
How much battery storage does a solar PV system need? - martinald
http://euanmearns.com/how-much-battery-storage-does-a-solar-pv-system-need/
======
phire
Battery storage is simply not meant to cover that kind of gap. It's intended
more to cover the overnight period, when you have no sun, or maybe for a few
days with bad weather.

You need to spec your system so at the very least it generates enough power on
the worst days of the year to cover your needs, and surplus power on good
days. You also should have a second means of power generation, wind works
nicely as it's uncommon to have a day that isn't either windy or sunny.

~~~
codecamper
The OP does not have a single clue about actually using solar. His price per
KWH is about 1 order of magnitude off (and then some).

I've used 100% solar for the past 2 years and there has not been a single day
that I needed a second source of power.

Also my system is quite modest. Just 500W of panels and about 600Wh of lithium
ion battery (lifepo4) energy storage.

The 500W of panels is plenty. Most days I charge my batteries fully before
noon.

The 600Wh of batteries is enough to get me through 3 days of heavy cloud
weather ( you get about 10% of your maximum generation on a heavily clouded
day ).

This system is for two people using our computers and 3g wireless for the
entire day. LED lights at night which are bright and warm, water pump, food
processor, stereo, iPad, hella gadgets, raspberry pi backup server, etc etc

If I changed something, I'd double the batteries that I use.. going to 24V.
You get more of the rated power of the solar panel when using a 24V system. I
am considering getting an electric bike & storing extra power there.

The total cost of the system was about $800.

In the summer I can generate about 2 kWh / sunny day -- I only need about
400Wh / day though. You do the math for the cost per kWh. It's damn cheap.

~~~
jacquesm
> The OP does not have a single clue about actually using solar.

Exactly. It's a total nonsense proposition to offset seasonal swing using
battery power. Running a PV system is about conservation first and foremost
and some planning second. Your battery is for the day/night cycle and overcast
days, _not_ for seasonal variations. Maybe he should run a PV system for a
while before writing articles like these.

~~~
raesene4
I think the original article was looking at the proposition of off-grid energy
use using purely solar, in which scenario seasonal variations matter

From the article "Even with these rough numbers it’s hard to see how anyone
living any distance from the Equator is going to be able to justify the cost
of installing enough batteries to go off-grid with a domestic rooftop solar PV
installation."

~~~
jacquesm
> Even with these rough numbers it’s hard to see how anyone living any
> distance from the Equator is going to be able to justify the cost of
> installing enough batteries to go off-grid with a domestic rooftop solar PV
> installation.

If you go off grid on purely solar your backup is not a battery, your backup
is a genset.

It's a nonsense proposition leading to nonsense conclusions based on nonsense
numbers. Very tiring.

~~~
raesene4
I'd agree, I was just pointing out the basis that the author was coming from
which was the reason why he was considering seasonal variations in battery
calculations.

With that said, from reading some articles about off-grid living, it's not
outwith the bounds of possibility that people are looking at pure solar (or
pure renewable anyway) off-grid living, for priniple-based reasons , rather
than a more pragmatic rationale.

~~~
jacquesm
Only if they don't care about their lives. Look, I've lived off the grid for a
number of years. 99% of the time things work just fine and all your plans come
out. And then, in the middle of January there is a 9 day spell without wind
and solar and you're going to have to fire up the genset. Tough, but
preferable over having all your waterlines freeze due to 'principles'.
Principles are nice to have but when you go off-grid out of principle the one
principle you don't want to throw overboard is reliability and a good
generator is worth gold when everything else you carefully planned out fails.
If there is one thing that you can predict about the weather then it is that
it is unpredictable.

~~~
codecamper
I guess a genset doesn't add much to the final bill?

~~~
jacquesm
A small one is better than nothing in a pinch and a 2 KW gas powered genset is
a few hundred bucks. I went the overkill route (diesel, 6.5 KW) but I also had
a machineshop attached to the house and it was nice to run the mill or the
lathe without draining the battery in a couple of hours.

