Generator Size Calculator
Enter the total running watts of what you want to power and the biggest extra starting surge. The calculator shows the running and starting watts a generator needs, with a safety margin.
How to use this Generator Size Calculator
- List the appliances you want to run at the same time and add their running watts.
- Find the appliance with the highest extra starting load, such as a pump or compressor, and enter its extra starting watts.
- Keep or change the safety margin. A margin leaves room so the generator is not run at its limit.
- Enter the rated running watts of a generator you are considering, if you have one.
- Press Calculate to see the running and starting watts to look for.
How to size a generator
A generator must cover the running load plus the starting surge of the biggest motor.
- Running watts with margin = running watts x (1 + margin)
- Starting watts = running watts + extra starting watts of the largest load
- Spare watts = generator running watts – running watts with margin
Worked example
You want to run lights, a fridge, a freezer and a well pump at the same time. The running loads add up to 3,000 W. The pump needs an extra 2,000 W for a moment when it starts. With a 20% margin the generator should supply 3,600 W continuously, and it must handle a starting load of 3,000 + 2,000 = 5,000 W. A 5,000 W running unit would leave 1,400 W spare.
Start the biggest motor first and add the others one by one so that surges do not overlap. If the generator is too small it can stall, trip or be damaged. If it is much too large it wastes fuel. Check labels, because starting watts are often not printed and may need the amps and a multiplier from the maker. Generators must be connected to a home with a proper transfer switch installed by a qualified electrician.
Assumptions & limits
- Running and starting watts are inputs from labels or data sheets. Starting watts differ widely between motors.
- The calculator assumes only the largest motor starts at a time.
- Rated watts of generators drop at altitude and in high heat.
- It does not size fuel, runtime or noise.
- Never connect a generator to a home without a transfer switch, and run it outdoors. Check the National Electrical Code and your local rules, and use a licensed electrician for real installations.
Frequently asked questions
How do I find what size generator I need?
Add the running watts of everything you want to run at once, add the largest starting surge, and leave a safety margin.
What are starting watts?
They are the extra power some motors need for a moment when they start.
Why add a safety margin?
So the generator is not run at its limit, which helps it last longer.
Can I connect a generator to my house?
Only through a proper transfer switch fitted by a qualified electrician.
Does altitude affect a generator?
Yes. Many generators produce less power at high altitude or in extreme heat.
Is a bigger generator always better?
No. Oversized units use more fuel and cost more.
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Last updated: November 2026