What Size Solar Battery Do I Need?

What size solar battery do I need?

For most US homes, the answer lands somewhere between 10 and 13 kWh of usable capacity — enough to store a typical evening and overnight load and run essential circuits through a short outage. But the honest answer is "it depends on your goal." Before you pick a number, decide what the battery is actually for: shaving your evening grid use (self-consumption) or keeping the lights on when the power goes out (backup). Those two goals size a battery very differently.

This guide walks through both methods, explains the capacity numbers that actually matter, works through a real example, and flags the oversizing trap that quietly wrecks the payback math.

First, pick your goal

There are two reasons to own a home battery, and they pull in opposite directions.

• Maximise self-consumption. Your panels overproduce midday and you want to use that solar in the evening instead of buying it back from the grid. Here you size to your evening and overnight consumption.
• Backup power. You want the house (or its critical loads) to keep running during an outage. Here you size to your essential loads multiplied by the hours you want to cover.

Many homeowners want both. That's fine, but size for whichever goal is larger and check the other still works.

Method 1: sizing for self-consumption

The number that matters is how much electricity you use after the sun stops producing — roughly from late afternoon through to the next morning. Pull a few daily figures from your utility bill or app, estimate the slice consumed in the evening and overnight, and that is your target usable capacity.

A typical US household uses around 29 kWh per day. A large share of that is daytime, which your panels cover directly. The evening-and-overnight slice is often 8 to 12 kWh — which is exactly why 10–13 kWh batteries are the popular sweet spot. Store that much and you ride through the night on your own solar instead of grid power. Going bigger rarely helps: once you can cover the evening, extra capacity sits empty most nights. How much you can produce in the first place depends on your array, which comes back to how many solar panels you need.

Method 2: sizing for backup

Backup sizing is a different sum. List the essential loads you want to keep alive — fridge, a few lights, internet, phone charging, maybe a well pump or medical device — add up their power draw, and multiply by the hours you want them to run.

A modest essentials set might draw 0.5 to 1 kW on average. Cover it for 10 to 12 hours overnight and you need roughly 5 to 12 kWh. Want to run the whole house including air conditioning? That can demand far more, and you may need multiple batteries plus an inverter rated for the surge. Be realistic: backing up everything is expensive, while backing up essentials is affordable and usually enough.

Usable vs nominal capacity (and depth of discharge)

Here is the catch that trips people up. A battery's headline rating is its nominal capacity, but you can't safely use all of it. The usable capacity is what you actually get, set by the depth of discharge (DoD).

• A 10 kWh battery at 90% DoD gives you 9 kWh usable.
• Modern lithium iron phosphate (LFP) batteries typically allow 90–100% DoD; older chemistries far less.

Always size against usable kWh, not the number on the box. When you compare products, compare usable capacity.

Round-trip efficiency

No battery returns every kWh you put in. Round-trip efficiency — energy out divided by energy in — is around 90% for good lithium systems. So storing 10 kWh of solar gives back about 9 kWh. Factor a roughly 10% loss into your sizing so the battery genuinely covers your evening load rather than falling just short.

Worked example

Say your evening-and-overnight use is 10 kWh and you also want light backup.

1. Target usable capacity: 10 kWh.
2. Add ~10% for round-trip losses: ~11 kWh usable needed.
3. At 95% DoD that's a ~11.5 kWh nominal battery.

A single 11–13 kWh battery covers it, doubles as backup for essentials, and matches what most installers quote for an average home.

The oversizing trap

Batteries are the most expensive part per kWh of a solar setup, and an oversized one is money parked idle. If your battery routinely wakes up still half full, you paid for capacity you never use — and payback stretches out for years. A right-sized battery cycles fully most days, which is where the savings live. Bigger is not better; matched is better. The economics also hinge on your local utility plan, so weigh storage against simply exporting under net metering where that's still generous.

FAQ

What size solar battery do I need? For an average US home, 10–13 kWh usable covers a typical evening and overnight load. Size to your post-sunset consumption, or to essential loads times backup hours.

Is one battery enough for backup? For essentials (fridge, lights, internet) over a night, yes. Whole-home backup with air conditioning usually needs multiple batteries and a larger inverter.

What's the difference between usable and nominal capacity? Nominal is the headline rating; usable is what you can draw, set by depth of discharge. Always size against usable kWh.

Can a battery be too big? Yes. An oversized battery sits partly empty most nights and lengthens payback. Match capacity to your real evening use.

Bottom line

Start with the goal, not the spec sheet. For self-consumption, size to your evening-and-overnight kWh; for backup, size to essential loads times hours. Compare usable capacity, allow ~10% for round-trip losses, and resist the urge to oversize. For most US homes that points to a 10–13 kWh battery — the workhorse range that pays back fastest. See more in our batteries hub.

Last updated June 2026. Informational only — sizing depends on your usage, tariff and equipment; confirm with a qualified installer.