Enter your total running watts, your largest motor's starting watts, and a safety margin to get recommended running watts, surge watts, and generator size in kW.
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Enter your values and press Calculate.
Sizing a generator comes down to two wattage figures and one safety buffer. Your running watts are the power your appliances draw continuously, while your starting watts are the brief surge the largest motor needs to kick on. This calculator multiplies both figures by your safety margin so the generator never runs flat out, then converts the surge figure into kilowatts and rounds up to the nearest 0.5 kW. The result is a generator size that handles your biggest motor starting while everything else runs. It works for a home backup, an RV, or a job site. For a typical home backup, the essentials usually total 3000 to 5000 running watts.
Recommended running watts = Running watts × (1 + Safety margin / 100). Recommended starting watts = Starting watts × (1 + Safety margin / 100). Generator size in kW = Starting watts with margin / 1000, rounded up to the nearest 0.5 kW. Only the single largest starting surge goes into the math, because motors almost never all start at the same instant. The margin, typically 20 to 25 percent, covers voltage sag on hot days, altitude losses, and the next appliance you switch on. Many buyers start from a generator sizing chart, but this three-step formula is all you really need.
Add up the running watts of everything you want powered at the same time. Find each appliance's running watts on its data plate or in its manual. Identify the largest motor in your list and note its starting watts, usually 2 to 3 times its running watts. Enter a safety margin; 25% is the default and a good choice for most homes. The calculator returns the recommended running watts, the recommended starting watts, and the generator size in kilowatts, rounded up to the nearest 0.5.
Suppose your essentials total 3500 W of running load, and your largest motor, the refrigerator compressor, needs 2200 W to start. With a 25% safety margin: recommended running watts = 3500 × 1.25 = 4375 W, recommended starting watts = 2200 × 1.25 = 2750 W, and generator size = 2.75 kW rounded up to the nearest 0.5 = 3.0 kW. A 3.0 kW portable generator would cover this outage kit with comfortable headroom.
Use these realistic ranges when you do not have the data plate handy. Refrigerator: 150 to 800 W running, 1200 to 2200 W starting. Window air conditioner: 1000 to 1500 W running, 3000 to 4500 W starting. Central air conditioner (3 ton): 3000 to 5000 W running, 8000 to 15000 W starting. Sump pump: 800 to 1050 W running, 1300 to 2150 W starting. Furnace fan blower: 750 to 1200 W running, 2300 to 2800 W starting. Well pump (1 HP): 1000 to 2000 W running, 3000 to 6000 W starting. Microwave: 1000 to 1500 W running with no extra starting surge. Electric water heater: 4000 to 4500 W. Space heater: 1500 W. LED TV: 100 to 400 W. Laptop: 50 to 200 W. LED lights: 10 W per bulb. Always confirm against your own appliance labels before buying, since models vary.
Portable generators suit occasional outages and job sites, while standby units switch on automatically and handle whole homes. Match the fuel to your situation: gasoline is easy to find, propane stores for years without going stale, and natural gas never needs refilling. If the air conditioner is the problem load, a soft start kit, also called a soft-start device, can cut its starting surge by 50 to 70 percent and drop you a full generator size. Remember that generators lose power at altitude and in extreme heat, so add extra margin if you camp above 3000 feet. Never run a generator indoors or in a garage: carbon monoxide is odorless and deadly, so keep the unit at least 20 feet from the house with the exhaust pointed away. Treat this page as a sizing guide, not a substitute for a licensed electrician on whole-home standby jobs.
Add up the running watts of everything you want to power at once, then account for the single largest motor's starting surge. Multiply both by a safety margin of 20 to 25 percent. This calculator does that math and rounds the result up to the nearest 0.5 kW so you buy a standard generator size.
Running watts are the continuous power an appliance draws while operating. Starting watts are the brief extra burst, lasting a few seconds, that an electric motor needs to start turning. A refrigerator might run at 700 W but need 2200 W to start its compressor, and your generator must handle that surge.
Outage essentials like a refrigerator, sump pump, furnace fan, lights, and a few outlets typically need 5000 to 7500 running watts, so a 5.5 to 7.5 kW portable works well. Add a window air conditioner or well pump and you move into the 7.5 to 10 kW range. Whole-home backup with central air conditioning usually needs a 14 to 24 kW standby generator.
The 80% rule says your continuous load should not exceed 80% of the generator's rated capacity. Running at 90 to 100% load causes overheating, shortens engine life, and increases fuel consumption. The safety margin in this calculator serves the same purpose by sizing the generator above your calculated load.
Usually not without help. A 3-ton central AC can surge to 9000 watts or more at startup, which overwhelms most portables. A soft-start kit reduces that surge by 50 to 70 percent and can make a 7500 W generator viable for a 1.5 to 2 ton system. For larger systems, a standby generator is the safer choice.
Runtime depends on tank size, generator size, fuel type, and how hard it is working. A common portable running at half load typically delivers 8 to 12 hours of fuel per tank of gasoline, which works out to roughly $2 to $8 per hour at typical fuel prices. Running near full load can cut that in half, which is another reason to size with a comfortable margin.
No. An oversized generator costs more to buy, burns more fuel at light loads, and can suffer from wet stacking on diesel models. Size to your actual loads plus a 20 to 25 percent margin. If your needs might grow, buy the next standard size up rather than doubling your capacity.