What size generator do I need? Running and starting watts
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Whether a generator is your only backup or the reserve behind a battery system, sizing it works the same way: add up what it must run at once, then allow for the moment motors start.
Step 1: running watts
List everything the generator must power at the same time, and add up their running watts. The label on each appliance shows its power. See how to find your appliances’ watts.
Example: lights 100 W, two fans 150 W, a fridge 150 W, a TV 60 W, a laptop 60 W and a water pump 750 W. That’s 1,270 W running.
Step 2: starting watts
Motors and compressors (fridges, freezers, pumps, air conditioners) draw several times their running power for a moment when they start. Add the extra starting power of the biggest motor, since two rarely start at exactly the same moment.
In the example, the pump is the biggest motor. If it starts at around three times its running power, that’s about 1,500 W extra, so the generator must briefly handle about 2,770 W.
The pump’s manual or rating plate may give its starting current. Use that if you have it.
Step 3: read the generator’s ratings
Generators usually show two numbers:
- Rated (continuous) output: what it can supply for hours. Your running watts must fit comfortably under this.
- Maximum (peak) output: what it can supply briefly. Your starting watts must fit under this.
Many generators are rated in kVA. To convert to watts, multiply by the power factor shown on the plate, often 0.8: a 3.5 kVA generator delivers about 2.8 kW. See kVA vs kW, which works the same way for generators.
Step 4: leave headroom
A generator running flat out for hours works hard and wears faster. Aim for your usual running load to be no more than about three-quarters of its rated output. In the example, 1,270 W running fits well within a generator rated at about 2 kW continuous, provided its peak rating covers the pump’s start.
Sizing a generator to charge batteries
If the generator mainly recharges a battery system during long outages, it has to supply:
the inverter-charger’s charging power + anything running in the house at the same time
The charging power is usually given in amps at battery voltage. Multiply to get watts: a charger set to 60 A on a 48 V battery draws about 2,900 W from the generator, before losses. If the generator is smaller, set the inverter’s charge current lower, so the generator isn’t overloaded. The batteries will just take longer to charge. See battery charging time.
Running a generator for a few hours at a steady load to charge batteries usually uses far less fuel than running it all evening to power a few lights. See inverter vs generator.
Inverter generators vs conventional generators
- Inverter generators produce clean, stable power that suits electronics and inverter-chargers, are usually quieter, and throttle down at light loads, which saves fuel.
- Conventional generators are cheaper for their size, but their output can be less stable. Some inverter-chargers are fussy about poor generator power, so check before buying.
Safety
- Never run a generator indoors, in a garage, or near windows, doors or vents. Exhaust contains carbon monoxide, which has no smell and kills. Fit a carbon monoxide alarm.
- Connect it properly. Plugging a generator into a house socket, or wiring it in without a changeover (transfer) switch, can send power back into the grid and endanger people working on the lines. Have an electrician install a proper changeover.
- Let it cool before refuelling, and store fuel safely away from the house.
The calculator gives you the running and starting watts for your appliance list, the same numbers you need to size a generator.