Generator sizing

Loads — add what has to run from the presets (kitchen, water, heating and cooling, EV, house, tools) or your own. Each has its running watts and a starting surge from starting watts, the motor’s locked-rotor amps (LRA, on the nameplate), its NEC code letter and horsepower, or for air conditioners and heat pumps the tonnage (running watts from the EER). Tick Soft start where a soft starter is fitted. Untick a load to see the plan without it. Duty % is the share of time it runs, for fuel.

Starting — the start order that keeps the peak lowest (biggest surge first, while little else runs), the usual sizing figure — everything running, then the largest motor starts — and what an automatic transfer does if everything starts at once.

Size — portable, inverter, air-cooled standby or liquid-cooled standby; the derating for altitude, heat and natural gas; the smallest standard size that carries the running load at 80 % and the peak within its surge ability; and how 120 V loads fall on the two legs of a 30 A or 50 A inlet.

Fuel — fuel per hour at each load and at your average load, and runtime from a gasoline or diesel tank or a propane cylinder or tank (natural gas: the flow the meter must supply).

Transfer & safety — what Article 702 asks: transfer equipment that can’t parallel the utility, sizing for manual transfer versus automatic transfer (the whole calculated load unless a load-management system sheds loads), the outdoor emergency shutdown, and carbon-monoxide safety.

Locked-rotor kVA per horsepower comes from NEC Table 430.7(B) (the middle of each letter’s range); a soft starter is taken to cut inrush to about 35 % of LRA, as makers claim. The best start order sorts by surge (start − run), which an exchange argument shows is optimal; the tests check it against every order for random load sets. Derating defaults to 3.5 % per 1,000 ft above 500 ft and 1 % per 10 °F above 77 °F, and natural gas at 90 % of the propane rating — typical of spec sheets; yours has its own. Fuel use is a straight line from no-load to full-load consumption, fitted to published figures for common sets, with fuel energy from the US DOE Alternative Fuels Data Center. The presets' watts are typical values from generator makers' guides; your nameplates are better.

For the whole-house calculated load an automatic transfer switch needs, see the dwelling load calculator; for motor branch circuits, the motor circuit tool; for conductors, wire size; for a workshop’s circuits, the shop power planner. A furnace or AC’s airflow is in HVAC ducts and airflow.

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