How big a battery to run 1,000 W?
To run 1,000 W for 5 hours you need about 6,173 Wh of LiFePO4: 483 Ah at 12 V, 242 Ah at 24 V or 121 Ah at 48 V. Lead-acid needs 11,111 Wh, because only half of it is usable.
| Hours | LiFePO4 energy | 12.8 V | 25.6 V | 51.2 V | Lead-acid |
|---|---|---|---|---|---|
| 2 h | 2,469 Wh | 193 Ah | 97 Ah | 49 Ah | 4,444 Wh · 371 Ah at 12 V |
| 4 h | 4,938 Wh | 386 Ah | 193 Ah | 97 Ah | 8,889 Wh · 741 Ah at 12 V |
| 6 h | 7,407 Wh | 579 Ah | 290 Ah | 145 Ah | 13,333 Wh · 1112 Ah at 12 V |
| 8 h | 9,877 Wh | 772 Ah | 386 Ah | 193 Ah | 17,778 Wh · 1482 Ah at 12 V |
| 10 h | 12,346 Wh | 965 Ah | 483 Ah | 242 Ah | 22,222 Wh · 1852 Ah at 12 V |
| 12 h | 14,815 Wh | 1158 Ah | 579 Ah | 290 Ah | 26,667 Wh · 2223 Ah at 12 V |
How it is worked out
- The battery feeds the load through an inverter or converter that is about 90 % efficient: 1000 W at the load is 1111.1 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 5.0 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.