge load. Washing machine. Huge torque surge load. Airconditioning.
That's basically a fridge?

So.. "home" solar setup.. home.. like fridges, aircon and washing machines...

Do you see where this is going?

~~~
thinkythought
The solution here is inverter drive appliances. I had a cheapo but great
thrift store inverter microwave. My cheapo junk 3000w inverter could run it
off a few hundred Ah of batteries without issue, still spitting out clean
power on the other outlets. A regular microwave(with a lower run wattage in
theory, by about 1/3rd) would instantly blow the thing up and set off its
obnoxious alarm.

With some carefully selected appliances you would probably be fine. This stuff
used to be super pricey, but now even the cheaper brands are putting out
inverter drive ACs for example. Not window units, but mini splits and home
systems

~~~
ggm
Really, if you build from the ground up, or, if you are skilled enough to
engineer it, this is a lowish burden if not a no brainer.

If you are a prepper, with your existing lifestyle and think a genset will
give you tide-over-until-the-zombies-die, I think you need to think harder: a
genny won't work, if you don't pre-design the load for it. Lots of things you
own right now probably won't be very happy with unstable volts, and lots of
things you own right now will suck hard on the generator, making it surge.

So yes. you _can_ design for it. But a _typical home solar setup_ (which is
what the article was about) is not going to just work, offline, with your
generator. It takes more work.

------
bacon_waffle
There are all sorts of issues with using a regular grid-tied inverter in an
off-grid situation; there are regulatory issues besides the anti-islanding
(which is a complicated topic all on its own), and technical challenges too. A
lot of the technical challenges go back to the inverter effectively being a
thing that bolts on to the existing house wiring; I think that if they could
be more of a pass-through thing (like a UPS) then the picture would be a lot
simpler.

An off-grid capable inverter needs to handle several state transitions
smoothly, and some of those are hard. For instance, starting up can be a lot
harder when there's no mains voltage. An inverter will have a maximum output
current, which is generally not too far from the maximum power output of the
inverter divided by minimum mains voltage. If you have a load that presents as
a constant resistance, like a water heater, then as the inverter's output
voltage goes from 0 to 240V (120V for areas with weenie power) the current
will smoothly go from nothing up to whatever the load normally draws, along
with the voltage, and there's no problem. But! A lot of loads aren't resistive
- things like computers, battery chargers, and inverter-driven motors (as
newer appliances often have) will tend to look more like constant-power loads.
So, once they decide it's OK to turn on, if the voltage happens to be half of
nominal at that point, they will try to pull twice their nominal current. If
that causes the total draw to exceed what the inverter can provide, it stops
and maybe tries again. It's practically impossible to measure what the load
characteristic of something like a house, from the perspective of an inverter,
so the best you can do is something like guess-and-check, and that's not a
great solution.

Then, how to handle reconnection with the grid; you either need to re-
synchronise before reconnecting, or ensure that the intrepid homeowner first
shuts off the house before turning the switch to reconnect.

Switches that can handle that job may need to be specified per-installation,
and there are all sorts of regulations around how they work and where they can
go. They are not cheap, as a rule. Some regions have the solar generation
stuff on a separate export power meter from the import one. This means that
the switch needs to be on he supply side of both meters, and that could be
regulatorily complicated.

What about multiple inverters? These things all need to play nice with each
other when solo, or installed with others. Perhaps you didn't design those,
and they might try to start up too, then fire up their anti islanding which
should promptly trip out again.

Remember this all has to work with variable input power available, and it
needs to be packaged up so that installation is straightforward, and operation
is basically hands-off. Oh, and inexpensive too!

------
irrational
What I would like to know is, if there was an emergency situation and the grid
was going to be down for an extended period of time (I'm thinking 3+ weeks,
months, years) would there be a way of altering the system so that the home
could have electricity, at least during daylight hours?

~~~
sounds
The article points out that the most cost-effective solution is to get an
internal combustion generator.

------
parimm
The solution to this is to use Hybrid inverters+batteries with a auto
disconnect on the mains.

------
kingkawn
Forgive my ignorance in advance; is there any viability to a system that pumps
water uphill to a storage tank then it flows downhill later to let the current
run a generator?

