What Size Generator for Home Backup: Room-by-Room Load

Start with the lights

Begin your load calculation in the rooms where lighting matters most: the kitchen, living room, hallway, and bedrooms. LED and CFL bulbs draw far less than older incandescent types, so count what you actually have installed. Add up the wattage of the bulbs you would turn on, not every fixture in the house.

Lighting is your baseline. Once you know the lighting total, move to the outlets that feed the electronics you need for communication, medical devices, or work. A modem, a laptop, and a phone charger together draw far less than a space heater or a window air conditioner, so keep those high-draw items on a separate line in your list.

Refrigerator and freezer

The refrigerator is the appliance most people insist on keeping alive during an outage, and it is the one that teaches the key lesson about starting watts. A fridge has a compressor motor that draws a burst of power for a second or two when it starts, then settles to a lower running draw. That burst is the starting surge, and it can be several times the running wattage.

To size correctly, you need the fridge's running watts and its starting watts. The running figure is what appears on the nameplate; the starting figure is not always listed, but a good rule is to plan for a surge of two to three times the running load. If you also have a chest freezer in the garage or basement, add it the same way, but remember that you can choose to run only one at a time if the generator is small. For the difference between the two ratings, see Generator Running Watts vs Starting Watts.

Kitchen and laundry circuits

Move through the kitchen and laundry room appliance by appliance. A microwave oven draws a steady load while running, but it has no motor surge, so its nameplate wattage is what you count. A dishwasher has a motor and a heating element; the heating element runs while the motor cycles, so count the full load on the nameplate. A washing machine has a motor that surges on start, and a dryer, if electric, draws a large steady load for its heating element.

You almost never need all of these at once during an outage. The practical approach is to build a list of what you will actually use, then decide which items you can run one at a time. If you have a gas water heater and a gas furnace, the house's electrical load drops sharply because those appliances use electricity only for controls and fans. See Generator Load Balancing for tips on avoiding overloads.

Heating and cooling

Heating and cooling can be the largest loads in a home, so they often decide the generator size. A forced-air furnace with a gas flame draws modest power for its blower fan and controls, but the blower motor has a starting surge. A heat pump or central air conditioner has a compressor and a fan motor, and the compressor surge can be large. A window air conditioner is smaller, but still has a motor that surges.

If your heating or cooling depends on a well pump, that pump can have the biggest surge in the house, because it starts against a column of water. Count it separately and understand that the pump and the furnace are unlikely to start at the same moment. For a full walkthrough of surge calculations, read Generator Inrush Current and Locked Rotor Amps Explained.

Well pump and sump pump

A well pump and a sump pump are both motor-driven, and both draw a heavy starting surge. The well pump starts against pressure, and the sump pump starts against a column of water, so each can briefly draw two to three times its running wattage. If you have both, size for the larger surge plus the running load of the other.

Because these pumps cycle on and off automatically, you should not assume they will run only when you are awake. The generator must be able to handle the starting surge whenever it occurs, even if the refrigerator also chooses that moment to start. Keep a margin of headroom so the combined surge does not trip the generator's breaker. The Generator Wattage Headroom article explains how much extra capacity is wise.

Electronics and communications

After you have counted the big appliances, add the electronics you need for information and connection: a router, a laptop, a phone charger, a cordless phone base, and a television. These draw relatively small amounts, but together they add up. If someone in the home relies on a CPAP, a nebulizer, or another medical device, check its nameplate and count it as an essential load.

Sensitive electronics can be damaged by distorted power, so if you plan to power them directly, consider an inverter generator, which produces cleaner output. See Can a Generator Power Sensitive Electronics and How Inverter Generators Work for guidance.

Put the list together

Write every essential item in a column with its running watts next to it. Add those running watts to get your running total. Then write the starting surge next to each motor-driven item. Add the largest surge to the running total of everything else; that is your peak demand. Match that peak to the generator's surge rating, and match the running total to the generator's continuous rating.

The Environmental Protection Agency notes that the average U.S. home uses about 10,500 kilowatthours of electricity per year, but that is the total for all uses, not the figure for a short outage. During an outage you will not run the water heater, the dryer, the oven, and the central air at the same time. Your essentials list is usually a fraction of the home's normal peak. For a more detailed method, see How Many Watts Do I Need a Generator For and Generator Wattage Headroom.

Safety and placement

Sizing the generator is only half the job; placing it safely is the other half. The U.S. Consumer Product Safety Commission warns that a portable generator produces as much carbon monoxide as hundreds of cars and can kill you in minutes. Never run a portable generator indoors, in a garage, basement, crawl space, or shed, and keep it outside well away from windows, doors, and vents.

The Centers for Disease Control and Prevention recommends running the generator outdoors more than 20 feet from windows, doors, and vents. Install battery-operated or battery-backup CO detectors near every sleeping area and check the batteries regularly. See Generator CO Poisoning Risks and Safe Placement Distances and What Does a CO Sensor on a Generator Do and How It Works for details.

Connect to the house

Once you know the size, you need a safe way to connect. A generator that plugs into a room via an extension cord can feed only the loads on that circuit. To power multiple circuits back to the panel, you need a transfer switch, which also prevents dangerous backfeeding. The size of the transfer switch depends on the generator's output. For guidance, see How to Size a Transfer Switch for Your Generator and Generator Connection Types.

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Questions

What is the difference between running watts and starting watts?

Running watts is what an appliance draws continuously after it is up and running. Starting watts, also called surge watts, is the brief burst of power a motor draws when it first turns on, often two to three times the running figure. You need a generator that can handle the starting surge of the largest motor plus the running load of everything else that is on.

Can I run a refrigerator and a freezer on one generator?

Yes, if the generator's running rating covers the combined running watts and its surge rating covers the larger of the two starting surges while the other is already running. If the numbers are close, run them one at a time or choose a larger generator.

How many watts does a well pump need?

A well pump's nameplate lists running watts, but the starting surge can be two to three times that. Because the pump cycles automatically, you must size for the surge, not the running figure. Exact numbers vary by pump and depth, so read the nameplate and keep a margin.

Is a 5,000-watt generator enough for a whole house?

For essential loads only, usually yes. Lights, a refrigerator, a freezer, a gas furnace blower, and communications often add up to under 5,000 running watts. A 5,000-watt generator is not enough to run an electric water heater, central air, or an electric dryer simultaneously. Match the generator's running rating to your essentials list.

Where should I place a portable generator during an outage?

The U.S. Consumer Product Safety Commission says never use portable generators indoors or in garages, basements, or sheds. The CDC recommends using generators outdoors more than 20 feet away from windows, doors, and vents. Keep the exhaust facing away from the house, and have a working CO alarm inside.

Should I connect the generator to the house panel?

If you want to power multiple circuits beyond one outlet, a transfer switch is the safe method. It isolates the house from the grid and prevents backfeeding. The transfer switch's amp rating must match the generator's output. See the transfer switch sizing article for details.