------
shalmanese
In this hypothetical scenario, instead of investing so much in batteries, why
not just double the size of your solar array so you're not storing 6 months
worth of power in batteries? That would only cost ~20K instead of the ~500K
for batteries.

~~~
jacquesm
You use the battery to offset the times when the sun does not shine so more
solar power does not work. You don't use batteries to offset the summer/winter
cycle, you use them to offset the day/night cycle and to store a bit more in
case the next couple of days are overcast.

The whole winter/summer storage thing is nonsense, nobody sizes a PV system
like that.

~~~
raesene4
well nobody apart from people looking for an off-grid lifestyle of course.
What I took from the article is that pure solar off-grid is impractical away
from equatorial latitudes.

~~~
jacquesm
If that's what you too from the article then you're right 'pure solar' is
impractical (anywhere). You always need a battery and you _always_ need some
kind of backup.

But if you're going to think like that grid power is _also_ impractical
anywhere, since it too could fail. Power is so important that no single source
should be relied on anywhere. In inner cities of course your only chance of
getting power is the local electricity company hookup, but if that should fail
for any extended period of time such a city would rapidly become a place where
you could no longer live, especially at latitudes where it tends to get very
cold in the winter.

So, nobody (in their right mind, at least) will do a 'pure' photovoltaic
system. There will always be some kind of backup-power mechanism, either the
grid, a wind-turbine, a battery to tide you over, a pelton wheel fed from a
stream, grid power or any combination of the above.

~~~
ghshephard
I really liked the article - it will be interesting to see it updated every
5-10 years as Battery Technology (slowly) improves.

In reality, what most people (today) who want to live off grid end up doing is
hybrid-systems. Natural Gas/Propane for heating, cooking, cooling, and Solar
for electrical use. (computer, lights)

~~~
jacquesm
People living off the grid tend to use wood for heating. Outdoor woodstoves
are the thing, and for cooling they open _two_ windows ;).

------
jacquesm
> 1\. Household consumption is 5,000 kWh/year. (About right for Western
> Europe, low for the US, high for most of the rest of the world.)

So, to stick with HN's tradition of making negative comments about articles:
This author is totally clueless, that's not at all how you size a PV system
(and the battery to go with it). Battery power in a solar system is not used
to offset seasonal variation but the day/night cycle and a possible overcast
day (or two).

Then the first line listing the assumptions made starts from the average
household but a PV system designer that is designing an actual system rather
than a theoretical one will start with throwing everything out that you can
miss, the household will be anything but 'average'. Electric heat, AC and a
whole raft of other big consumers get thrown right out before you even start
to think about what your consumption will be. No romantic array of
incandescent spots for you, and no baseboard heat. Everything you can save you
won't have to generate and store and that's your first gain. This will cut the
consumption in half or even better and _then_ you can start wondering about
how much PV you need, if you're going to have wind or water to augment that,
what size you genset will have to be (in case all of the above aren't
available for a longer period) and what kind of storage battery you need. (And
of course what size inverter(s) to convert your DC back to AC tied into the
distribution panel of your house, you really don't want to run your household
on DC)

A good rule of thumb for battery storage for a system that is off-grid and
totally self-reliant is to have 5 days worth of consumption leading to 33% or
20% of battery discharge depending on how nice you want to be to your
batteries. That's a _whopping_ large battery, most likely larger than what
your friendly car company can sell you in one chunk so you'll need multiple
units ganged together via some kind of bus system.

When you go off-grid your life can depend on the working of the system, if
you're snowed in and you can't get fuel, the sun lets you down and there is no
wind then you fire up your trusty genset (preferably a diesel, you might be
running it for a week straight in an emergency) to put some much needed charge
into your batteries. Of course you anticipated this and you have two weeks
worth of fuel stored.

Also, in very cold weather battery capacity tends to be reduced.

So, in short design your house and your life around your energy budget and
even then you'll find you need a rather very large battery.

~~~
ZeroGravitas
He's not clueless, he's intentionally writing propaganda to undermine solar
power, to help boost his hobby-horse of nuclear. He has no genuine interest in
answering the question posed in the headline.

~~~
maxerickson
Read the follow up linked here:

[https://news.ycombinator.com/item?id=10512199](https://news.ycombinator.com/item?id=10512199)

------
ra
The amount of battery storage required for a household on-grid is much smaller
when you can think of the grid as a load balancing device.

Of course we'd need to get away from the huge mismatch between feed-in tariffs
and consumption tariffs.

Ideally the difference would be a small spread (like in a currency exchange
rate) and a nominal daily connection fee.

In which case a 10KW Tesla powerwall is a pretty handy size.

