Battery Backup Calculator
Work out the battery you need for a backup — from the load and the hours — in amp-hours, or see how long a battery you have will last.
A battery is not its label capacity — it is the label times the depth of discharge times the efficiency. This calculator is that honest number, in amp-hours.
Enter the load and the hours; the battery follows.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Enter the load in watts and the hours you need.
- Enter the battery voltage, depth of discharge and efficiency.
- Enter a battery capacity to see how long it lasts.
- Read the Ah needed — and the backup hours from your battery.
Worked example
| Load | 500 W for 4 hours |
|---|---|
| Energy | 500 × 4 = 2,000 Wh |
| Usable factor | 80% × 90% = 0.72 |
| Required Ah at 12 V | 2,000 ÷ (12 × 0.72) = 231.48 Ah |
| A 200 Ah battery lasts | 200 × 12 × 0.72 ÷ 500 = 3.46 hours |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
What is depth of discharge?
How much of a battery you can use before it is empty. Lead-acid batteries last longer if you only use about 50%; lithium can use 80–90%. Enter your battery's figure.
Why is efficiency less than 100%?
Because the inverter and wiring lose energy — about 10% in a typical system. A 2,000 Wh load needs about 2,222 Wh from the battery before efficiency is counted.
What voltage should I use?
12 V for small systems, 24 V or 48 V for bigger ones — higher voltage means lower current for the same power. Use your system's voltage.
Is this a substitute for electrical design?
No. This is a capacity estimate for planning. Battery, inverter and circuit sizing require qualified professionals and local code compliance — see the safety notice.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.