Solar panels make electricity while the sun shines. Your home uses a large share of its electricity after the sun goes down — cooking dinner, running the television, charging phones, keeping the lights on. A home battery bridges that gap, storing daytime solar energy for the evening. But how big a battery do you actually need?

What a battery actually does

A home battery has two jobs. The first is night use: storing surplus solar power generated during the day so you can use it in the evening instead of buying electricity from the grid. The second is backup: keeping essential loads running during a power cut — the fridge, some lights, the internet router, perhaps a medical device.

These two jobs point to different sizes. Night use is a daily cycle governed by how much electricity you use after dark. Backup is governed by which loads matter to you and how long an outage might last. A battery sized for evening use will usually cover a short outage as well, but a battery sized only for backup may be far too small to shift your evening consumption.

Start with your nightly electricity use

The key number is not your total daily consumption but the share of it that falls after your panels stop producing — roughly from late afternoon to the following morning. For many households this is about half of daily use, though it varies: a home where everyone is out during the day might use 60 to 70% at night, while a household with someone home all day might use far less. If you do not have hourly usage data from a smart meter, 50% of daily use is a reasonable starting assumption.

Note that a battery can only store what your panels produce in surplus. Battery sizing and panel sizing belong together — check your expected daytime surplus with our solar panel calculator before choosing a battery size.

Depth of discharge: the capacity you can actually use

Batteries are sold by nominal capacity — 10 kWh, 13.5 kWh, and so on — but you cannot use all of it. The usable share is governed by the depth of discharge (DoD), the proportion that can safely be drained before recharging.

Modern lithium batteries, which dominate home solar storage, typically allow a depth of discharge of around 80 to 90% for everyday cycling, with lithium iron phosphate (LFP) models often reaching 90 to 95%. Older lead-acid batteries are far more limited, at around 50%. A 10 kWh lithium battery at 90% DoD gives you about 9 kWh of usable energy, while a 10 kWh lead-acid bank at 50% DoD gives you only about 5 kWh. Always compare batteries on usable capacity, not the headline number.

Do not forget inverter losses

Energy is also lost in conversion. Electricity from your panels is direct current (DC); your home runs on alternating current (AC). Converting between the two typically loses around 5 to 10% each way, giving a round-trip efficiency of roughly 85 to 90% — so a battery that stores 10 kWh may deliver only about 9 kWh to your appliances.

A practical sizing rule follows from this. Take your nightly need, divide by the depth of discharge, and divide again by the round-trip efficiency:

Nominal battery size = nightly kWh ÷ DoD ÷ efficiency

A worked example

Take a home using 30 kWh a day, with about 50% of that — 15 kWh — falling at night. Using a lithium battery with 90% depth of discharge and about 90% round-trip efficiency:

  • 15 kWh ÷ 0.9 (DoD) = 16.7 kWh
  • 16.7 kWh ÷ 0.9 (efficiency) = 18.5 kWh

So this household would need a battery of roughly 20 kWh nominal capacity to cover a typical night in full — for example, two 10 kWh units. A single 10 kWh battery would cover only about half the night’s use, which may still be worthwhile: covering the expensive evening peak alone can capture much of the benefit. Try your own figures with our solar battery calculator.

Common battery sizes

Most homes end up with one of a handful of sizes. A 10 kWh battery is the most common single unit: it typically covers the evening peak — cooking, lighting, television — for an average household, and runs essential loads for several hours in an outage. A 13.5 kWh unit, the size made familiar by the Tesla Powerwall, covers a larger share of the night and gives more backup headroom.

Larger homes, homes with an EV charged overnight, and homes in areas with frequent outages often go to 20 kWh or more, usually by stacking two or more units. Batteries are modular, so starting with one unit and adding another later is usually possible — worth confirming with your installer before you buy. Size for your typical night, not for the most extreme night of the year.

When a battery is worth it — and when it is not

A battery earns its keep fastest where the gap between what you pay for grid electricity and what you are paid for exported solar is large. If you buy electricity at 30 cents per kWh but are paid only 5 cents for exports, every kWh you store and use yourself instead of exporting is worth 25 cents. Where export payments are close to retail prices, the financial case weakens, because exporting surplus power already pays almost as well as storing it.

Time-of-use tariffs strengthen the case further: if your utility charges much more in the evening peak than at midday, a battery lets you dodge the expensive hours. And if your area suffers regular outages, the backup value alone may justify the cost.

A battery is harder to justify if you use most of your solar power during the day already, if your export rate is generous, if your electricity is cheap, or if your panels rarely produce a surplus. In those cases the battery will cycle shallowly and save little — and the money may be better spent on more panels, or on shifting loads like laundry into daylight hours, which costs nothing at all.

Batteries add cost and lengthen payback

A battery makes a solar system more expensive and, in purely financial terms, usually slower to pay back. A typical home battery adds several thousand dollars to a project, while the savings it generates — the difference between import and export prices on the energy it cycles — are modest per kWh. It also has a shorter life than the panels themselves: while panels are typically warranted for 25 years, batteries are usually warranted for 10 to 15 years, so a replacement may be needed during the life of the system. Buy one for the right reasons — evening self-consumption where the price gap is wide, or backup power you genuinely need — and run the numbers on your own import and export rates first.

Figures in this article: lithium depth of discharge of around 80 to 90% for everyday use, and round-trip efficiency of roughly 85 to 90%, reflect typical manufacturer specifications for modern home lithium systems. Check the datasheet for the specific battery you are considering.