Generator Size Calculator

How generator sizing works

A generator has to handle two different numbers for every appliance: running watts and starting (surge) watts. Running watts are the steady draw once something is already on — a refrigerator compressor, for example, runs at around 150W. Surge watts are the short burst a motor pulls when it first starts, which can be several times higher. That same refrigerator's compressor can pull roughly 800W for a second or two on startup.

The correct way to size a generator is to add up the running watts of everything you want to run at once, then add only the single largest surge value among those appliances — not the sum of every surge. That's because motors rarely all start at the exact same instant in real use; the generator only has to absorb one surge on top of the steady running load. This calculator uses that method (the same logic behind the generatorSizeKw function that powers it), then adds a safety margin — 20% by default, and editable above — to leave headroom instead of sizing to the exact edge.

Essential circuits vs. whole-house backup

There are two common strategies. Essential circuits means backing up a short list: the refrigerator or freezer, a sump pump if flooding is a risk, the furnace blower (the fan, not the gas flame itself), a well pump if you're not on municipal water, and basic lighting. This is the cheaper, smaller option and covers the things that cause real damage or safety issues if the power stays off. Whole-house backup means the generator can run everything at once, including central air conditioning and an electric range — loads that are individually large enough to double or triple the recommended size on their own.

A worked example

Check the default "Essential circuits" preset above — refrigerator (150W running, 800W surge), sump pump (800W running, 1,300W surge), gas furnace blower (700W running, 1,400W surge), well pump (1,000W running, 2,100W surge), and basic house lighting (90W) — and the running watts add up to 2,740W. The largest single surge among them is the well pump's 2,100W. With the default 20% margin, that's about 5.8 kW. The three closest real published models that cover it are the Champion 8.5kW Home Standby Generator (natural gas/LP dual-fuel, model 100199, aXis technology, transfer switch sold separately) (7.5 kW natural gas), the Generac Guardian 10kW (natural gas/LP dual-fuel home standby, model 7171/G0071710) (9 kW natural gas), and the Kohler 14RCAL (natural gas/LP dual-fuel home standby, 14kW, with bundled automatic transfer switch) (12 kW natural gas) — based on published specs, not hands-on testing. If you're on municipal water and sewer, uncheck the well pump and sump pump and the recommendation drops further.

Common sizing mistakes

The biggest mistake is adding up every appliance's surge watts instead of taking just the largest one — that overshoots the real requirement and leads to buying a bigger, more expensive generator than needed. The second is ignoring surge watts entirely and sizing only to running watts, which can trip a generator's breaker or stall it the moment a compressor or pump kicks on. A third is skipping the safety margin: a generator running flat-out at 100% of its rated output constantly wears down faster and has no room for a slightly higher real-world draw than the spec sheet lists.