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# Generator size calculator

Size a generator from your appliance list with running watts, a margin, and the largest motor start, plus altitude and heat derating.

List your appliances to size the set.

The sizing rule is the largest motor start, the way generator makers write their charts: total running watts plus a margin, plus the single biggest start surplus, not every start at once. In practice two motors rarely start on the same second, but if yours can, put them in one row or raise the margin until the numbers feel honest. Derating uses fixed steps of 300 m and 5.5 °C above the thresholds you set, and shrinks the nameplate you must buy. Fuel is interpolated between the two data-sheet points; below half load the figure falls toward zero, which is a straight-line guess, not a data-sheet curve. What this page does not do: reactive load studies, voltage drop, three-phase imbalance, THD limits for electronics, transfer switch or grounding design, or a fuel curve for the specific set you are buying. Read its manual.
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