~~~
crdoconnor
>Of course we'd need to get away from the huge mismatch between feed-in
tariffs and consumption tariffs.

Maybe first we could nationalize the whole outfit and run it on a cost-plus
basis. There is way too much profit being made from being a monopoly grid
operator.

That probably has more to do with why solar is such a good deal than the green
energy subsidies do.

~~~
maxerickson
In the US I'm under the impression that generation and transmission are pretty
heavily regulated. Do you have a favorite example company that makes egregious
profits?

------
anovikov
Of course the example is wrong because for most of the places it is resolved
by simply having a larger PV array so that even winter generation is
sufficient. That will probably not work for 60N, but easily for 40N: an array
about the size calculated for 60N, used at 40N, will not require any seasonal
storage.

And of course, nobody uses storage like that. Off-grid system must combine
solar and wind, wind typically gets stronger in winter, plus about 24 hour
worth of energy consumption in storage (a couple Tesla systems), and finally a
diesel generator for a few percent of the total generation for the unlucky
days when sun doesn't shine and the wind doesn't blow.

------
Johnythree
The usual approach is to take a conventional house design, and then design a
solar supply to meet its needs.

This results in a crazy large system due to our appalling wasteful house
designs (which in turn are due to historically cheap electricity).

A better approach is to decide how much you can spend on solar, then design a
house which will function within that limitation.

To succeeded you will need to change house designs, select different type and
size of appliances, and most importantly, make some personal sacrifices.

If you can live within these limits, then yes, you will enjoy having Solar. If
not, you would be better to continue with your existing expensive lifestyle.

------
ghouse
How many 9s of reliability do you want to pay for?

Speaking only regarding residential energy: In North America, other than
Hawaii as of a couple weeks ago, solar generation banks credit for electricity
exported to the grid. In effect, a 100% efficient, and free, battery. In
Austrlia, exported power receives credit only equal to the offset generation
-- less than 1/3 of the total price of electricity. In those cases, there is
an economic evaluation.

Otherwise, batteries are for reliability. And apparently there are more
residential backup generators in the US than there are residential rooftop
solar.

~~~
jacquesm
Going off-grid when the grid is present and reliable is dumb. Even without
net-metering being 100% as long as you cover your own use 90% of the time it
is still much cheaper than battery power (charge/discharge costs more than
what power from the grid costs, batteries have a life-span).

So in that situation you try to cut your consumption when you generate little
power or you take the hit by paying for grid power.

The larger chunk of your bill in that case will be the termination fees, the
electricity bill typically has two components, one where you pay a fixed fee
for the connection whether you use it or not and another you pay per used KWh.
The first component can handily outstrip the second making those very
expensive KWs, especially when you're close to running of your own power most
of the time.

Still, if you think of it as a replacement for both your genset _and_ your
very large battery it may make economic sense to do this anyway. It all
depends on the location and reliability of the grid. Where I lived our PV
system was many times more reliable than the local grid so we ended up
ditching the hookup but that's a very exceptional situation.

~~~
chris_wot
That's what this doesn't take into account. If you install batteries, you use
them for only part of the time to make up for times the PV doesn't generate
electricity. This offsets costs of drawing from the grid. But when you need to
draw from the grid, you do so.

------
aexaey
You really shouldn't plan battery storage for winter/summer cycle. Batteries
are for day/night cycle. If stretched a bit, then maybe batteries will cover
"that rainy day, when I need to do laundry" and similar. But not much more, as
you've seen yourself - this is prohibitively expensive.

For winter/summer cycle, if you're south of 40° N (and north of 40° S) just
oversize PVs. Otherwise, you need a second power source - grid, wind or
geothermal.

Not to mention that having a second source is really good if you ever plan for
maintenance or emergency repairs.

------
alephnil
One thing this article shows, is how different solar is on high vs low
latitudes. Up to around 40 degrees, solar is practical, and you only need to
take the nights and possibly cloud cover into account, at 60 degrees, the
seasonal variation is too high to be stored in a parctical way, even with his
assumption of flat consumption through the year, which is quite unrealistic
with the short days and low temperatures at high latitudes. It means that off
the grid solar is unrealistic in Northern Europe, but it can be a possibility
many other places in the world.

