How to Work Out What Size Generator You Need

What you need

The list of loads you want the generator to run, their running watts (from the appliance labels), and their starting multipliers — motors typically draw two to three times their running watts when they start. The generator size calculator takes up to three loads and does the rest.

Step 1 — List the loads

Write down what must run: a fridge (150 W), an air conditioner (1,200 W) and lights (200 W) in the example below. Take the running watts from the appliance labels, not from memory — the label is the fact, and the size of the generator depends on it. Decide in advance what you will NOT run: a generator that tries to run everything is a generator that is too small for everything.

Step 2 — Add the running watts

Add the running watts together: 150 + 1,200 + 200 = 1,550 W. That is the load once everything is running — but it is not the size of the generator, because motors do not start at their running watts.

Step 3 — Add the starting watts

Only the biggest motor starts at once, so add the extra starting power of the largest motor only. The air conditioner runs at 1,200 W and starts at 2.5 times that — 3,000 W — so the extra is 3,000 − 1,200 = 1,800 W. Starting watts are 1,550 + 1,800 = 3,350 W. The generator size calculator does this for up to three loads, each with its own starting multiplier.

Step 4 — Add the margin and convert

Add a safety margin — 20% is the usual figure — and convert to kilowatts and kVA. The recommended size is 3,350 × 1.20 = 4,020 W, which is 4.02 kW, and in kVA (dividing by a power factor of 0.8) that is 4.02 ÷ 0.8 = 5.02 kVA. Generators are sold by their rated output, so buy at or above the recommended size — never below it.

Step 5 — Choose and place the generator

Pick a generator rated at or above the recommended size, and place it outdoors, downwind and at least 20 feet (6 metres) from the house — never indoors, in a garage, or near a door, window or vent. Plan the connection before you buy: a transfer switch, installed by a qualified electrician, is the safe way to connect a generator to a house. Never plug a generator into a wall socket.

Worked example

Loads: a fridge at 150 W with a ×3 starting multiplier, an air conditioner at 1,200 W with ×2.5, and lights at 200 W with ×1. Running watts: 150 + 1,200 + 200 = 1,550 W. Starting watts: 1,550 + (3,000 − 1,200) = 3,350 W. Recommended size with a 20% margin: 3,350 × 1.20 = 4,020 W = 4.02 kW. In kVA: 4.02 ÷ 0.8 = 5.02 kVA. Those are exactly the numbers the generator size calculator gives you.

Common mistakes

• Sizing on running watts only. A generator that can run your fridge cannot necessarily start it — and a fridge that never starts is a fridge full of spoiled food.
• Forgetting the starting multiplier. Motors draw two to three times their running watts on start-up. A load list without multipliers sizes a generator that trips on the first start.
• No safety margin. A generator run at its absolute limit overheats and fails early. The 20% margin is what keeps it running.

Safety

Carbon monoxide kills: it is colourless and odourless, and a generator's exhaust can fill a house through a door, a window or a vent. NEVER operate a generator indoors, in a garage, or near any opening — always outdoors, downwind, at least 20 feet (6 metres) from the house, with the exhaust pointing away. Never plug a generator into a wall socket: backfeeding energises the grid and can kill the line workers repairing it. A transfer switch must be installed by a qualified electrician. Keep fuel away from the generator while it runs, let it cool before refuelling, and never run a generator in rain without proper cover.

Try these calculators

• Generator Size Calculator — running and starting watts, kW and kVA.
• Energy Usage Calculator — kWh and cost for up to three appliances.
• Watt to kWh Calculator — watts and hours to kilowatt-hours.