Generator Size Calculator for Your Home
List what you want to power during an outage and see the running and starting watts a generator must deliver.
Sample numbers only, to show how it works. Replace them with the labels on your own appliances.
Add the running watts of the first thing you want to power (it is on its label).
This is an estimate, not a professional calculation. See the disclaimer and the methodology.
How it is calculated
Add the running watts of everything that will run at the same time. That is the power the generator must deliver continuously.
Motors, such as a refrigerator compressor or a sump pump, draw a surge when they start, often several times their running watts. The calculator takes the largest extra surge of any one item (its starting watts minus its running watts) and adds it to the running total. That gives the starting watts the generator must deliver at the worst moment.
This is the common method generator makers use. It is not an official standard, and it assumes the motors do not all start at the same moment. If two motors often start together, such as two pumps, treat them as one item with their combined numbers.
The watts of an appliance are on its label or in its manual. If a label lists volts and amps, multiplying them gives volt-amps, which can overstate the watts of a motor, so check the manual. A plug-in watt meter shows what an appliance draws while it works.
An optional margin adds extra capacity to both numbers. We do not set one for you, because the right amount depends on how sure you are of the numbers.
Safety comes first. The U.S. Consumer Product Safety Commission says to operate portable generators outside only, at least 20 feet from the house, with the exhaust directed away from the home and any building people could enter. This calculator does not cover how to connect a generator to your home wiring. A licensed electrician should do that.
Worked examples
A refrigerator, a sump pump and some lights
A refrigerator at 200 running and 800 starting watts, a sump pump at 800 and 2000, and 300 W of lights and chargers (all assumed). Running total: 1,300 W. The sump pump has the largest surge, 1,200 W over its running watts, so the generator must deliver 2,500 W at start.
Adding a 25 percent margin
The same list with a 25 percent margin (assumed) comes to 1,625 running and 3,125 starting watts.
Adding a space heater
Replace the sump pump with a 1,500 W space heater (assumed), which has no surge. The running total becomes 2,000 W, and the starting need is 2,600 W, driven by the refrigerator's surge.
Frequently asked questions
What size generator do I need for my house?
It depends on what you want to power. List those items and enter their running and starting watts. The calculator adds the running watts and the largest starting surge.
What are running watts and starting watts?
Running watts is the power an item draws while it works. Starting watts is the surge a motor draws for a moment when it starts. Generators list both ratings.
Where do I find the watts of an appliance?
On its label or in its manual. A plug-in watt meter shows the real draw. Items with motors usually list starting watts separately, or you can look in the manual.
Can I run a generator in the garage?
No. The CPSC says to operate a portable generator outside only, at least 20 feet from the house. Opening a door or window does not make it safe indoors, and it must not run in a garage or on a porch.
How do I connect a generator to my house?
This calculator does not cover it. Use a licensed electrician: connecting a generator to home wiring wrongly can send power back to the grid and hurt utility workers.
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Sources and last update
Last updated: November 3, 2026. See methodology and sources.