Short answer: work out your daily energy use in kWh, decide how many days you want to run without much sun, then divide by the battery’s usable capacity. The maths is simple; getting honest numbers for winter is what most people skip.
The basic sizing formula
Battery capacity (kWh) = daily use (kWh) × days of autonomy ÷ (usable depth of discharge × inverter efficiency)
Worked example (illustrative numbers only): a home using 8 kWh a day, wanting 2 days of autonomy, with lithium iron phosphate (LiFePO4) batteries at 80% usable depth of discharge and 90% inverter efficiency needs about 8 × 2 ÷ (0.8 × 0.9) ≈ 22 kWh of nominal battery. Change any input and the answer changes a lot.
Step 1: measure your real daily use
List every appliance, its power in watts and the hours you use it. Multiply to get watt-hours, add them up and divide by 1,000. Separate essential loads (fridge, lighting, water pump, communications) from discretionary ones (kettle, heaters, tumble dryer). Electric heating and cooking are the loads that break most first plans. Our Power & Battery Load Planner does this step in a spreadsheet.
Step 2: choose days of autonomy
Autonomy is how long you can run without recharging from sun, wind or a generator. Rules of thumb commonly quoted are two to three days, more in gloomy climates. In the UK, winter generation from solar can be a small fraction of summer, so many systems include a backup generator instead of an enormous battery.
Step 3: depth of discharge and battery type
- LiFePO4 (lithium iron phosphate): commonly used to about 80% depth of discharge, with several thousand cycles quoted by manufacturers.
- Lead-acid: cheaper upfront but usually kept nearer 50% depth of discharge to preserve life, so you need roughly twice the nominal capacity.
Lifespans depend on temperature, charging and how deeply you cycle; treat manufacturer figures as best case.
Step 4: system voltage and amp-hours
Convert kWh to amp-hours by dividing watt-hours by system voltage (commonly 12, 24 or 48 volts). Higher voltage means lower current, thinner cables and less loss, which is why larger systems usually use 48 V.
Add a margin
Real systems lose energy in cables, inverters and cold weather, and batteries age. Many designers add a safety margin of roughly 10–15%. Also plan for growth: the appliances you buy next year will change the numbers.
Safety and professional help
Large batteries store a lot of energy. Poor fusing, wiring or ventilation is a fire and shock risk. Use a competent installer for final design, protection devices and inspection. Read our off-grid solar cost guide and solar panels and wind turbines too.
FAQ
How long do off-grid batteries last? Anywhere from a few years to well over a decade depending on chemistry, use and care.
Can I run a house on 10 kWh of battery? Only if your daily use is low and you have reliable charging; test against your own numbers.
Sources
Educational planning information, not electrical design or safety advice. Have any final system specified and installed by a competent person. Last checked: September 2026.
