Inverter Sizing Calculator
Work out the continuous inverter rating you need to run a set of loads, with a safety margin and a power-factor allowance to convert watts to volt-amps (VA), plus a surge check for motors and compressors.
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Range 0–1
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the total continuous load in watts — add up the running wattage of everything that could be on at once.
- Set a safety margin (default 25%) for headroom and future loads, and a power factor (default 0.8) to convert watts to VA.
- Optionally enter the surge/starting load so you can check it against the inverter's peak rating; read off the recommended standard inverter size.
How it works
Required continuous power = load × (1 + margin/100). Because inverters are usually rated in volt-amps, the required apparent power = required watts ÷ power factor. The tool then recommends the next inverter up from a standard series (300–15000 W). If you enter a surge load, it is reported separately because an inverter's short-duration peak rating is what must cover motor inrush, not its continuous rating.
Worked example
Worked example. A 2000 W continuous load with a 25% margin needs 2000 × 1.25 = 2500 W continuous. At a 0.8 power factor that is 2500 / 0.8 = 3125 VA. The next standard inverter above 2500 W is 3000 W.
Common mistakes
- Sizing to the continuous load with no margin — no headroom for extra appliances or hot-day derating.
- Forgetting motor/compressor inrush: a fridge or pump can surge to several times its running watts for a second or two, which the continuous rating may not cover.
- Ignoring power factor and treating watts as VA — inductive loads need more VA than their watt figure suggests.
Frequently asked questions
What power factor should I use?
Use about 0.8 for a mix of appliances or inductive loads (motors, pumps, fridges). For purely resistive loads such as heaters, kettles and incandescent lamps, use close to 1.0.
Why does the surge load matter?
Motors, compressors and pumps draw a brief inrush current several times their running wattage at start-up. The inverter needs a peak/surge rating above that momentary load even though its continuous rating is lower — otherwise it can trip or shut down on start-up.
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Cite this tool
Zerdly. (2026). Inverter Sizing Calculator. Retrieved from https://www.zerdly.com/tools/inverter-sizing-calculator/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.