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Inverter Size Calculator

Work out the right inverter size from your realistic peak simultaneous load, with an appropriate safety margin depending on whether your loads include motors (fridges, pumps) that surge on startup.

Recommended inverter size1800 W

How to measure your inputs

  • Peak load: add up the wattage of everything that could realistically run at the same time, not your full appliance list.

Methodology

Standard practice: inverter size (W) = peak load (W) × safety margin. 20-25% margin is the commonly cited real-world range, higher for loads with motor-start surges (fridges, pumps, power tools).

Sizing an inverter to your real peak load, not your total appliance list

Peak load, not total connected load

An inverter needs to handle the highest wattage drawn at any one moment — not the sum of every appliance in the building. If a fridge, a kettle, and lights might all run together but a well pump never runs at the same time as the kettle, size for the realistic worst-case combination, not literally everything added up.

Why motor-start loads need extra headroom

Motors (fridges, pumps, power tools) draw a brief surge — often 2-3x their running wattage — at the instant they start up. An inverter sized only for running wattage can trip or fail at that exact moment, which is why this calculator asks whether your load includes motor-start appliances before applying a margin.

Continuous vs. surge rating

Inverters are typically rated for both a continuous output and a brief higher surge output. The number this calculator gives you is a target continuous rating with margin built in — when shopping, check that a specific inverter’s surge rating also comfortably covers your largest single motor-start event.

Frequently asked questions

What counts as my peak load?

The highest total wattage of everything that could realistically be running at the exact same moment — not every appliance you own added together. Think through your actual usage pattern rather than a worst-case-everything-at-once scenario, unless that genuinely could happen.

Why does my fridge or pump need extra inverter margin?

Motors draw a brief startup surge well above their steady running wattage — often 2-3x. An inverter sized exactly to running wattage with no margin can fail to start the motor at all, or trip its overload protection.

What's the difference between continuous and surge inverter ratings?

Continuous rating is what an inverter can sustain indefinitely; surge rating is a much higher output it can briefly supply (usually a few seconds) for motor starts. Both matter — size continuous rating to your steady peak load, and check surge rating separately against your largest motor-start event.

Is it safe to undersize an inverter to save money?

No — an undersized inverter can trip under load, fail to start motor-driven appliances, or run hot and degrade faster under sustained overload. The margin this calculator applies exists specifically to avoid that.