Generator Size Calculator

Generator shopping goes wrong in one predictable way: people add up what they want to run, buy a unit that covers it, and then watch it stall the first time the refrigerator compressor kicks on with the furnace already running. The number that matters is not just the running load. It is the running load plus the surge the biggest motor draws for a couple of seconds when it starts.

This uses the sizing worksheet utilities hand out. Pick what you want to keep on, from the refrigerator, sump pump, furnace blower, and lights up through a window air conditioner and the water heater, say whether you are on a well, and the calculator adds the running watts and then the single largest starting surge, the way the worksheet says to. Then it names the portable class or standby size whose rating and surge both clear those numbers.

Whole house is a different question. Add up every circuit in a home and you get a number nobody installs, because nothing runs at once and standby systems manage loads. For that path the calculator uses the sizes generator makers themselves publish by home size, and it says plainly that no portable covers a whole house.

Essentials and comfort: the worksheet method. Utilities publish a sizing worksheet: list what you want to power, add up the running watts, then find the single item with the highest starting surge and add that one number. The result is the starting watts a generator must produce for a few seconds and the running watts it must hold. The calculator uses the worksheet's own appliance chart: a refrigerator at 700 running and 2,200 additional to start, a one-third horsepower sump pump at 800 and 1,300, a furnace blower at 700 and 1,400, a half-horsepower well pump at 1,000 and 2,100, a 10,000 BTU window air conditioner at 1,200 and 3,600, an electric water heater at 4,000 with no surge, and a washer at 1,150 and 2,250. Lights and electronics are a 900-watt allowance.

Matching a generator. A generator has two ratings: the watts it can supply continuously and the surge it can produce briefly. The calculator picks the smallest common portable class, from 3,500 to 15,000 watts, whose rating covers the running total and whose surge, taken as 1.25 times the rating the way Generac's GP series is rated, covers the starting total. It does the same across the standby lineup of 10, 14, 18, 22, 24, and 26 kilowatts.

Whole house: the manufacturer ladder. For a whole home the makers publish sizes by home size: 10 to 14 kilowatts for a smaller home's essentials, 18 to 22 for a home around 2,000 square feet with 22 as the standard, and 26 or more for larger homes with heavy air conditioning. That ladder assumes the transfer switch manages loads, which is how a 22 kilowatt unit runs a house whose central air alone can demand 15 to 18 kilowatts to start. The calculator reports the ladder size and, on the portable path, explains that the hardwired loads in a whole house need a standby.

Total starting watts = total running watts + the single largest additional starting watts. Essentials: refrigerator 700 + sump pump 800 + furnace blower 700 + lights and electronics 900 = 3,100 running watts. The largest starting surge on that list is the refrigerator's 2,200, so the generator must produce 3,100 + 2,200 = 5,300 starting watts.

Match to a generator. The 3,500-watt class surges to about 4,375, short of 5,300. The 5,500-watt class rates 5,500 running and surges to 6,875, so 5,500 watts is the portable answer; a 10 kW standby covers it easily.

Add a well pump. A half-horsepower well pump adds 1,000 running watts and its 2,100 surge is still below the refrigerator's, so essentials on a well are 4,100 running and 6,300 starting. The 5,500 class surges to 6,875, so it still fits, but with only about 9 percent of surge to spare, which is why the result page suggests going a class up when you land this close to the line.

Comfort. Add a window air conditioner (1,200, surge 3,600), an electric water heater (4,000, no surge) and the washer (1,150, surge 2,250): 9,450 running, and the largest surge is now the air conditioner's 3,600, so 13,050 starting watts. That takes a 12,000-watt portable class (15,000 surge) or a 14 kW standby.

Checking the worksheet's own example. The utility's worked example lists six items totaling 12,500 running watts with a highest surge of 2,300 and concludes "at least 12,500 Total Running Watts and 14,800 Total Starting Watts." The calculator applies exactly that arithmetic.

📎Sources:Fall River Electric Cooperative — Standby Generator Sizing Guide (worksheet method and wattage chart),Generac — What Size Generator Do I Need? (kW ladder by home size, AC starting loads),Generac — Air-cooled home standby generator sizes (10–26 kW Guardian lineup),Honda Power Equipment — Generator Wattage Chart (running and starting watts by appliance)