Solar Battery Size Calculator
A typical US home uses about 30 kWh of electricity per day — at 80% depth of discharge, backing that up for one full day takes roughly 37.5 kWh of lithium battery storage. This calculator sizes your battery bank three ways: from daily usage, from your appliances, or from amp-hours at your system voltage.
Most US homes use 25–35 kWh per day (EIA). Check a few power bills and use your own average for a tighter size.
Estimates only. Battery sizing depends on installer design, local electrical code, and your real loads. Get a professional design before buying equipment.
How the solar battery size calculator works
Batteries are sold by nameplate capacity, but you can only use part of it. A 10 kWh lithium battery at 80% depth of discharge gives you 8 usable kWh; a 10 kWh lead-acid bank at 50% gives you 5. The core formula is simple: required nameplate kWh = daily kWh × days of backup ÷ depth of discharge.
Days of autonomy is the judgment call. One day covers the overnight gap when panels recharge daily. Two to three days carries you through cloudy stretches — the standard for off-grid design. More than that and a small backup generator is usually cheaper than extra batteries.
battery kWh = daily kWh × days ÷ DoD · Ah = kWh × 1000 ÷ volts · usable kWh = nameplate × DoD
Related tools
Keep exploring: size the full array with the Off Grid Solar System Calculator, or pick a backup power source with the What Size Generator Do I Need Calculator.
Solar battery size calculator FAQ
How many kWh of battery do I need for my house?
A typical US home uses about 30 kWh per day. At 80% depth of discharge, one full day of backup takes 30 ÷ 0.8 = 37.5 kWh of lithium storage. Most households that only back up essentials (fridge, lights, internet, fans) get by with 10–15 kWh.
What is depth of discharge (DoD)?
Depth of discharge is the share of a battery's rated capacity you can actually use. Lithium (LiFePO4) batteries safely use 80–90%; lead-acid batteries should stay near 50% or their lifespan collapses. Always divide your usable need by the DoD — that is what the calculator does.
How long will a 10 kWh solar battery last in an outage?
A 10 kWh lithium battery holds about 8 usable kWh at 80% DoD. Running 800 W of essentials (fridge, lights, laptop, fans), that lasts roughly 10 hours. A 1,500 W space heater alone would drain it in about 5 hours.
Lithium vs lead-acid solar batteries — which is better?
Lithium costs more upfront but lasts 10+ years at 80%+ DoD with almost no maintenance. Lead-acid is cheaper to buy, lasts 3–5 years, and only gives 50% usable capacity. Over a decade, lithium usually wins on lifetime cost per usable kWh.
How many solar panels do I need to charge my battery bank?
Size panels from daily use: panel kW ≈ daily kWh ÷ peak sun hours ÷ 0.8. A 30 kWh/day home with 4.5 sun hours needs about 8.3 kW of panels — roughly twenty-one 400 W panels. The Off Grid Solar System Calculator does the full array-plus-battery math.
What size inverter do I need with my solar battery?
Size the inverter to your peak simultaneous load, plus margin: a home that can draw 6 kW at once wants at least a 7–8 kW inverter. Undersized inverters trip when the well pump or AC compressor kicks in.
Do solar batteries work during a power outage?
Only with the right inverter. Standard grid-tied systems shut down in outages for safety. You need a hybrid or off-grid inverter with islanding capability for the battery to carry your home when the grid drops.