What size generator do you need?
List what you want to run, get the running and starting watts instantly.
Starting watts: 0 for things without a motor — lights, TVs, heaters.
What size generator do I need?
Add up the running watts of everything you want on at once, then add the biggest extra surge of a single motor as it starts. A refrigerator (400 W running, 1,200 starting), a ½ HP sump pump (1,050 / 2,150), a ½ HP furnace fan (800 / 1,600), a TV (400) and ten 15 W LED bulbs run on 2,800 W; the sump pump starting adds 1,100 W, so you need at least 2,800 running and 3,900 starting watts.
What are running and starting watts?
Running watts are what a device draws once it is on. Motors and compressors — refrigerators, pumps, air conditioners — briefly draw much more to start. Generators list both a running (rated) and a starting (peak or surge) wattage; your load must fit under each.
Where do I find my appliance's wattage?
On the nameplate or in the manual. If it lists amps, multiply by the voltage: 5 A at 120 V is 600 W. The defaults here are the upper ends of Champion Power Equipment's wattage chart — replace them with your own.
Should I add extra capacity?
Many buyers leave some headroom so the generator is not running flat out and there is room for one more device. Starting motors one at a time also keeps the peak down — that is why only the single largest surge is added.
How many amps is that?
Divide the watts by the voltage: 3,900 W is 32.5 A at 120 V or 16.3 A at 240 V. A transfer switch or inlet must be rated for the generator's output — have an electrician connect a generator to a house.
What if two motors start at the same moment?
This calculator adds only the single largest surge, which assumes motors start one at a time. If two will start together — two pumps on one switch, say — add both surges to the running total by hand. Starting the big motors one after another, with the generator already running, keeps the peak lower.
Can I judge the size from a breaker panel or from amps?
Watts are volts times amps, so a 20 A, 120 V circuit can carry up to 2,400 W and a 30 A, 240 V circuit up to 7,200 W. That is the most a circuit allows, not what is actually running, so a list of real appliances gives a better size than adding up breakers.
How the totals are built
Running watts are the sum of (running watts × number of units) over every appliance on the list. The peak is that sum plus one extra: for each appliance, take its starting watts minus its running watts, and add only the largest such difference. Appliances without a motor, such as lights and a TV, enter with a starting value of 0 and add nothing at start-up. Amps are the peak watts divided by the voltage, 120 V or 240 V.
Take the list in the calculator as it opens. The running watts add up to 2,800 W. The sump pump needs 2,150 W to start but runs on 1,050 W, so it adds 1,100 W, the largest surge on the list. The peak is 2,800 + 1,100 = 3,900 W, or 32.5 A at 120 V and 16.3 A at 240 V. A generator rated below 2,800 W running or below 3,900 W starting would not carry this list.
Mistakes that change the answer
Adding every starting figure together is the most common one. For this list the starting watts of all five entries come to 5,500 W, about 1,600 W more than the 3,900 W the list needs, because the refrigerator, pump and fan are not all in their start-up surge at the same instant. Entering the running watts of a motor as its starting watts has the opposite effect: the surge disappears and the peak comes out too low.
Another is copying a watt figure from a nameplate that actually shows amps or volt-amps. If only amps are printed, multiply by the voltage first. Also remember that the quantity column multiplies the running watts, so ten 15 W bulbs enter once with a quantity of 10, not as ten surges. Heaters and ovens on 240 V count the same way; their amps at 240 V are half the amps at 120 V for the same watts.
Safety with a portable generator
The CDC advises running a portable generator outdoors, more than 20 feet from windows, doors and vents, and never inside a home or garage, even with the doors open. It also advises a battery-powered or battery-backup carbon monoxide detector in the home whenever a generator is running. Size the generator for the appliances you will use, and have an electrician install a transfer switch or inlet rather than back-feeding an outlet.
Sources: the default appliances come from Champion Power Equipment's generator wattage chart; the safety advice is paraphrased from the CDC's carbon monoxide guidance for portable generators. Related tools: watts to amps, battery runtime and solar panel calculators.