How this calculator works
An inverter changes battery power into household current. Its continuous rating has to cover everything that runs at once, with a margin so that it is not at its limit all the time. It also has to cope with the surge when a motor starts, which most inverters can do for a few seconds at about twice their rating. So we size the inverter from the larger of the running load plus the margin and half of the surge, and round up to a standard size. The inverter draws current from the battery in proportion to the load and in inverse proportion to the voltage and its efficiency, and the battery bank has to store the energy for the hours you want to run.
Worked example: a running load of 1,000 W with a 25% margin is 1,250 W, and a 2,000 W surge needs a 1,000 W inverter, so the inverter is 1,500 W. On a 12 V battery at 90% efficiency it draws 93 A at 1,000 W and 139 A at its full rating, so the fuse is 175 A. On a 24 V bank the full-power current is 69 A. Running 4 hours needs 4,444 Wh from the battery, which is 463 Ah of lithium at 12 V, or 741 Ah of lead-acid. A 300 W load with a 600 W surge needs a 500 W inverter, and a 2,000 W load with a 5,000 W surge on 24 V needs 2,500 W (116 A, 150 A fuse). With no margin and a 1,000 W surge the 1,000 W load fits a 1,000 W inverter.
Inverter size for common loads (25% margin, 12 V, 90% efficient)
Battery current and fuse at full inverter power.
| Running load | Inverter | Current at 12 V | Fuse | Current at 24 V |
|---|---|---|---|---|
| 200 W | 300 W | 28 A | 40 A | 14 A |
| 500 W | 750 W | 69 A | 100 A | 35 A |
| 800 W | 1,000 W | 93 A | 125 A | 46 A |
| 1,000 W | 1,500 W | 139 A | 175 A | 69 A |
| 1,500 W | 2,000 W | 185 A | 250 A | 93 A |
| 2,000 W | 2,500 W | 231 A | 300 A | 116 A |
| 3,000 W | 4,000 W | 370 A | 500 A | 185 A |
Tips before you buy
- Add up the running watts of every load that could be on together, and then add a 20 to 25% margin so the inverter does not run flat out.
- Check the surge. A fridge compressor, a pump or a power tool can draw 2 to 4 times its running watts for a second or two.
- A higher battery voltage (24 V or 48 V) halves or quarters the current, so the cables are thinner and the losses lower. Choose 24 V or 48 V above about 1,500 W.
- Keep the battery cable short and thick, and fuse it near the battery. A short on a battery bank can start a fire.
- Use a pure sine wave inverter for motors, medical devices and sensitive electronics. A modified sine wave inverter can make them hot or noisy.
Tools & materials for this project
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Frequently asked questions
What size inverter do I need?
Add up the running watts of everything on at once, add about 25%, and check the start-up surge. A 1,000 W load with a 25% margin needs 1,250 W, so a 1,500 W inverter.
How many amps does a 1000 W inverter draw?
About 93 amps from a 12 V battery at full 1,000 W with 90% efficiency, or 46 A on 24 V.
What size fuse for a 2000 W inverter?
A 2,000 W inverter draws 185 A from a 12 V battery at full power, so a 250 A fuse (125% of that). On 24 V it is 125 A.
How big a battery for a 1000 W inverter?
For 4 hours at 1,000 W you need 4,444 Wh from the battery: 463 Ah of lithium or 741 Ah of lead-acid, at 12 V.
Do I need an inverter bigger than my load?
Yes, by a margin: about 25% above the running load, and enough surge capacity for the biggest motor. Motors can draw 2 to 4 times their running watts at start-up.
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