
Home Solar Resiliency - luu
https://www.jefftk.com/p/home-solar-resiliency
======
sxates
This is something I've been thinking about recently due to a potential new
policy from PG&E (Northern California electric utility) in which they would
preemptively shut down electricity to high fire risk areas when red flag
conditions are present. I live in the Oakland hills which would be affected by
that plan. PG&E says they expect such an outage would occur 2-3 times per year
in October/November, and that because they have to do a lot of safety checks
before restoring power, that we should expect those outages to last 3-5 days.

So 3-5 day outages 2-3 times a year sounds like a right pain the ass, and
certainly has me looking at adding a powerwall to our PV system. I had wanted
one anyway for backup in the event of an earthquake or other event that might
disrupt power for an extended time, but now we're almost guaranteed those
outage events, rather than it being a hypothetical.

~~~
amluto
Having tried and failed to work with Tesla for solar, I strongly suggest you
skip the powerwall. Use LG Chem’s RESU instead. It’s slightly cheaper, and
they are willing to let third parties install it. It pairs with SolarEdge’s
StorEdge.

If you want serious off-grid functionality, you need a beefier inverter.
Outback sells such things, but the resulting system is a good deal more
complicated.

~~~
sxates
SolarCity installed the PV system, so I figured it'd be easier for them to
continue with the upgrade. But it sounds like things are kind of a mess over
in Tesla/SC land these days, so who knows...

~~~
amluto
Another factor is that the Powerwall 2 DC coupled model got canceled. And I
don’t see how an AC coupled battery will give as reliable of a backup system
without complicated integration with the inverter and without extra automatic
switches that can isolate the battery from the loads.

In particular, how does an AC coupled system handle the case where the battery
was fully discharged before sunrise? The inverter needs to turn on when the
array starts producing, but the battery can’t charge without the backed up
loads being powered, and the backed up loads can’t be powered until there’s
enough battery charge to supply the inrush current.

A DC coupled system like the Powerwall 1, RESU, or pretty much any lead-acid
system doesn’t have this problem at all.

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zaarn
I've been looking into a bit of DIY solar lately. Not for powering the house
but for lights and other low-voltage (9 to 24V) tools, probably even charging
my laptop if I find a good adapter and a voltage regulator that can handle a
few amps.

Handling DC is much easier than AC here since you don't need to keep the grid
in sync with the inverter and you win efficiency due to not needing the
inverter either.

I'll probably switch out all my lamps for LED strips, RGBW and controlled by
arduino, so I could light up the house purely from solar without having to
think about the grid, store energy into a battery and get light overnight too.
The efficiency of the LEDs would help reduce power usage too.

I think about 200W should be peak usage if I do everything and charge my
laptops and use some tools like my soldering iron plus the wifi router. Should
be manageable at 24V (10A cables aren't too expensive). If all else fails, go
to 48V.

~~~
cbr
I looked into this some and found some of the most useful advice was for
people going to Burning Man. For example:
[https://www.theplayalabs.com/shopping-list-
solar](https://www.theplayalabs.com/shopping-list-solar)

My understanding is that for anything big like a fridge (even a minifridge)
the efficiency and reliability of standard well-optimized 120V AC appliances
means it's worth it inverting to AC instead of sticking to DC. Lots of RVs
use/used 12V DC systems so things are available, they're just not very good.
Inverters are very efficient these days. Plus it's really helpful to have a
way to run AC things you already have if you need to.

Though if you _only_ want lights, I do see how DC could make more sense if you
can either find or DIY the right electronics to handle the voltage issues.

~~~
zaarn
I'll probably not hookup fridges or anything large like that. My laptop would
be fine, it uses a switching power supply to 19V (though it charges of 12 to
24V just fine).

Most of what I want to power basically uses a power supply to go to DC already
including router and wifi, so it's not a problem, I just need to either find
the right DCDC converter or solder one myself (I'm confident I can solder
something together that should be able to hold 4 or 5 amps, above might need
thicker copper plating on the PCB).

------
gwbas1c
I just built my house, and it has a lot of solar panels. We planned to
generate enough to offset heating with electric heat pumps and charging two
cars.

What I did was set up a sub-panel with a generator interlock. For now, we're
going to use a portable generator in extended outages. Once batteries are a
bit cheaper, the batteries will run the sub-panel.

What about heat in a power outage? That's why we have a wood stove. You never
know when an ordinary fossil fuel furnace will fail!

