What is the difference between continuous and surge rating?

Continuous rating is the wattage an inverter can supply indefinitely, while surge or peak rating is a brief burst, usually for a few seconds, to start motors. Motors can draw three to seven times their running watts at startup, so the surge rating must cover that inrush.

Why convert watts to VA?

Inverters and AC equipment are often rated in volt-amps because reactive loads draw more apparent power than real power. Dividing watts by the power factor gives VA. At a 0.8 power factor a 1000 W load is 1250 VA, so the inverter must be rated for the higher figure.

When do I need a pure-sine inverter?

Pure-sine output is required for sensitive electronics, motors, microwaves, CPAP machines, and many modern appliances. Modified-sine inverters are cheaper but can cause buzzing, overheating, or failure in those loads, so the tool flags pure-sine when motor loads are present.

How much headroom should the inverter have?

Size the continuous rating about 20 to 25 percent above your real continuous load so the inverter is not running flat out, which shortens its life and lowers efficiency. The surge rating must comfortably exceed the largest motor's startup draw.

What surge multiplier should I use?

Resistive loads have no surge. Capacitor-start motors and compressors typically need three to five times running watts, and some pumps and air conditioners up to seven. If unsure, use the manufacturer's locked-rotor amps or pick a conservative higher multiplier.

Inverter Sizing Calculator

An inverter must do two jobs: supply your continuous load indefinitely and survive the brief inrush surge when a motor starts. This calculator sizes both the continuous and peak ratings, converts to VA, and tells you whether a pure-sine unit is required.

How it works

The recommended continuous rating adds headroom to your real continuous load. The peak rating must cover the largest motor’s startup surge on top of the other running loads:

Continuous rating = continuous load × (1 + headroom)
Largest motor surge = motor watts × surge multiplier
Peak rating = (continuous load − motor watts) + largest motor surge
VA = watts ÷ power factor

Subtracting the motor’s running watts before adding its surge avoids double-counting that motor.

Example

A 1500 W continuous load with a 500 W fridge surging 4× needs a peak of (1500 − 500) + (500 × 4) = 3000 W. With 20 percent headroom the continuous rating is 1800 W, or 2250 VA at a 0.8 power factor.

Tips

Use pure-sine for motors, microwaves, and medical or audio equipment.
Check the motor nameplate for locked-rotor amps to refine the surge figure.
Do not run an inverter continuously near its full rating.