~~~
jacquesm
I ran an off-the-grid fairly normal house in Northern Canada, solar was > 70%
of our power. Seasonal variations are not leveled by batteries but by properly
dimensioning the system and reduced consumption. So off the grid solar is
_very_ realistic in higher latitudes, especially if you take into
consideration that panels will generate a lot more power when they're cold
than when they are warm. The most power I ever saw generated instantaneously
was on a terribly cold day in February (but that was exceptional). Sure the
days are shorter but you adapt to the reduced energy budget fairly easily.

Off the grid solar is unpractical the world over if you don't adapt your
consumption.

~~~
codecamper
Wow that is great that you were 70% solar in North Canada. Yes I've heard that
panels can output actually more than their rated capacity when sunny & cold.

I think the big problem people have with going PV, is that there isn't a good
manual on how to go solar. & much of it is changing and improving as we speak.

~~~
jacquesm
There may not be a manual simply because every situation is different (and
it's a personal thing too), but there is a huge community of renewable energy
enthusiasts at [http://www.otherpower.com/](http://www.otherpower.com/) , they
have a discussion board at
[http://www.fieldlines.com/](http://www.fieldlines.com/) . Collectively they
have a huge amount of knowledge about anything related to renewables and
anybody considering going off grid could do a lot worse than to become a
member and to start reading forum threads. They range all the way from people
interested but not actually doing anything to people operating fair sized
solar and wind farms and everything in between.

------
nkurz
The author wrote followup article in which he estimates costs for a more
reasonable system that is sized for winter production rather than annual
storage:

A Potential Solution to the Problem of Storing Solar Energy – Don’t Store It.

[http://euanmearns.com/a-potential-solution-to-the-problem-
of...](http://euanmearns.com/a-potential-solution-to-the-problem-of-storing-
solar-energy-dont-store-it/)

As commenters here and on the blog suggest, this is a much more cost effective
approach (although still prohibitively expensive in extreme latitudes).

~~~
ZeroGravitas
Across two epically long blog posts, that's a lot of words, numbers and graphs
to say "it's sunnier nearer the equator" and "electric grids are handy, don't
abandon them for no reason", all delivered with a lot of _attitude_ for no
obvious reason.

Is there some context outside of these two posts that makes them reasonable
things for someone to have spent so much time and effort on?

------
chris_wot
5,000kW per year is _low_ in the U.S.?

That is extraordinarily wasteful. In most of Australia the average is about
2300kW per quarter, and even that is probably a high estimate. But if 5000kW
is low then I'm assuming the average has to be 6000kW, or 6500kW! That a HUGE
amount of power!

What on earth are people in America using?!?

~~~
zurn
You might have mixed per-quarter and per-year numbers there.

According to [http://shrinkthatfootprint.com/average-household-
electricity...](http://shrinkthatfootprint.com/average-household-electricity-
consumption) the average (in MWh/a) is 11.7 for USA and 7.2 for Australia,
both at the high end of first-world countries. It's 3.5 for Germany for
instance.

I always wondered why we measure consumption in those units, instead of just
using average power directly. 11.7 MWh/a = 1.33 kW.

~~~
chris_wot
No, not at all. I'd like to know where they got those figures! In NSW,
Australia's largest state, IPART (the Independent Pricing and Regulatory
Tribunal) who until about a year ago set electricity prices, gives different
figures for 2010:

"Households with 1-2 residents use about 1,400 kWh of electricity per quarter

Households with 3-4 residents use about 2,100 kWh of electricity per quarter,
and

Households with 5 or more residents use at least 2,700 kWh of electricity per
quarter."

[http://www.ipart.nsw.gov.au/Home/For_Consumers/Compare_Energ...](http://www.ipart.nsw.gov.au/Home/For_Consumers/Compare_Energy_Offers/Typical_household_energy_use)

Edit: I must be tired. My calculations are way off. Sorry about that.

------
crdoconnor
IIRC the Tesla batteries are actually really pricey for what you get because
of the brand. I think you can get something from a 'lesser' brand for about
2/3 the price, possibly even less.

In other news, name brand sneakers are also expensive and probably not really
worth it.

