How big a battery to run 500 W?
To run 500 W for 5 hours you need about 3,086 Wh of LiFePO4: 242 Ah at 12 V, 121 Ah at 24 V or 61 Ah at 48 V. Lead-acid needs 5,556 Wh, because only half of it is usable.
| Hours | LiFePO4 energy | 12.8 V | 25.6 V | 51.2 V | Lead-acid |
|---|---|---|---|---|---|
| 2 h | 1,235 Wh | 97 Ah | 49 Ah | 25 Ah | 2,222 Wh · 186 Ah at 12 V |
| 4 h | 2,469 Wh | 193 Ah | 97 Ah | 49 Ah | 4,444 Wh · 371 Ah at 12 V |
| 6 h | 3,704 Wh | 290 Ah | 145 Ah | 73 Ah | 6,667 Wh · 556 Ah at 12 V |
| 8 h | 4,938 Wh | 386 Ah | 193 Ah | 97 Ah | 8,889 Wh · 741 Ah at 12 V |
| 10 h | 6,173 Wh | 483 Ah | 242 Ah | 121 Ah | 11,111 Wh · 926 Ah at 12 V |
| 12 h | 7,407 Wh | 579 Ah | 290 Ah | 145 Ah | 13,333 Wh · 1112 Ah at 12 V |
How it is worked out
- The battery feeds the load through an inverter or converter that is about 90 % efficient: 500 W at the load is 555.6 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 2.5 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.