How it works
An inverter turns battery DC power into household AC power, and it has two ratings. The continuous rating is what it can supply all day; the surge (peak) rating is what it can supply for a few seconds while motors start. Fridges, freezers, pumps and power tools draw two to seven times their running watts for a moment at start-up.
The calculator adds up the running watts of everything you might use at once and adds 25% headroom, because inverters run hotter and less efficiently near their limit. It then checks the surge: most inverters surge to about twice their rating, so a big motor can push you up a size. Finally it works out the current the inverter draws from the battery, which decides cable and fuse sizes.
Choose a pure sine wave inverter for anything with a motor, compressor or sensitive electronics such as a CPAP or laptop charger. Modified sine wave inverters are cheaper but can make motors run hot and some devices fail.
The formula
continuous watts = Σ running watts × 1.25 surge watts = Σ running watts + biggest (starting − running) inverter size = next standard size ≥ larger of continuous and surge ÷ 2 battery amps = watts ÷ (battery volts × 0.9 inverter efficiency)
Worked example
A van or cabin running LED lights, phone charging, a laptop and a full-size fridge from a 12 V battery.
| Step | Working | Result |
|---|---|---|
| Running watts | 40 + 20 + 65 + 150 | 275 W |
| With 25% headroom | 275 × 1.25 | 344 W |
| Surge (fridge starting) | 275 + (1,000 − 150) | 1,125 W |
| Inverter size | surge 1,125 ÷ 2 = 563 W | 600 W |
| Battery current at this load | 275 ÷ (12 × 0.9) | 25.5 A |
Here the fridge's start-up surge, not the running load, sets the size. Add a 1,000 W microwave (which draws about 1,500 W) and the load jumps to 1,775 W: a 3,000 W inverter that would pull about 278 A from a 12 V battery at full output. At that point a 24 V or 48 V battery bank is the better choice.
Frequently asked questions
What size inverter do I need to run a refrigerator?
A modern full-size fridge runs at about 150 W but can draw around 1,000 W for a second when the compressor starts. A 600 W to 1,000 W pure sine wave inverter with a surge rating of at least 1,200 W usually handles it; check the fridge label.
What size inverter do I need for my house?
For essentials during an outage, 3,000 to 5,000 W is typical; running a well pump, microwave and fridge together needs about 5,000 W. Whole-home backup with central air usually means a hybrid inverter of 8 to 12 kW, installed by an electrician.
What is the difference between continuous and surge watts?
Continuous watts is what the inverter can deliver indefinitely; surge is a short burst, usually for a few seconds, to start motors. Size the continuous rating for your running load and check the surge rating covers the biggest motor starting.
Can a 12 V battery run a 3,000 W inverter?
Yes, but at full output it draws close to 280 A, which needs very thick cables, a large fuse and a battery able to supply that current. Above about 2,000 W, 24 V or 48 V systems are safer and more efficient.
Do I need a pure sine wave inverter?
For fridges, pumps, CPAP machines, laptops and most modern electronics, yes. Modified sine wave inverters suit simple resistive loads such as heaters and incandescent bulbs.
Sources
- Appliance running and starting watts: typical figures from generator and inverter makers' wattage guides.
- US Department of Energy, estimating appliance energy use.
- Inverter efficiency of about 90% and surge of about twice the continuous rating are typical for current pure sine wave models; check your model's datasheet.
Estimates are for planning only and are not financial advice. Get written quotes before you buy.