How big a battery to run 100 W?
To run 100 W for 5 hours you need about 617 Wh of LiFePO4: 49 Ah at 12 V, 25 Ah at 24 V or 13 Ah at 48 V. Lead-acid needs 1,111 Wh, because only half of it is usable.
| Hours | LiFePO4 energy | 12.8 V | 25.6 V | 51.2 V | Lead-acid |
|---|---|---|---|---|---|
| 2 h | 247 Wh | 20 Ah | 10 Ah | 5 Ah | 444 Wh · 38 Ah at 12 V |
| 4 h | 494 Wh | 39 Ah | 20 Ah | 10 Ah | 889 Wh · 75 Ah at 12 V |
| 6 h | 741 Wh | 58 Ah | 29 Ah | 15 Ah | 1,333 Wh · 112 Ah at 12 V |
| 8 h | 988 Wh | 78 Ah | 39 Ah | 20 Ah | 1,778 Wh · 149 Ah at 12 V |
| 10 h | 1,235 Wh | 97 Ah | 49 Ah | 25 Ah | 2,222 Wh · 186 Ah at 12 V |
| 12 h | 1,481 Wh | 116 Ah | 58 Ah | 29 Ah | 2,667 Wh · 223 Ah at 12 V |
How it is worked out
- The battery feeds the load through an inverter or converter that is about 90 % efficient: 100 W at the load is 111.1 W from the battery.
- Energy = power × hours, then divided by the share you may use: 90 % of a LiFePO4 bank, 50 % of a lead-acid one (deeper discharge shortens its life sharply).
- Amp-hours = watt-hours ÷ the bank's voltage. Round up, and add margin for cold weather and an ageing battery.
Check the 5-hour figure in the runtime calculator · Open it on the benchMy house sizes the whole backup system for 0.5 kWh a day and builds it.
For planning and learning. Use the appliance's nameplate (amps, MCA and MOCP) when you have it, follow your local electrical code, and have a licensed electrician do work on a building's wiring.