What Size Inverter Do I Need? Watts, Surge and Battery Chart
Part of the project planner: Planning backup power, batteries and solar.
An inverter has to carry everything you run at once, with a margin, and the surge when a motor starts. A 1,000 W load with a 25% margin is 1,250 W, so the inverter is 1,500 W. On a 12 V battery it draws 139 A at full power, so it needs a 175 A fuse and thick cables. The inverter size calculator works out your own loads, the battery current, the fuse and the battery bank.
How to size an inverter
- Add up the running watts of everything that could be on at the same time. The label gives volts and amps (multiply them) or watts.
- Add a margin of about 25%, so the inverter is not at its limit all the time. It runs cooler and lasts longer.
- Check the surge. Motors, compressors and pumps draw two to four times their running watts for a second or two. Most inverters can surge to about twice their rating for a few seconds, so the inverter must be at least half the peak surge.
- Round up to the next standard size: 300, 500, 750, 1,000, 1,500, 2,000, 3,000, 4,000 or 5,000 W.
If you are not sure how many watts something uses, a plug-in watt meter gives the real figure.
Common loads and the inverter they need
Typical figures: check the label of your own appliance. The surge multiple is how many times the running watts the load draws at start-up.
| Load | Running | Surge multiple | Peak at start | Inverter size |
|---|---|---|---|---|
| Laptop and phone chargers | 100 W | 1× | 100 W | 300 W |
| LED lights (ten bulbs) | 100 W | 1× | 100 W | 300 W |
| TV and streaming box | 150 W | 1× | 150 W | 300 W |
| Coffee maker or kettle | 1,000 W | 1× | 1,000 W | 1,500 W |
| Microwave oven (1,000 W cooking) | 1,500 W | 1× | 1,500 W | 2,000 W |
| Full-size refrigerator | 200 W | 6× | 1,200 W | 750 W |
| Window air conditioner | 600 W | 3× | 1,800 W | 1,000 W |
| Sump pump (1/3 hp) | 800 W | 3× | 2,400 W | 1,500 W |
| Circular saw or power tool | 1,400 W | 2× | 2,800 W | 2,000 W |
| Well pump (1/2 hp) | 1,000 W | 3× | 3,000 W | 1,500 W |
Resistive loads such as kettles and heaters have no surge, but they use a lot of power, so they are the loads that most often need a bigger inverter than you expect. A surge-heavy load can set the size even when the running watts are low: with a 1,000 W running load and a 2,600 W surge the inverter has to be 1,500 W.
Battery current and fuse by inverter size
At full inverter power, with 90% efficiency. The fuse is 125% of the current, in the next standard size:
| Inverter | Load it carries (with 25% margin) | Current at 12 V (fuse) | Current at 24 V (fuse) | Current at 48 V (fuse) |
|---|---|---|---|---|
| 300 W | 240 W | 28 A (40 A) | 14 A (30 A) | 7 A (30 A) |
| 500 W | 400 W | 46 A (60 A) | 23 A (30 A) | 12 A (30 A) |
| 1,000 W | 800 W | 93 A (125 A) | 46 A (60 A) | 23 A (30 A) |
| 1,500 W | 1,200 W | 139 A (175 A) | 69 A (100 A) | 35 A (50 A) |
| 2,000 W | 1,600 W | 185 A (250 A) | 93 A (125 A) | 46 A (60 A) |
| 3,000 W | 2,400 W | 278 A (350 A) | 139 A (175 A) | 69 A (100 A) |
| 5,000 W | 4,000 W | 463 A (too high: use a higher voltage) | 231 A (300 A) | 116 A (150 A) |
A current of 100 A or more needs very heavy cable. Use the wire size calculator for the battery cable, and keep it as short as you can.
Battery bank for the hours you want to run
Amp hours of lithium (LiFePO4, using 80% of the battery) at 12 V. Lead-acid needs about 1.6 times as much, since only half of it should be used:
| Load | 2 hours | 4 hours | 8 hours |
|---|---|---|---|
| 100 W | 23 Ah | 46 Ah | 93 Ah |
| 300 W | 69 Ah | 139 Ah | 278 Ah |
| 500 W | 116 Ah | 231 Ah | 463 Ah |
| 1,000 W | 231 Ah | 463 Ah | 926 Ah |
| 1,500 W | 347 Ah | 694 Ah | 1,389 Ah |
| 2,000 W | 463 Ah | 926 Ah | 1,852 Ah |
The battery capacity calculator converts between amp hours, watt hours and runtime for any battery.
Pure sine wave or modified sine wave?
- Pure sine wave inverters give clean power like the grid. They run motors, laptops, CPAP machines and electronics properly, and are the right choice for most uses.
- Modified sine wave inverters cost less, but can make motors hot and noisy and may damage some electronics.
Fitting an inverter safely
- Put a fuse or a DC breaker near the battery on the positive cable, rated for the cable and no more than the inverter allows.
- Use cable of the right size, keep it short, and crimp the lugs properly. Loose or small cable gets hot.
- An inverter also draws a little power when it is on with no load, so switch it off when you are not using it.
- Ventilate the inverter and do not cover it. It makes heat at full load.
- For a fixed installation or a connection to a house, have a qualified electrician do the wiring.
If you also want solar to charge the battery, see the solar charge controller calculator. For a ready-made battery with an inverter inside, compare it with the power station size calculator. These are estimates: follow the manufacturer's instructions and local electrical codes.
Frequently asked questions
What size inverter do I need?
Add up the running watts of everything on at once, add about 25% as a margin, and check the start-up surge. A 1,000 W load with a 25% margin is 1,250 W, so a 1,500 W inverter.
How many amps does a 1500 W inverter draw?
At full power, about 139 amps from a 12 V battery (69 A on 24 V and 35 A on 48 V), with 90% efficiency. That is why the cables are thick and the battery is fused.
What happens if the inverter is too small?
It shuts down with an overload alarm, especially when a motor starts. A motor draws two to four times its running watts for a moment, so the surge matters as much as the running load.
Should I use a 12V, 24V or 48V system?
A higher voltage cuts the current: a 1,500 W inverter draws 139 A at 12 V and 69 A at 24 V. Above about 1,500 W, 24 V or 48 V is easier on the cables and the connections.
How big a battery for a 1000 W inverter?
For 4 hours at 1,000 W you need 4,444 Wh from the battery, which is 463 Ah of lithium at 12 V, or 741 Ah of lead-acid, since lead-acid should use only half its capacity.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.