Generator Size Calculator

How to size a generator correctly

Sizing comes down to two numbers. First, continuous load — add up the watts of everything that will run at the same time. Second, starting surge — find the single largest motor in your list and multiply its running watts by its start factor, because that one motor determines how much surge capacity the set needs.

Add a 20% margin and round up to the nearest standard size. This calculator does all three steps for you.

The three mistakes that matter

• Sizing on running watts alone. A 1,300 W air conditioner can pull close to 4,000 W for a second or two when the compressor starts.
• Confusing kW with kVA. Alternators are rated in kVA. If your load has a power factor below 1 and you size on kW, you will overload the alternator.
• Running at very light load. Below roughly 30% load a diesel engine never reaches operating temperature, which causes wet stacking and carbon build-up.

Common questions

What size generator do I need for a house?

A typical household with a refrigerator, lighting, a television and one air conditioner needs roughly 2–3 kW of continuous power and 5–6 kW of starting capacity. List your own appliances above for the exact figure.

Should I buy a bigger generator than I need?

A small margin helps, which is why we include 20%. Beyond that, oversizing wastes fuel and money. If your load will grow, size for the future now rather than replacing the set later.

What if my total falls between two standard sizes?

Always round up. A generator running at 80% load is far happier than one running at its limit, and it will last longer.