Home Generator Size Calculator: Running and Starting Watts

Find the generator size your home needs from the appliances you actually want to run. The calculator adds up running watts, finds the largest motor starting surge, adds headroom and suggests a size class. Edit any wattage to match your appliance labels.

How to use this tool

1. Keep the appliances you need during an outage and remove the rest with the x button. Add rows for anything missing.
2. Replace the typical wattages with the running and starting watts from your appliance labels or manuals where you can.
3. Set headroom. Twenty percent is a sensible default so the generator is not running flat out.
4. Read the running and starting watts, then compare them with the rated (running) and starting (surge) watts on generator spec sheets.

Worked example

The default list (refrigerator, half-horsepower sump pump, furnace blower, LED lights, internet, device charging, television and microwave) totals 4,020 running watts. With 20 percent headroom the calculator rounds up to 4,900 running watts. The largest extra starting surge is the furnace blower at 1,550 watts, so the starting requirement is 5,600 watts. That points to a large portable generator connected through a transfer switch or interlock.

Understanding your results

Running watts must not exceed the generator’s rated output. Starting watts must not exceed its surge or starting rating. Only the single largest surge is added because motors rarely start at the same instant; you can help by turning loads on one at a time when power comes up. If your result is above roughly 14,000 watts, you are in standby generator territory.

Common mistakes to avoid

• Using the generator’s starting watts as its capacity. Size by running watts.
• Forgetting well pumps and central air, which can need far more than the defaults.
• Assuming propane output equals gasoline output. Dual fuel generators usually produce less on propane; check both ratings.
• Ignoring altitude and heat, which reduce engine output. Check the manufacturer’s derating guidance.

Next steps

Decide how to connect the generator with the Transfer Switch Selector, and read the Backup Generators hub for types and safety.

Frequently asked questions

What is the difference between running and starting watts?

Running watts are what an appliance draws continuously. Starting watts are the brief extra power a motor needs to start, often two to three times its running watts for a second or two.

Will a 5,000 watt generator run a house?

It can run many essentials such as a refrigerator, sump pump, furnace blower, lights and internet if you manage loads, but not central air, an electric range or an electric water heater at the same time.

Where do I find my appliance wattage?

Check the label inside the door or on the back of the appliance, or the owner’s manual. Labels often list amps; multiply amps by volts to estimate watts.

This tool is for planning and education only. It does not store your entries. Work on home wiring must be done by a licensed electrician. Read our disclaimer.