Inverter Sizing Calculator
Enter continuous and surge loads to get the minimum inverter VA rating — with the power-factor conversion and surge margin that prevent nuisance shutdowns.
Last updated: 2026-09-28
How the calculation works
- Continuous load is multiplied by 1.25 for margin, then divided by power factor — inverters are VA-rated while loads are watt-described.
- The surge requirement (biggest motor start) is compared against the continuous figure; the larger wins.
- Low power factor inflates the VA need: an 800 W load at PF 0.8 demands 1000 VA, not 800.
Formula
Required VA = max(continuous × 1.25, surge) ÷ PF
| Symbol | Meaning | Unit |
|---|---|---|
S | Inverter rating | VA / W |
PF | Load power factor | — |
Worked example
Interpreting the result
Inverters fail two ways: sustained overload trips them during normal use (undersized continuous), and motors stall at start (insufficient surge rating). Size continuous generously — future loads always arrive — and verify the surge spec in the datasheet, not the marketing headline. Idle draw is the silent budget killer on small systems: 20 W of standby consumption is 0.5 kWh/day, more than the fridge.
Assumptions
- Single inverter, sine-wave output assumed for motor and electronics loads.
- Power factor of the load mix, not the inverter's own rating convention.
Limitations
- Does not size battery current draw: DC amps = VA ÷ battery voltage — verify cables and BMS limits separately.
- Parallel/stacked inverter configurations have their own surge-sharing rules.
Frequently asked questions
What size inverter for a fridge?
A fridge running at 150 W typically surges 400-600 W at start — a 1000 VA pure-sine inverter handles it with margin. Never use modified-sine for a fridge compressor: it runs hotter and noisier.
Why divide watts by power factor for inverter sizing?
Inverter ratings are in VA (voltage × current capacity) while loads are described in watts (real power). Current depends on VA: a 0.8 PF load drawing 800 W pulls 1000 VA through the inverter's transistors and cables.