Calculating your battery capacity needed is straightforward once you know the Battery Bank Size formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Battery Bank Size Calculator.
What is Battery Bank Size?
The Battery Bank Size calculation tells you your battery capacity needed from a few simple inputs. The figure you are solving for here is the battery capacity needed.
The Battery Bank Size formula
The core formula is:
Battery capacity needed = Daily energy use × Days of backup ÷ (Depth of discharge ÷ 100 × Battery bank voltage)Here is what each input means:
- Daily energy use — a value measured in Wh. Example: 2,000 Wh.
- Days of backup — a number. Example: 2.
- Depth of discharge — a percentage, such as an annual rate. Example: 5%.
- Battery bank voltage — a value measured in V. Example: 12 V.
How to calculate it step by step
- Write down the daily energy use (for example, 2,000 Wh).
- Write down the days of backup (for example, 2).
- Write down the depth of discharge (for example, 5%).
- Write down the battery bank voltage (for example, 12 V).
- Apply the formula above to get your battery capacity needed.
- Double-check the result with the Battery Bank Size Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Daily energy use | 2,000 Wh |
| Days of backup | 2 |
| Depth of discharge | 5% |
| Battery bank voltage | 12 V |
| Battery capacity needed | 667 |
With daily energy use of 2,000 Wh, days of backup of 2, depth of discharge of 5% and battery bank voltage of 12 V, the battery capacity needed works out to 667.
Example 2
With daily energy use of 4,000 Wh, days of backup of 2, depth of discharge of 5% and battery bank voltage of 12 V, the battery capacity needed works out to 1,333.
| Result | Value |
|---|---|
| Battery capacity needed | 1,333 |
Example 3
With daily energy use of 1,000 Wh, days of backup of 2, depth of discharge of 5% and battery bank voltage of 12 V, the battery capacity needed works out to 333.
| Result | Value |
|---|---|
| Battery capacity needed | 333 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
Prefer not to do the maths by hand? — the Battery Bank Size Calculator does it instantly, for free, with the formula and a worked example built in.
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