~~~
taylortbb
Yes, in fact they're already in use. They're called pumped storage systems,
[https://en.wikipedia.org/wiki/Pumped-
storage_hydroelectricit...](https://en.wikipedia.org/wiki/Pumped-
storage_hydroelectricity)

------
foxhop
It's also not safe for line workers who think power is off to a main and get
zapped by a small fleet of home solar in The neighborhood.

------
gwbas1c
That article is TLDR for the target audience.

The end part that basically says, 'solar panels need batteries and a more
expensive inverter to run without the grid" needs to be at the beginning of
the article.'

The details of power curves then come for those who want more details about
why.

------
randyrand
instead of shutting off when the current is exceeded- why can’t you just use a
current limiting device and cap it?

modern bench top power supplies can limit current

------
stevewilhelm
Install a Tesla Powerwall. Problem solved.
[https://www.tesla.com/powerwall](https://www.tesla.com/powerwall)

------
tzs
I've been looking into backup solutions to keep my fridge and well pump [1]
going during outages. Anyone tried something like the following?

The obvious approach is a propane generator. Propane rather than gasoline
because we only get a small number of outages a year, and most are fairly
short, but there is occasionally one that is 10+ hours. I'd want to store
enough fuel for at least 10 hours of generator use, but most years would only
actually use a fraction of that, and so I'd have to be constantly rotating the
fuel (probably into my car, as the only other gas powered thing I have, my
lawn mower, only uses about a gallon a year) to keep it fresh enough to still
work for the generator. Propane does not go bad, so storing 10 hours worth of
propane for years is no problem.

But both of the things I want to power (fridge and pump) are very intermittent
loads. I don't want the generator running all the time during an outage,
wasting fuel, just to be ready when the fridge or pump needs to actually run.
Even idling the generator will be consuming fuel. I could leave the generator
off except when I need to actually have the fridge or well run, but that would
be quite annoying as I'd have to go outside to start and stop the generator--
and power failures are usually when it is quite unpleasant outside.

So what I've been thinking of is having a dedicated battery and inverter for
the fridge, and a dedicated battery and inverter for the well pump.

When power fails, I can start watching the fridge temperature (it has a
battery powered wireless temperature monitor, so I can see the temperature
without opening the door). When it warms up to the high end of its normal
variation, I can unplug it from the wall outlet, plug it into the inverter,
and start the inverter running off the battery. When the fridge cools back
down, turn off the inverter. Repeat throughout the outage.

Similar for well pump. When I run out of water, go run the pump from the
inverter and battery until the tank is full.

I'd still have the generator, but now it would be used to recharge batteries,
not power any house circuits or devices. Over time, I'd probably add
batteries, eventually reaching a point where I have enough that I can get
through most outages without needing the generator.

(Actually, I'd probably do the well directly from the generator instead of
inverter and battery. When I go out to turn on the generator to recharge
batteries, I'd also plug the well pump into the generator to refill the well).

I could start with one inverter and a battery or two, without a generator.
This would probably get me through a large fraction of the outages. Then over
time I could add more batteries, more inverters, and the generator to extend
the number of things I can use during an outage and how long an outage I can
get through, in a nice budget friendly way.

[1] I probably don't actually need the well pump--as soon as an outage starts
I fill a couple water bottles to have drinking water, and fill the bath tub to
have water for flushing the toilet, and that will almost always be good
enough. Why immediately fill things instead of just leaving the water in the
well tank and hot water heater until I actually need it? Because I have no
idea how much water I have. The outage could have happened right after the
pump finished filling the tank, or it could be just before the pump was going
to start and fill a nearly empty tank. Filling the water bottles and bathtub
right away lets me know what I've got.

------
HIPisTheAnswer
Off grid power != AC power.

Duh! Wtf do I want AC for? People think theyre all going to have a stupid
fridge which opens from the side to let as much heat in as possible and with a
highly inneficient compressor which is tucked _behind_ it where the heat can
only escape through it. Or a giant motor to shake their clothes in
(electrically heated) water! Ahahah! I swear i keep telling people the KWh is
going to cost 150-300 mg of gold (0.10 - 0.20 gold $) and their eyes just
glaze over; they think it sounds nuts? If they had off grid power, they might
be forced to agree. So the average person will be able to afford a couple or
three KWh per month. Two low-powered computers and some lights. We light the
whole place with less than 5w. The mere fact that we so often use measures
like _kilo_ watts is a good tell tale of the current power glut. The powergrid
rate where we are is about 0.0015 gold $ per KWh, or 100 times below sensible
prices.