~~~
cbr
_> What about heat in a power outage?_

Right now we'd have no heat. I'm thinking of putting together a small general
purpose battery backup that we can normally charge via the wall but can also
charge via the SPS on the solar system we're installing. I'd use it for
whatever was highest priority; depending on time of year that could be boiler,
sump pump, fans, or unexpected things.

------
IvyMike
Curious about SPS + A/C. Due to a medical condition in my household, having
A/C even during an extended power outage would make life a lot easier.

~~~
rbritton
Air conditioning is incredibly power-hungry when powered from an inverter. My
solar experience is with an RV, but even with that lower-powered AC unit, you
can realistically expect only 30-60 minutes with the batteries an RV can
carry.

A house won’t have the same space or weight restrictions, but that doesn’t
make them any cheaper. FLA batteries are the least expensive currently
available, and I’d ballpark it at around $4000 in batteries alone to get a few
hours of air conditioner.

~~~
coding123
An alternative to this is to have a generator that works with either propane
or car-gas.

------
politician
I'd also like to understand the reason for this restriction on whole house
power when the grid is down but the sun is shining. Does anyone know why that
might be the case?

~~~
sathackr
Most solar systems are grid-tie, in that they synchronize their output to your
local electric grid, and feed any excess power into that grid. They do this by
increasing their voltage output until all power generated by the panels is
consumed, either by your house or by the attached grid.

Because of the way these systems are designed, they do not work well when the
grid is not present. If grid power is down, and your solar system feeds power
into your home electrical system, you will also be 'backfeeding' power into
the grid. Since you likely cannot possibly generate enough power to satisfy
your entire neighborhood's power usage, this will in effect overload your
system. There is a second safety issue of feeding voltage into a grid system
that the power workers are expecting to be dead. Electrical code requires
these grid-tie systems to cease feeding power into the grid if the grid power
is removed.

Most of these systems do this by just shutting down -- unlike backup
generators with auto-transfer switches -- they usually do not have an
automated method to disconnect the grid power while still powering your house.

If you were to provide some sort of disconnect, due to the nature of these
systems, they would try to 'push' all available power into your house, because
they're designed to force all available solar power into the electrical
system. The grid can absorb this, but generally your home cannot.

They do have models that can function off-grid, and even models with attached
batteries to handle surge loads and night-time operation, but these models are
not as widely installed. Likely due to cost and/or owner education.

~~~
ballenf
Would a generator be able to supplant the grid? In effect have an auto-
transfer but to the diesel/natural gas/propane generator? The generator would
be idling except when power needs exceeded solar panels' capacity.

Might feel like a step backwards in terms of environmental impact, but
presumable the fuel used would be minimal while the sun was shining or
batteries not empty.

~~~
ridgeguy
> Would a generator be able to supplant the grid?

Yes.

Our neighbor across the street depends on a respirator. We live in a rural Bay
area location that has grid dropouts several times a year. Some last a few
seconds, some more than a day.

His setup: A transfer switch operates ~ 3 seconds after grid failure, followed
~ 2 seconds later by autostart of a propane generator. His house stays on
generator power until the grid comes back up for at least 5 continuous
minutes. Then the transfer switch flips back to the grid and the generator
shuts off.

Why propane? Because gasoline slowly degrades (polymerization) and needs to be
drained, disposed of, and replaced with new (typically every year or so).
Propane doesn't degrade over time.

~~~
cbr
_> Why propane? Because gasoline slowly degrades (polymerization) and needs to
be drained, disposed of, and replaced with new (typically every year or so).
Propane doesn't degrade over time._

If you have natural gas piped to your home then a natural gas generator is a
solid option. Expensive, though.

~~~
rootusrootus
A good bit cheaper than propane, though. The only people I know who use
propane do so because they don't have NG pipes to their house.

------
8bitsrule
_a blackout lasting more than a week..._

At 12 volts, a one-amp lamp draws 12 watts. A 24kwH battery ($5500 last year)
will power this lamp (neglecting losses) for 2000 hours. Say, 80 days.

So, used all day, 48 watts will last last 20 days, 96 watts 10 days. Used
half-time, you'd get about a week at 200 watts. Two hundred-watt light bulbs
for 10 hours a day.

Point being: solar power is great, but for -survival- purposes, our
2000kWh/month habits can't be sustained.

