What size generator do I need for my house?

Two numbers matter: running watts and starting watts

Running watts (sometimes called rated or continuous watts) are what an appliance draws once it is going. Starting watts (surge or peak) are the short burst a motor needs to get moving. Fridges, freezers, pumps, furnace blowers and air conditioners all have that kick. Lights, routers and chargers don’t.

Generators are labelled the same way, with a running figure and a higher starting figure. You need both to cover your list. A generator can handle your total running load and still trip when the well pump kicks on, if the starting figure is too low.

The usual way to estimate the surge, and the way our calculator works, is to add the extra starting watts of only the single biggest motor; Honda’s sizing guide makes the same point, telling you to start big appliances at different times. That assumes motors start one at a time. If two can start together, plan for both.

Household generator wattage chart

These are the typical planning figures our calculator and iPhone app use. Real appliances vary, sometimes a lot, so treat them as a starting point and swap in the numbers from your own labels where you can.

On a small screen, scroll the table sideways to read every column.

The four-step method

1. List what must run at the same time. Be honest about the outage you are planning for. Most people start with food, heat, water and a few lights.
2. Add up the running watts. That is your steady load.
3. Add the biggest extra surge. Find the item with the largest gap between starting and running watts and add that gap. The result is your peak.
4. Leave headroom. Running a generator flat out all night is hard on it and leaves no room for a surprise. Our calculator plans for your load to sit at no more than 80% of the generator’s running rating, then rounds up to the next 100 W.

Worked examples for a 3-bedroom house

These use the chart figures above. Your house will differ, which is the point of running your own list.

1. Essentials: food, heat and a dry basement

Fridge, freezer, furnace fan, sump pump, LED lights, Wi-Fi router, a phone charger and a TV.

• Running: 200 + 180 + 600 + 800 + 80 + 15 + 10 + 120 = 2,005 W
• Biggest extra surge: the fridge, 1,200 − 200 = 1,000 W. Peak: 3,005 W
• Look for at least 2,600 W running and 3,100 W starting on the label.

2. Essentials plus a well pump and a window AC

Everything above, plus a well pump and one small window air conditioner for a bedroom.

• Running: 2,005 + 1,000 + 600 = 3,605 W
• Biggest extra surge: the window AC, 1,800 − 600 = 1,200 W. Peak: 4,805 W
• Look for at least 4,600 W running and 4,900 W starting.

3. A more comfortable outage

Example 2 plus the microwave.

• Running: 3,605 + 1,000 = 4,605 W
• Peak: 4,605 + 1,200 = 5,805 W
• Look for at least 5,800 W running and 5,900 W starting.

Notice how quickly the number climbs. The cheap trick is to stagger: run the microwave or kettle when the well pump isn’t filling, and start the big motors one at a time with the fridge first.

What about the whole house, or central air?

Central air, an electric range, an electric water heater or an electric dryer each add thousands of watts, and some run on 240 V. Once those are on the list you are usually looking at a standby generator, not a portable one. Our calculator flags anything over 16,000 W running as standby territory. Standby units are sized and installed by a licensed electrician, who will look at your panel and your actual equipment, so use these numbers to start that conversation rather than to finish it.

To power circuits in the house rather than running extension cords to appliances, you need a transfer switch or interlock fitted by an electrician. Never plug a generator into a wall outlet. That backfeeds the lines and can kill a utility worker.

Check your own appliances

Look for the rating plate inside the fridge door, on the back of the unit or in the manual. If it gives amps and volts, multiply them for an approximate wattage, as the US Department of Energy explains in its guide to estimating appliance energy use. Starting watts are rarely printed; the manufacturer or the chart above is the next best source.

Then put your real numbers into the generator size calculator. You can add your own items with running and starting watts, check a generator you already own, and print or share the plan.