Generator Sizing Calculator
Add what you want the generator to carry from a searchable library of household loads. The calculator adds up running watts, adds the biggest motor start on top, and tells you the running and starting ratings to look for on a portable or standby generator.
The short version
- Search 140 household loads with typical running and starting watts, add your list, and see the running total, the starting peak, and the generator size range to look for.
- Typical estimates, not measurements.
Generators are sized in two numbers. Running watts is what the machine can hold all day. Starting watts, sometimes printed as surge or peak, is the short burst it can give a motor for a second or two while it spins up. Fridges, well pumps, sump pumps, furnace blowers and air conditioners all ask for that burst. Undersize the start and the motor stalls or the breaker trips, even when the running total looks fine.
How to use
- 1Find your appliances. Type in the search box, like fridge, modem or well pump, or tap a category such as Kitchen or Medical. Tap any result to add it to your list as a card. Every item comes with typical watts filled in, so you do not have to look them up.
- 2Check running and starting watts. Each card shows running watts and starting watts. Starting watts only matter for motors and compressors. If your appliance label lists a different number, or the LRA on an air conditioner nameplate, use your own figure.
- 3Set the duty. Duty is the share of the whole day the appliance is actually drawing power. A fridge compressor cycles, so about 33 percent. A router is on all the time, so 100 percent. A microwave used for 15 minutes a day is about 1 percent. Drag the slider or type a number.
- 4Switch cards on or off. Use the switch on each card to try your list with and without a load. Tap Remove to drop it, Add a custom load for anything not in the library, or Reset to go back to the starting list.
- 5Read the results. The dark panel shows your running total, the starting peak with the biggest motor kicking on, and a generator size to shop for. The colored bar shows which loads take the most running watts. If you know your generator’s fuel use and tank size from its spec sheet, type them in for a runtime estimate.
The calculator
3Your results
Optional fuel runtime. Use the numbers printed on your own generator’s spec sheet.
About these numbers.
Generator size here is the larger of your running total plus a 25 percent cushion, or your starting total, rounded up to the next 500 W. Starting watts assume only one motor starts at a time, which is how you should run a generator anyway: let the fridge settle before the well pump kicks on. Runtime is only your spec sheet’s fuel rate divided into your tank. Real burn changes with load, temperature and altitude.
Why starting watts decide the size
A fridge that runs on about 150 watts can ask for well over 1,000 watts for a moment when its compressor starts. A half horsepower sump pump can double its running draw on start. Add up only running watts and you will buy a machine that hums along until the moment the pump needs it. The calculator takes the single biggest start surge on your list and adds it to everything else that is already running. That is your starting peak.
240 volt loads need a different setup
Well pumps, central air, electric water heaters and electric dryers usually run on 240 volts. Those cards carry a 240 V tag. Powering them takes a generator with a 240 volt outlet and a transfer switch or interlock installed by a licensed electrician. Never backfeed a dryer outlet with a cord. If several big 240 volt loads are on your must keep list, start with the standby generator buying guide and the standby vs portable comparison.
Fuel runtime, honestly
We do not guess fuel burn for you. It depends on the engine, the load and the weather. Most generator spec sheets print gallons per hour or hours per tank at a stated load, often half load. Type that rate and your tank size and the calculator divides one into the other. If your list runs the machine harder than the spec sheet load, expect fewer hours.
Nobody here metered your appliances. These are typical estimates plus arithmetic, and a licensed electrician should size anything wired into your panel. Method: how we work · corrections: contact.