How the generator size is calculated
Add up everything that runs at the same time for the running load, and add the largest motor start on top of it for the peak. Both get a safety margin, giving the running rating the generator must sustain and the starting burst it must survive. For example, 2,400 W of running loads with an air-conditioner start peaking at 4,500 W and a 25% margin needs 3,000 W running and 5,625 W starting — a common 4,000 W unit covers the running side but falls short on starting, so the suggestion moves up to 5,000 W.
What the model leaves out
Two real-world effects are deliberately not modelled in v1. Altitude and heat derate engine output — roughly 3% per 300 m of elevation plus around 1% per 5 °C above 25 °C, so a mountain cabin needs the next size up. And fuel consumption varies too much between engines to estimate honestly; the manufacturer's half-load figure is the number to compare. Starting capability also differs by design — the suggestion assumes a typical 1.25× headroom, but the datasheet's starting-watts figure is the one that counts.
Generator plus battery is often smaller
A generator sized for the worst-case motor start idles oversized the rest of the time. Pairing a smaller generator with an inverter and battery bank lets the battery carry the surges and the generator run in its efficient band — size that system with the Inverter Sizing and Battery Bank Sizing calculators.
Methodology
The calculator UI, the size-check table and the displayed formula all call the same calculation function. Common sizes reflect the portable-generator market; the sources below carry the sizing conventions.
- Running vs starting watts sizing method — Generator manufacturer sizing guides (e.g. Honda, Generac, Champion)
- Motor start-up draw of 2–6× running power — NEMA MG-1 motor locked-rotor code letters and appliance start-up specifications
- Altitude/heat derating ≈ 3% per 300 m — Small-engine manufacturer derating guidance (carburetted engines)
Assumptions last reviewed on 27 August 2026. If a source disagrees with your battery or equipment datasheet, trust the datasheet.
Frequently asked questions
What is the difference between running and starting watts?
Running watts is what a generator sustains continuously; starting watts is the short burst it can deliver for motor start-up. Your loads have the same two numbers, and the generator must cover both.
Why should a generator not run at full load continuously?
Continuous duty near the rating shortens engine life, raises fuel use per kWh and leaves no headroom for the next motor start. 50–80% load is the healthy band — the safety margin builds that in.
When is an inverter generator worth the extra cost?
When sensitive electronics — laptops, CPAP machines, modern TVs — run on it. Inverter units produce a clean sine wave, run quieter and can usually be paralleled for more power.