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Battery Runtime

Solar Battery Calculator

Size the battery bank for an off-grid or backup solar system. Enter how much energy you use each day and how many days the batteries must last without sun.

Updated · Free, no sign-up · Runs in your browser

Quick answer

Battery bank (kWh) = daily use (kWh) × days of autonomy ÷ efficiency ÷ usable share. A home using 10 kWh a day needs about 12.3 kWh of LiFePO4 for one day of autonomy, which is about 257 Ah at 48 V.

Size your solar battery bank

kWh

From your bill: monthly kWh ÷ 30.

days

Days the battery must cover with no sun.

System voltage
How the load is powered

Battery you plan to buy

Ah
V
Advanced settings
%
%

Battery capacity you need

257 Ah

at 48 V, or 12.3 kWh of rated capacity

With your batteries

3 × 100 Ah 48 V

1 in series × 3 in parallel = 14.4 kWh, runs about 28 h

Average draw at the battery
463 W
Energy you will use
11.1 kWh

Rounded up to whole batteries. Add 10–20% if the batteries will sit in the cold or you expect to add loads later.

How to size a solar battery bank

  1. Find your daily energy use. Divide your monthly kWh from the electricity bill by 30, or add up each appliance’s watts × hours per day. For backup only, count just the essential circuits.
  2. Choose days of autonomy: how long the batteries alone must carry the load through bad weather or a grid outage.
  3. Choose the battery type and system voltage. The calculator adds inverter loss and keeps the battery within its safe depth of discharge.
  4. Enter the battery you plan to use, such as a 48 V 100 Ah rack battery, to see how many you need.

Solar battery bank size chart

Rated storage needed for one and two days of autonomy, with a 90% efficient inverter.

Daily useLiFePO4, 1 dayLiFePO4, 2 daysLead-acid, 1 day
5 kWh6.2 kWh12.3 kWh11.1 kWh
10 kWh12.3 kWh24.7 kWh22.2 kWh
20 kWh24.7 kWh49.4 kWh44.4 kWh
30 kWh37 kWh74.1 kWh66.7 kWh

Battery size is only half of a solar system

The battery has to be refilled every day, so the solar array must produce your daily use plus charging losses (roughly 10–15%), on top of what the house uses while the sun is up. In winter, with fewer sun hours, that can mean a much larger array than summer suggests. A battery bank larger than the panels can recharge just sits partly empty.

For a single appliance or a short outage, the battery size calculator works in watts and hours instead. To check how long an existing battery bank will last at a given load, use the battery runtime calculator, and see how battery runtime is calculated for the method.

Frequently asked questions

How do I calculate solar battery size?

Battery size (kWh) = daily energy use (kWh) × days of autonomy ÷ inverter efficiency ÷ usable depth of discharge. For a home using 10 kWh a day that wants one day of backup with LiFePO4 batteries (90% usable) and a 90% efficient inverter: 10 × 1 ÷ 0.9 ÷ 0.9 ≈ 12.3 kWh.

How many solar batteries do I need to power a house?

Find your daily use from your electricity bill (monthly kWh ÷ 30). An average home using 30 kWh a day needs about 37 kWh of LiFePO4 storage for a full day without grid or sun. Most grid-tied homes size for the essential loads only (fridge, lights, internet, some outlets), often 5–10 kWh a day, which needs about 6–12 kWh.

How many days of autonomy do I need?

For a grid-tied home with backup, one day covers most outages. For an off-grid cabin used year-round, 2–3 days covers runs of cloudy weather; very cloudy climates or critical loads may need 3–5 days, or a generator as backup instead of a much larger bank.

Is LiFePO4 or lead-acid better for solar storage?

LiFePO4 is now the default for solar: you can use about 90% of its capacity daily, it lasts thousands of cycles and needs no maintenance. Lead-acid costs less up front but you should only use about 50% of it, so you need nearly twice the rated capacity, and it wears out much sooner under daily cycling.

Does the solar battery need to be 48V?

Not always, but 48 V is the standard for home systems above about 3 kW of inverter output because it keeps currents and cable sizes manageable. Small cabin and RV systems often use 12 V or 24 V. The energy you need in kWh is the same at any voltage.