Size a standby generator from running and motor starting loads.
Motor starting current, not total running load, usually sets generator size. Only the largest motor is assumed to start at once, since simultaneous starts are unlikely and would be prohibitively expensive to design for. A generator that carries the running load but cannot start the well pump or air conditioner is functionally useless during an outage, which is why the surge figure matters most.
Generator Sizing
Governing load = max(running load, running load − motor + motor × starting multiplier)
Governing load = max(running load, running load − motor + motor × starting multiplier) Motor starting current, not total running load, usually sets generator size. Only the largest motor is assumed to start at once, since simultaneous starts are unlikely and would be prohibitively expensive to design for.
A generator that carries the running load but cannot start the well pump or air conditioner is functionally useless during an outage, which is why the surge figure matters most.
This calculator takes 5 inputs: Total running load, Largest motor running watts, Motor starting multiplier, Power factor, Altitude and temperature derate. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.