How battery size is calculated
Size a battery for the night, not the day. No signup, no email, and nothing leaves your browser — the numbers below are calculated on your device.
Nightly use ÷ depth of discharge ÷ inverter efficiency. The calculator takes your daily kWh, multiplies by the share you use at night (50% is typical), then divides by the battery’s usable share — 0.9 for lithium — and by inverter efficiency (0.95). A home using 30 kWh a day with half its use at night needs 15 ÷ 0.9 ÷ 0.95 = 17.54 kWh of usable storage, rounded to a 20 kWh battery (for example, 2 × 10 kWh).
Lithium is now standard because it safely discharges to about 90%; older lead-acid batteries only reach about 50%, so they need twice the rated size for the same usable energy. Battery cost is estimated at about $600 per kWh installed — a 20 kWh battery is about $12,000.
A 10 kWh battery delivers about 9 kWh of usable energy: enough for an evening of lights, TV, fridge and charging, but not a full night of air conditioning or electric heating. Batteries add cost and lengthen payback, so many homes start with panels only.
Worked example: a home using 30 kWh a day
Follow the calculator’s own defaults. Daily use is 30 kWh and half of it falls at night, so the battery must cover 15 kWh of night use. Lithium can safely give up 90% of its rated capacity, and the inverter loses about 5%, so the rated size must be 15 ÷ 0.9 ÷ 0.95 = 17.54 kWh. Batteries are sold in whole units, so the calculator recommends 20 kWh — two 10 kWh units — at about $600 per kWh, or $12,000 installed. One 10 kWh unit on its own delivers about 9 kWh of usable energy, which covers an evening, not a full night of heating or cooling.
| Battery worked example | Figure |
|---|---|
| Daily electricity use | 30 kWh |
| Night share | 50% |
| Nightly use to cover | 15 kWh |
| Depth of discharge (lithium) | 0.9 |
| Inverter efficiency | 0.95 |
| Usable storage needed | 17.54 kWh |
| Recommended battery | 20 kWh (2 × 10 kWh) |
| Usable energy from one 10 kWh unit | About 9 kWh |
| Battery cost | $12,000 ($600 per kWh) |
What changes the answer
Your night share is the first thing to get right: a household that runs a dishwasher and charges a car after dark may use 60% or more of its electricity at night, while a home that is empty in the evening may need far less. Chemistry matters next — a lead-acid battery at 50% depth of discharge needs almost double the rated size of a lithium one for the same usable energy, which is why installers now quote lithium by default. Purpose matters too: a battery bought for backup during outages is sized for your essential loads on the worst day, while one bought to use more of your own solar is sized for a typical evening. Winter shortens the solar day, so a battery that covers a summer night may fall short in December. And prices are still falling, so if the maths does not work today, check again when you get a quote — a smaller battery now can be extended later.
Size the panels first with the solar panel calculator, or work out your savings with and without a battery.
Sources
- U.S. Department of Energy — solar-plus-storage basics and battery guidance for homeowners.
- EnergySage — US battery costs per kWh and storage sizing benchmarks.
- Solar Energy Industries Association (SEIA) — US solar and storage market data.
By Shehzad Hussain. Published November 9, 2026. Last updated November 11, 2026.