Watts, VA & power factor calculator
Convert a measured or specified AC load between real power and apparent power, then compare its watts and VA with an inverter's continuous ratings.
Use the inverter sizing calculator for headroom, starting surge, and battery current. This tool checks steady-state AC demand; AC amps are not battery DC amps.
Enter the load and continuous ratings
Formulas and assumptions
VA = watts ÷ power factor
watts = VA × power factor
single-phase RMS amps = VA ÷ RMS volts
rating utilization % = load ÷ continuous rating × 100Power factor describes real watts relative to apparent VA. It is different from conversion efficiency. For multiple loads, use measured aggregate W and VA or a justified aggregate power factor; do not average individual power factors without accounting for their loads. This tool does not model three-phase or split-phase load distribution.
Manufacturer data can list separate W and VA limits; see Victron's inverter specifications for an example. Use your own equipment's applicable ratings.
Worked example
A 1,200 W load at power factor 0.8 needs 1,500 VA. At 120 V it draws 12.5 A AC. Against a 1,500 W / 1,500 VA inverter, it uses 80% of the watt rating but 100% of the VA rating. At 230 V, the same load draws about 6.52 A; its W and VA demand are unchanged.
Watts and VA FAQ
Is power factor the same as inverter efficiency?
No. Power factor relates real to apparent AC power. Inverter efficiency relates useful AC real power to battery DC input power. Do not divide by power factor to estimate battery energy losses.
Can I assume every appliance has a 0.8 power factor?
No. Use a manufacturer specification or suitable true-power measurement under the actual operating conditions. Loads can change power factor with operating mode.
Does passing both ratings prove compatibility?
No. Continuous ratings alone cannot establish motor starting performance, permitted crest factor, waveform compatibility, or battery current capability. Check the complete system and manufacturer guidance.