Sizing a Generator for Home Backup: Step by Step

Why sizing matters

When the power goes out, a generator is only useful if it can handle the loads you connect to it. A unit that is too small may struggle to start appliances with motors, while one that is far larger than needed may cost more than necessary and use more fuel. The goal is to match the generator's stated wattage to the loads you actually plan to power.

This article walks through the steps to estimate your home backup wattage needs. It explains the terms you will see on listings and maker pages, and it points you to the questions you should ask an electrician or installer. It does not provide a specific wattage recommendation for your home, because that depends on your appliances, your wiring, and your local codes.

Understand running and starting watts

Every generator listing shows wattage figures, usually two numbers. The first is often the starting watts, sometimes called peak or surge watts. The second is the running watts, sometimes called rated watts. Starting watts are the extra power needed for a few seconds when an appliance with a motor, such as a refrigerator or a sump pump, first turns on. Running watts are what the appliance uses continuously.

When you read a listing, note which figure is which. The maker's page for the same model usually states both numbers clearly. If a listing only shows one figure, check the maker's page before you compare it to your load estimate. For a deeper look at these terms, see our guide on running watts vs starting watts.

List the appliances you want to power

Start by writing down the essential items you want to run during an outage. Common choices include a refrigerator, a freezer, lights, a sump pump, a well pump, a furnace fan, a space heater, a television, and chargers for phones and laptops. You do not need to power everything in your house at once; most people choose a few circuits.

For each item, find the wattage on its label or in its manual. Many appliances list the running watts and the starting watts. If you cannot find the starting watts, a general rule is that motor-driven appliances may need two to three times their running watts to start, but the exact figure is on the label or in the manual. Write down both numbers for each item.

Add up the load

Add the running watts of all the items you plan to run at the same time. This total is your continuous load. Then add the starting watts of the item with the highest starting requirement to that total, because you will need enough starting watts to handle that surge while everything else is running. The result is your peak load.

For example, if you want to run a refrigerator, a few lights, and a sump pump, add their running watts for the continuous load, then add the sump pump's starting watts for the peak load. Compare the continuous load to the generator's running watts and the peak load to its starting watts. The generator's figures must be equal to or greater than your totals. You can also use our wattage calculator to help with the math.

Consider the type of generator

The type of generator affects how you use it for home backup. A portable generator, whether inverter or conventional, is moved outside and connected to appliances with extension cords or to a transfer switch installed by an electrician. A standby generator is permanently installed and starts automatically when the power goes out.

For a standby unit, an installer and an electrician determine the correct size based on your home's electrical panel and the circuits you want to back up. They will also handle the transfer switch and any permits. For a portable unit, you have more flexibility, but you still need to match the wattage to the loads you connect. See our guide on portable generators and our guide on whole house generators for more.

Ask the experts

Sizing a generator for home backup is not a do-it-yourself calculation for a permanent installation. A licensed electrician and the generator maker can tell you what size you need, how to connect it, and what local codes require. For a portable generator, the maker's manual explains how to connect appliances and what the wattage figures mean.

If you are considering a standby generator, contact an authorized dealer or installer. They will perform a site survey and recommend a model. The maker's website often has a dealer locator and a sizing tool. Use those resources alongside the steps in this article. You can also browse our best generator picks to see what is available.

Safety first

Carbon monoxide from a generator is deadly. Never run a portable generator indoors, in a garage, basement, crawlspace, or shed, or near a window or vent. Follow the maker's manual and the guidance from the CPSC, CDC, and EPA on generator placement and carbon monoxide alarms.

Connecting a generator to your home's wiring is a job for a licensed electrician. Do not attempt to backfeed or use a double-male cord. The electrician, the maker, and the local permit office decide how to connect the generator safely. For more on safety features, see our guide on generators with CO sensors.

What to pick for your situation

Questions

What is the difference between running watts and starting watts?

Running watts are what an appliance uses continuously. Starting watts are the extra power needed for a few seconds when a motor starts. A generator's listing shows both figures; the starting watts are usually higher.

How do I find the wattage of my appliances?

Look for a label on the appliance that lists watts or amps. The manual may also list running and starting watts. If you only see amps, multiply by the voltage (120 or 240) to get watts.

Can I run my whole house on a portable generator?

A portable generator can power selected circuits, not the entire house. To connect it to your home's wiring, you need a licensed electrician to install a transfer switch. The generator's wattage must cover the loads on those circuits.

Who should size a standby generator for my home?

An authorized dealer or installer performs a site survey and recommends a model. A licensed electrician handles the connection and permits. The maker's website often has a dealer locator.

What if my generator is too small?

If the generator's starting watts are less than the peak load of the appliances you connect, the generator may struggle or trip a breaker. You may need to run fewer appliances at once or choose a larger generator.

Is it safe to run a generator in my garage?

No. Never run a portable generator indoors, in a garage, basement, crawlspace, or shed. Carbon monoxide can build up and be deadly. Use the generator outside, far from windows, doors, and vents.