Power Inverter Sizing Calculator
A power inverter turns battery DC into household AC, and it must be rated for both the steady running load and the brief surge that motors draw at start-up. It also pulls more current from the battery than it delivers to the load because of conversion losses. Enter your total running watts, a headroom margin, a surge factor, the inverter efficiency, and the battery voltage; this calculator returns the recommended continuous rating, the peak surge power, the DC input power, and the battery current the inverter will draw.
Inverter sizing formula
Recommended rating = running load * headroom margin
Peak surge power = running load * surge factor
DC input power = running load / efficiency
Battery current = DC input power / battery voltage
Efficiency is entered as a percentage and divided by 100. Choose an inverter whose continuous rating exceeds the recommended figure and whose surge rating exceeds the peak surge power.
Inverter sizing context
- Size the continuous rating above the total running load with a headroom margin, often 20 percent.
- Motor and compressor loads need a surge rating two to three times their running watts.
- Conversion losses mean the battery delivers more power than the load consumes.
- Battery current rises sharply at low battery voltage, so 12 volt systems draw heavy current for large loads.
- Confirm wiring and fuse ratings can carry the calculated battery current safely.
Power inverter sizing: frequently asked questions
How do I size a power inverter?
Add up the running wattage of everything you will run at once, then add a headroom margin (commonly 20 percent) so the inverter is not run at its limit. Devices with motors also need a surge rating, often two to three times their running watts, to handle start-up.
Why does an inverter draw more DC current than the load?
Inverters are not perfectly efficient, so they pull a little more power from the battery than they deliver to the load. The DC input power equals the AC load divided by the efficiency, and the DC current equals that input power divided by the battery voltage.
What surge rating do I need?
Motors, compressors, and pumps draw a large surge for a fraction of a second when they start. A surge factor of two to three times the running watts is typical. Enter your surge factor and this calculator reports the peak power the inverter must handle.
How do I find the DC current from the battery?
Divide the inverter's DC input power by the battery voltage. The input power is the AC load divided by efficiency. For example, a 600 watt load at 90 percent efficiency on a 12 volt battery draws about 55.6 amperes.
What efficiency should I assume?
Modern inverters are commonly 85 to 95 percent efficient at moderate load, with pure sine-wave units toward the higher end. Efficiency is a user-editable input so you can match your unit's data sheet at the load you expect to run.
Official sources
- U.S. Department of Energy: Federal Energy Management Program (power and efficiency).
- NIST: SI electrical units, the watt, volt and ampere (SP 811).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.