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Inverter size calculator

Add everything you might run at the same time. We total the running watts, add the biggest startup surge, and recommend a standard inverter size.

Everything that may run at the same time

Remove anything you'll never run together with the rest.

Running load

1,752 W

Surge needed

2,802 W

running load + biggest startup

Recommended inverter

2,500 W

continuous rating

  • Continuous rating with 25% headroom above your running load, so the inverter is not working at its limit.
  • Its surge (peak) rating should be at least 2,802 W. Check the spec sheet: surge is usually quoted for a few seconds or less.
  • Choose a pure sine wave inverter for fridges, pumps, CPAPs and electronics.

How the inverter size is calculated

  1. Running load: the running watts of everything on at the same time, added together.
  2. Surge: the running load plus the extra startup watts of the single biggest motor. We assume motors start one at a time, which is usually true.
  3. Recommended size: running load × 1.25, rounded up to a standard inverter size, so it isn't running flat out.
continuous rating ≥ running watts × 1.25 · surge rating ≥ running watts + largest startup surge

Watch the surge rating

Inverter spec sheets quote a continuous rating and a surge (peak) rating. Surge ratings are only held for a short time, from milliseconds to a few seconds depending on the brand, and some budget inverters quote optimistic figures. For motor loads like well pumps and air conditioners, choose an inverter whose surge rating clearly exceeds the number above.

Power stations have inverters built in

If you're choosing a portable power station rather than a DIY system, its AC output rating is the inverter size. Use the runtime calculator to check both output and runtime in one go. For DIY systems, pair this with the battery size calculator.

Large inverters draw very high battery currents (a 2,000 W inverter on 12 V draws about 170 A or more). Use correctly sized cables and fuses, and have anything connected to home wiring installed by a licensed electrician with a transfer switch.

Frequently asked questions

What size inverter do I need to run a refrigerator?
A household fridge runs at 100–250 W but can surge to 600–1,200 W at startup. A 1,000 W pure sine wave inverter with a surge rating of 2,000 W handles most fridges, plus a few small loads alongside.
Is a bigger inverter always better?
Not quite. Bigger inverters cost more and draw more power at idle (often 10–50 W), which drains batteries even with tiny loads. Size for your real needs with some headroom.
Pure sine wave or modified sine wave?
Pure sine wave. Motors, compressors, CPAPs, chargers and many electronics run poorly or get damaged on modified sine wave power. The price difference is now small.

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