How big a battery to run 300 W?
To run 300 W for 5 hours you need about 1,852 Wh of LiFePO4: 145 Ah at 12 V, 73 Ah at 24 V or 37 Ah at 48 V. Lead-acid needs 3,333 Wh, because only half of it is usable.
| Hours | LiFePO4 energy | 12.8 V | 25.6 V | 51.2 V | Lead-acid |
|---|---|---|---|---|---|
| 2 h | 741 Wh | 58 Ah | 29 Ah | 15 Ah | 1,333 Wh · 112 Ah at 12 V |
| 4 h | 1,481 Wh | 116 Ah | 58 Ah | 29 Ah | 2,667 Wh · 223 Ah at 12 V |
| 6 h | 2,222 Wh | 174 Ah | 87 Ah | 44 Ah | 4,000 Wh · 334 Ah at 12 V |
| 8 h | 2,963 Wh | 232 Ah | 116 Ah | 58 Ah | 5,333 Wh · 445 Ah at 12 V |
| 10 h | 3,704 Wh | 290 Ah | 145 Ah | 73 Ah | 6,667 Wh · 556 Ah at 12 V |
| 12 h | 4,444 Wh | 348 Ah | 174 Ah | 87 Ah | 8,000 Wh · 667 Ah at 12 V |
How it is worked out
- The battery feeds the load through an inverter or converter that is about 90 % efficient: 300 W at the load is 333.3 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 1.5 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.