Solar Battery Calculator
Work out how many kWh of battery you need for daily use or for backup, and what it should cost.
Batteries are priced by the kWh and chosen by the load. This calculator starts from either your daily energy use or a backup load in watts, then adds the days you want to ride through, the energy lost in the battery and the share of it you can safely use.
For most homes, a small battery that covers the essentials is enough: the fridge, lights, internet and a few outlets. Backing up an entire house costs several times more.
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Planning estimates only. Your roof, equipment, utility rules and weather change the real numbers, so get a written quote before you buy. Data as of October 2026.
How this calculator works
Energy to store is the energy you use per day times the days of backup. A battery wastes some energy going in and out (round-trip efficiency), and shouldn’t be emptied all the way (depth of discharge). The tool divides by both: battery size is the stored energy, divided by efficiency, divided by the usable share.
The defaults are 90% usable and 95% efficient for lithium, and 50% usable and about 80% efficient for lead-acid. These are common figures. Your battery’s data sheet is the final word, so change them if it says otherwise.
For lithium, the cost range uses the middle 60% of home battery installs recorded in Berkeley Lab’s Tracking the Sun for 2024 and 2025. That sample is small, so use the range as a guide and not as a quote. There’s no public price data for lead-acid, so enter your own price per kWh.
Amp-hours and voltage
Many batteries are sold in amp-hours at a set voltage. Watt-hours equal amp-hours times volts. The tool shows the amp-hours for the voltage you pick so you can match what’s on a data sheet. Above about 3,000 W, 48 V is the usual choice because it keeps the current and the cable size down.
Where the numbers come from
- Berkeley Lab, Tracking the Sun (public data file of 28 July 2026, installs through 2025), data as of July 2026.
Questions
How many kWh of battery do I need?
For a night of essential loads, 5 to 10 kWh is common. The average US home uses around 30 kWh a day, so backing up everything for a day needs far more. Start with the loads you can’t do without.
How long will a 10 kWh battery last?
Take the usable energy, about 8.5 kWh for lithium after losses, and divide by your load in kW. At 1.5 kW it runs about 5.7 hours.
Lithium or lead-acid?
For most new systems, lithium iron phosphate. It costs more per kWh, but you can use more of it, it wastes less energy and it lasts through many more cycles. Lead-acid only wins on a tiny budget or a rarely used backup.
Is a battery worth it with solar?
It depends on your utility. If it credits extra power at full price, a battery mostly buys backup during outages. If it pays little for exports or charges more for evening power, a battery can also save money. The Solar Savings Calculator doesn’t model batteries.
What system voltage should I pick?
12 V suits an RV or a small cabin. For a home system above about 3,000 W, use 48 V.
How many days of backup should I plan for?
One day covers most outages. Off-grid homes plan for two or three days without sun, and many add a generator rather than buy a much bigger battery.
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