Brand-neutral · free · shows the math
What size generator do you actually need?
Pick the things you want running in an outage. We add the running watts, then the single biggest starting surge, and tell you which size class fits, with every number editable to match your nameplates.
Tools
Five calculators, no brand behind them
Generator size calculator
Pick the appliances you need during an outage. We add running watts, then the single largest starting surge, and show the size class that fits.
Open →RV amp calculator
Check whether your RV loads fit a 30-amp or 50-amp service and how many generator watts you need to feed it.
Open →Fuel run-time estimator
Use your generator’s own spec-sheet numbers to estimate hours per tank and fuel for a multi-day outage.
Open →Watts, amps and volts converter
Turn a nameplate amp rating into watts (or back), with an optional power factor for motors.
Open →Outage prep checklist
A tick-off checklist for before, during, and after a power outage. It saves in your browser.
Open →Average time without power per U.S. electricity customer in 2024, the most in ten years of tracking.
U.S. EIATypical starting watts for a household refrigerator, versus 150–400 running watts.
Champion wattage chartKeep a generator more than 20 feet from windows, doors, and vents, and never run it in a garage.
CDCHow to size a generator
The three-step method we use
- List what must run at the same time. Think refrigerator, freezer, furnace fan or window AC, sump or well pump, lights, internet, and any medical device. Leave out loads you can run in turns, like a water heater or dryer.
- Add running watts, then the biggest surge. Motors need a short burst to start. Add the running watts of everything, then add only the largest extra starting demand. That is your peak. See starting vs running watts.
- Leave headroom and check voltage. Pick a generator whose running rating is about 20% above your running total, whose starting rating covers your peak, and which offers 240 V if you need a 240 V well pump or a transfer switch.
Our generator size calculator does all three steps and shows the math. Its default wattages are the high end of published manufacturer ranges, so you will not undersize by accident, and every number can be replaced with the figure on your appliance label.
Guides
Start with these
Starting watts vs running watts, explained
Why motors need a surge to start, how to add starting watts correctly, and why you only count the biggest one.
Read →What size generator do I need for my house?
A step-by-step way to size a generator for essentials, most of the house, or the whole house, with typical size classes explained.
Read →What size generator runs a refrigerator?
Refrigerator running and starting watts, how to read your fridge's label, and the smallest generator that can keep food cold.
Read →Load management: run more on a smaller generator
How to take turns with big loads so a mid-size generator covers the essentials, with a sample outage schedule.
Read →Sizing a generator for a well pump
Well pump running and starting watts by horsepower, 120 V vs 240 V pumps, and how to find your pump's specs.
Read →Generator carbon monoxide safety: where to put it and why
How far a generator must be from your house, why garages are never safe, CO alarm placement, and the symptoms to act on.
Read →Transfer switch vs interlock kit: how to connect a generator to your house
The safe ways to power household circuits from a portable generator, how manual transfer switches and interlocks differ, and why backfeeding is dangerous.
Read →Choosing extension cords for a generator
Wire gauge, length, and ratings for generator cords, plus when a cord is the wrong answer and you need a transfer switch.
Read →Inverter vs conventional generator: which do you need?
How inverter and open-frame generators differ in power quality, noise, fuel use, and size, and which fits home backup or RV use.
Read →Why brand-neutral matters
Most sizing tools are built by generator makers and end on their own product pages. We do not sell generators, and we do not rank models. We show the arithmetic, cite the published ranges we used, and point out when a smaller unit plus a little load management would do. Read our editorial standards for how we source every figure.