~~~
gh02t
I agree that high energy consumption is a problem, but this analysis is overly
pessimistic because you're not factoring in the fact that the system is
recharging during the day. As long as the average load is lower than your
average power draw you can sustain it indefinitely. The battery capacity only
needs to be there to compensate for your worst case scenario of bad
weather/reduced light exposure. So maybe not surviving a nuclear apocalypse
kind of scenario, but otherwise workable.

Also, a 100 watt LED "bulb" is ridiculously bright. That's far more light than
you'd get out of a tungsten bulb, 200 W of LEDs is [more than] enough to light
an entire house. Electric heating and cooling is the real problem when it
comes to energy consumption.

~~~
snowwindwaves
In a lot of places blackouts or grid failures tend to coincide with bad
weather that doesn't supply much solar charging. The bad weather is quite
often windy though so a small wind turbine to diversify the generation sources
is helpful.

Living off of solar and batteries means foregoing things like the toaster and
the blender.

~~~
gh02t
True, which is why your battery needs to be large enough to supply power
during your worst-case scenario for bad weather (or have back up systems like
wind/thermal like you mention). You also definitely want a plan for cutting
consumption to only essentials during such a blackout. A toaster / blender
really doesn't take _that_ much energy; sure they're 500 W or so, but you
don't typically operate them for long periods of time. The majority of energy
usage in most homes is devoted to heating and cooling, which are high power
_and_ run for long periods of time.

My point was that it's important to distinguish that long blackouts with
severe inclement weather are extreme events, and on average a solar power
system should be fairly stable as long as your usage is kept to essentials. I
completely agree with OP that you really do have to cut back on power-hungry
luxuries for solar-only to be really viable, but you should be able to at
least keep the lights on for quite a while. In terms of holding out during
more extreme events, yes... relying on any single energy source leaves you
vulnerable in that respect.

~~~
snowwindwaves
Here's a video of a family that lives off grid you might find interesting
[https://www.youtube.com/watch?v=v8Pe_u_4q5M](https://www.youtube.com/watch?v=v8Pe_u_4q5M)

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jamesrom
Most other commentators here have hit the nail on the head, in summary:

\- Backfeeding into the grid is unsafe

\- Backup generators will throttle to not produce too much electricity for
your home, so you can safely use a transfer switch

\- Solar can't be throttled, excess power needs to be used, stored somewhere,
or sent to the grid

~~~
toomuchtodo
Newer inverters _can_ throttle solar, as there are some markets where no
export is permitted to the grid (the inverter optimizes the MPPT to
effectively shut certain solar modules down).

[https://www.solaredge.com/us/solutions/feed-in-limitation-
an...](https://www.solaredge.com/us/solutions/feed-in-limitation-and-metering-
solution#/)

~~~
sathackr
I have such an inverter(SolarEdge HD Wave)[1], but you have to specify a
specific power level to throttle to -- and it requires some additional
supporting hardware to measure what it is feeding the grid with, and still
requires at least a sync signal from the grid.

I have not found an off-grid mode in its settings yet.

[1] [https://www.solaredge.com/us/products/pv-inverter/single-
pha...](https://www.solaredge.com/us/products/pv-inverter/single-phase#/)

~~~
toomuchtodo
It’s not user friendly (export limit setup), as it should be set by your solar
installer at system provisioning. There’s only a few markets in the world it
applies to (Arizona, Hawaii, parts of Australia).

In short, newer inverters can’t be overloaded if there isn’t enough load and
too much sun.

~~~
sathackr
I kicked the solar installer out after he did the physical work, I did the
rest of the provisioning and configuration after the inspections cleared and
the meter was swapped.

He was insisting on installing a cellular modem on my inverter and maintaining
remote access and control over it. I did not want this, so I did not allow
them to.

~~~
bonzini
Here they required installing a data logger which is really a Raspberry Pi
running a web server + a USB to RS-485 adaptor. It was required for warranty
beyond the legal minimum because they want to monitor the environment
temperature in the room where the inverter is installed. However it is not on
the public network and does not call home.

------
syntaxing
Does anyone have a good tutorial or a site that sells a off-grid small kit
(600Wish with 1-2kWh battery) with battery backup? I want to get one for my
house if its affordable enough as an insurance on top of a generator.

