Generator Load Calculator

Enter the appliances you want to run at the same time and how many of each. The page returns the running load, the start-up peak, the generator output in kW and the capacity in kVA.

A generator has to cover two very different numbers: the watts everything draws once it is running, and the much larger spike when a motor starts. Tick the appliances and counts below; the page adds the running load, finds the single worst start-up surge and converts the result into the kVA figure printed on generator labels.

What will run at the same time

Enter how many of each appliance you want to run together (0–10). Leave anything you will not use at 0. Only appliances with a count above 0 take part in the start-up check.

Fridge / freezer LED bulb (each) TV and router Electric water heater Water pump / booster pump Space heater / air conditioner Rice cooker / microwave

Running load –

Start-up peak –

Recommended output –

Recommended capacity –

Biggest start-up surge –

How this was worked out

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Typical appliance power

Appliance	Running	Start-up

Five rules before you buy

Motors draw 3–5 times their running watts for a second or two when they start. Fridge, pump and air conditioner are all motors, so always plan with the start-up figure.
Start the largest motor first, then switch on the rest one at a time. Starting a pump on top of a running air conditioner is what trips a small generator.
Keep the running load under about 80% of the generator’s rated output. The last 20% is headroom for a hot day, a long extension lead and the next appliance someone plugs in.
Output falls roughly 3% for every 300 m above sea level. A 5 kW set at 1,500 m behaves like a 4.3 kW set, so buy one size up in the mountains.
The exhaust must go outdoors, well away from windows, doors and air intakes. Never run a generator in a garage, basement or closed balcony — carbon monoxide is invisible and odourless.

Practical notes

Measure rather than guess. A plug-in watt meter on the fridge or pump for a day gives a real running figure; the nameplate is often the worst case.
Stagger the big loads. Water heater first, then the pump, then the air conditioner, with a minute between each, keeps the peak well below the sum of every start-up.
Keep the set outdoors on a dry, level surface, with at least one metre of clear air around it and the exhaust pointing away from the house.
Run it unloaded for a few minutes before plugging anything in, and check the oil at every refuel. Fuel up only when the engine is off and cold.

Four common mistakes

Adding up the numbers printed on the labels and buying that. Nameplate watts are worst-case, and adding every appliance assumes they all start at once.
Looking only at running watts. A 150 W fridge that starts at 1,200 W can stall a set sized from the running figure alone.
Letting several motors start together. Two pumps or a pump plus the air conditioner can double the surge the generator has to survive.
Backfeeding a socket with a male-to-male lead. It puts live pins where they should never be and can kill a utility worker — use a proper transfer switch fitted by an electrician.