How to size an inverter
An inverter has two ratings. The continuous rating must cover everything running at once, plus a margin so it doesn't run at its limit. The surge (peak) rating must cover the brief spike when a motor or compressor starts.
Example
A fridge (150 W), TV (100 W), lights (50 W) and microwave (900 W) total 1,200 W. With 25% margin you need 1,500 W, so a 1,500 W inverter. The fridge compressor adds about 1,000 W at startup, so the surge rating should be at least 2,200 W. At 12 V the inverter draws about 111 A from the battery.
Typical appliance watts
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | 150–250 | 1,200–2,000 |
| Microwave | 800–1,500 | – |
| Coffee maker | 800–1,200 | – |
| RV air conditioner (13.5k BTU) | 1,300–1,800 | 2,800–3,500 |
| Well pump (1/2 hp) | 1,000 | 2,000–3,000 |
| Laptop | 50–90 | – |
Frequently asked questions
What size inverter do I need to run a refrigerator?
At least 1,000–1,500 W continuous with a 2,000 W+ surge rating, since compressors draw 5–8 times their running watts at startup.
Should I choose a 12 V, 24 V or 48 V inverter?
Above about 2,000 W, 24 V or 48 V keeps battery current and cable size manageable.
Do I need a pure sine wave inverter?
Yes for motors, compressors, medical devices and most electronics. Modified sine wave is only acceptable for simple resistive loads.