~~~
ghshephard
I'd love to see a citation. All I recall people saying when the Tesla
Batteries came out was, "How can they sell the system for so little money?"
and that "There was nothing out there at that price point"

[http://www.forbes.com/sites/jeffmcmahon/2015/05/05/why-
tesla...](http://www.forbes.com/sites/jeffmcmahon/2015/05/05/why-tesla-
batteries-are-cheap-enough-to-prevent-new-power-plants/)

------
orasis
Troll post.

------
knappador
Let's look at chemical storage with hydrocarbons in the mix, allowing some as-
yet non-commercial (air-metal & combined cycle home generators) to make good
food for speculation.

A consumer to operate a small gas turbine and store their energy in the form
of hydrocarbons. If this is to be sensible, then 43MJ/kg would have to be
enough, because that's what they're going to get with kerosene/jet fuel.

Now let's consider lithium. The problem with metal-air batteries has mostly
been poisoning by unwanted gases. Let's assume for a moment that a membrane
material is developed that only allows high purity oxygen through, but suffers
low throughput. This is no problem for our off-grid solution because we're
talking about large storage relative to power, similar to some flow battery
applications. Using some method of specific ion or biological style solution,
such a membrane is not implausible.

Lithium-air weighs in at 40MJ/kg specific energy. How much does it cost? Given
that a metal-air battery uses lithium oxide, the lithium carbonate and lithium
hydroxide prices are not quite valid guides, but the price of $7/kg is
obtainable for the carbonate of "battery grade." If the pure oxide can be the
material used in manufacture, then possibly the $7 value is in the ballpark.

Jet-A is $0.45 per kilogram. 1.41 $/gal / 3.78541 l/gal / 0.820 kg/l

Higher efficiency must be allowed because even with combined cycle at high
latitudes where it's cold, we're looking at 90% round-trip for a battery vs
~75% for our Brayton cycle + home heating.

There are external costs to a large tank of kerosene and a pile of lithium
carbonate. One requires a gas turbine and the other requires a battery &
significant manufacturing. Excepting those two things, we're left with a
roughly 7:1 factor of cost for the raw material, so a seven year payoff it is.

Lithium might not be the cheapest rechargeable precursor. It might be great
for EV's (high power output is a requirement) and maybe something else will
show up as the cheapest energy storage without requiring off-grid people to
rely on ARES gravity trains.

Perovskites (new paper just suggested that hot-carrier phonons can be used to
raise theoretical efficiency to 66%) throw yet another wrinkle into solar.
Both energy storage and generation could become quite a bit cheaper.

All hypothetical, yes, but my napkin does tell me not to rule out off-grid,
not that I'm sympathetic to off-grid, anti-social Ayn Rand acolytes =D
Whatever matters for off-grid matters for a ton of settings, such as places
where there is no grid still. It's tough to say definitively that a straight-
renewable energy source won't become cost-competitive even in grid areas of
high latitude within the decade.

Source on Jet A specific heat:
[http://www.exxonmobil.com/AviationGlobal/Files/WorldJetFuelS...](http://www.exxonmobil.com/AviationGlobal/Files/WorldJetFuelSpecifications2005.pdf)

Source on Lithum Air theoretical specific energy:
[https://en.wikipedia.org/wiki/Lithium%E2%80%93air_battery](https://en.wikipedia.org/wiki/Lithium%E2%80%93air_battery)

Jet A price: [http://www.indexmundi.com/commodities/?commodity=jet-
fuel](http://www.indexmundi.com/commodities/?commodity=jet-fuel)

Lithium Carbonate Price (ev grade):
[http://www.globalstrategicmetalsnl.com/_content/documents/40...](http://www.globalstrategicmetalsnl.com/_content/documents/405.pdf)

