Generator size calculator

Introduction

To size a generator, add up the running watts of everything you want to power, then add the extra surge that the largest motor draws for a few seconds when it starts. The generator's running rating must cover the first number with some margin, and its starting rating must cover the second. A fridge, sump pump, furnace fan, lights, microwave and TV run on about 4 450 W, but need 6 000 W for the moment the furnace fan starts.

This calculator does that sum for any list of loads, for a home backup generator, an RV, a job site or a cabin. It gives the running and starting watts, the minimum ratings to look for on a generator, and the kVA rating of a permanently installed standby set.

Formulas

Running load and starting surge:

W_run = Σ W_running
surge = max(W_starting − W_running)       motors starting one at a time
surge = Σ (W_starting − W_running)        all starting together
W_peak = W_run + surge

Minimum ratings:

running rating ≥ W_run / k_max
starting rating ≥ W_peak
standby kVA = running rating / 0.8

• W_running, W_starting — running and starting watts of each load, from the nameplate or manual
• k_max — the highest share of its rating you want the generator to run at, 80 % by default
• 0.8 — the power factor at which standby generators are rated (ISO 8528-1)

Assumptions and limitations

• The default list is an example. Real running and starting watts vary by model, so read them from nameplates or manuals. A motor that shows only amps draws about volts × amps watts while running.
• Motors usually start one at a time. After a power cut several may start together when the power returns; choose "all together" or use a transfer switch that staggers them.
• Running a generator at no more than about 80 % of its rating leaves room for loads switching on and spares the engine. Inverter generators handle surges less well than their running rating suggests, so check their surge rating too.
• Altitude and heat reduce an engine generator's output. A permanently connected standby generator must be installed with a transfer switch by a licensed electrician.

Worked example

A house wants to back up a fridge (700 W running, 2 200 W starting), a 1/2 hp sump pump (1 050 W, 2 150 W), a 1/2 hp furnace fan (800 W, 2 350 W), 600 W of lights, a 1 000 W microwave and 300 W of TV and internet. What generator is needed?

1. Running watts: 700 + 1 050 + 800 + 600 + 1 000 + 300 = 4 450 W.
2. Surges: fridge 1 500 W, sump pump 1 100 W, furnace fan 1 550 W. The largest is 1 550 W.
3. Peak while the furnace fan starts: 4 450 + 1 550 = 6 000 W.
4. Running rating at 80 % loading: 4 450 / 0.8 = 5 563 W.

So look for a generator rated at least about 5 600 running watts and 6 000 starting watts; a common 7 500 W portable model covers both. If all three motors could start together, the peak would be 8 600 W.

FAQ

Related calculators

• Generator load, kVA and fuel — loading and fuel of generator sets running in parallel.
• Watts to amps — the current of each load.
• kW to kVA — the kVA rating for a load in kW.
• Breaker size calculator — the breaker for the generator circuit.