Inverter calculator

Inverter Size Calculator

Size from the loads that can run together, then check the largest startup event. A single watt total is not enough when motors, compressors or poor power factor are involved.

Minimum continuous inverter rating1,875 W2,083 VA at the entered running power factor

Calculation breakdown

Calculation breakdown
Required startup surgeOther running loads plus the largest motor at startup	2,400 W
Candidate continuous check	Pass
Candidate surge check	Pass
Entered candidate	2000 W continuous · 3000 W surge

Continuous W = running W × margin; surge W = other running W + largest motor starting W
= 1500 × 1.25; (1500 − 300) + 1200
= 1,875 W; 2,400 W surge

A surge rating is valid only for the manufacturer's stated duration and conditions. Verify that duration against the appliance startup requirement.
Continuous VA matters for lower-power-factor loads even when their real watts fit the inverter.

Before you rely on this

Verify the inverter's temperature-derated continuous rating, surge duration, waveform, grounding, protection and compatibility with every load.

Method

How the number is reached

The continuous requirement begins with all loads that can run simultaneously and adds the entered design margin. Apparent power in VA is calculated from the running-load power factor.

For one starting motor, its running watts are removed from the simultaneous total and replaced with its starting watts. If multiple motors can start together, enter their combined event instead.

Continuous W = simultaneous running W × (1 + margin)
Continuous VA = continuous W ÷ running power factor
Surge W = running W − motor running W + motor starting W

Symbols

W: Real power
VA: Apparent power

Worked examples

The same maths, applied

Example

1,500 W running load with a compressor startup

The compressor uses 300 W while running and 1,200 W at startup. The design margin is 25% and running power factor is 0.9.

Continuous = 1,500 × 1.25 = 1,875 W
Continuous apparent power = 1,875 ÷ 0.9 = 2,083 VA
Startup = 1,500 − 300 + 1,200 = 2,400 W

Result: At least 1,875 W continuous, 2,083 VA and 2,400 W surge.

Limits of the model

What it assumes, and where it stops

Assumptions

The entered running load contains every device that can operate at the same time.

Not covered

Does not predict motor inrush or startup duration; use equipment data or measurement.

Sources

Where these figures come from

Inverter Smart — Technical specificationsVictron EnergyManufacturer ratings that distinguish continuous watts, continuous VA, temperature derating, peak power, maximum efficiency and zero-load consumption.
IEC 60050 — International Electrotechnical Vocabulary (Electropedia)International Electrotechnical CommissionStandard terminology for electrical quantities, including apparent power and power factor.
Electrical Installation Guide (IEC-based)Schneider ElectricReference for single-phase and three-phase power relationships and power factor.

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Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.