How many amps does a whole house generator need?

The honest answer is that a whole house generator does not have one amp rating. It has a running amp capacity and a surge amp capacity, and the number you actually need depends on how much of your house you want to keep powered at the same time.

The honest answer is that a whole house generator does not have one amp rating. It has a running amp capacity and a surge amp capacity, and the number you actually need depends on how much of your house you want to keep powered at the same time. Most homes that want true whole house coverage land somewhere between 100 and 200 amps of generator capacity at 240 volts, which works out to roughly 24,000 to 48,000 watts. Smaller homes with gas heat and modest appliances can often get by with less. Larger homes with electric heat, electric water heating, or multiple air conditioners usually need more.

If you want the short version: add up the amps of everything you need to run at once, add a cushion for motor startup, and size the generator to that number. The rest of this article shows you how to get there without guessing.

1. Amps, watts, and volts are the same conversation

Generator shoppers see both amps and watts, and that confuses people. They are two ways of describing the same capacity. At 240 volts, watts divided by 240 gives you amps. A 24,000 watt generator is a 100 amp generator. A 48,000 watt generator is a 200 amp generator.

Why does this matter? Because your electrical panel and your utility service are rated in amps, and your appliances are usually rated in watts or in amps on the nameplate. Being able to convert between them is what lets you compare a generator's spec sheet to the load you actually have.

2. Running amps and starting amps are not the same number

This is the part that trips up most homeowners. A motor draws far more power in the first second or two of startup than it does while running. A central air conditioner might run at 15 amps but need 60 to 80 amps for a split second when the compressor kicks on. A well pump, a furnace blower, a refrigerator compressor, and a sump pump all behave the same way.

Generators are rated two ways to match this reality:

• Running watts (or running amps): what the generator can supply continuously, hour after hour.
• Surge watts (or starting amps): a short burst of extra capacity, usually available for a few seconds, meant to cover motor startup.

If you size a generator only to the running load, it will trip or stall every time a big motor starts. If you size it only to surge, you will overspend. The right approach is to size to the running load plus the largest single motor startup you expect to happen while everything else is running.

3. Typical household loads in amps

The table below gives realistic ranges for common home loads at 240 volts. Your exact numbers will vary by model and age, but these ranges are close enough to plan with. A licensed electrician will confirm actual values from the nameplates before final sizing.

Load	Typical running amps at 240V	Typical starting amps at 240V
Refrigerator	1 to 2	6 to 12
Freezer	1 to 2	6 to 12
Furnace blower (gas heat)	3 to 6	10 to 18
Central AC, 3 ton	12 to 16	50 to 75
Central AC, 5 ton	20 to 26	80 to 120
Well pump, 1/2 HP	4 to 5	15 to 20
Sump pump, 1/3 HP	3 to 4	10 to 15
Electric water heater	18 to 20	18 to 20
Electric range or oven	20 to 30	20 to 30
Electric dryer	20 to 24	20 to 24
Microwave	6 to 7	6 to 7
Lights and outlets (whole home)	5 to 15	5 to 15

Notice that heating and cooling dominate. If your home has gas heat, gas water heating, and a gas dryer, your electrical load is mostly motors and lights, and a smaller generator covers a lot of ground. If your home has electric heat, an electric water heater, or an electric range, the load climbs fast.

4. How to add it up

You do not need to add every single appliance in the house. You need to add the ones that will realistically run at the same time during an outage. Here is a practical way to do it.

• List the loads you consider essential: heat or AC, refrigeration, well or sump pump, lights, internet, and a few outlets for phones and a computer.
• List the loads you would like to keep if capacity allows: range, dryer, water heater, microwave, garage door.
• Add the running amps of everything in the essential list.
• Add the running amps of the optional items you want to keep, but only count the largest one at a time if they are unlikely to run together.
• Add the starting amps of the single largest motor in the whole list, on top of the running total.

That final number is your target. Round up to the next common generator size.

As a rough guide, a home with gas heat and gas appliances, one central AC, and normal lighting typically lands around 60 to 80 running amps and needs a generator in the 100 to 150 amp range to handle startup. A home with electric heat, electric water heating, and two AC units can easily push past 150 running amps and need a 200 amp generator or a managed approach where some loads are locked out while others run.

5. Whole house does not always mean every load at once

There is a middle path that many homeowners overlook. Instead of buying a generator large enough to run everything simultaneously, you can install a load management system. It watches total demand and briefly sheds large loads like the water heater, the dryer, or a second AC while the rest of the house keeps running. The homeowner rarely notices. The generator can then be one or two sizes smaller, which saves real money on the unit and on the fuel or gas line feeding it.

This is common with 100 amp and 150 amp generators on homes that would otherwise need 200 amps. It is also how many homes with electric heat get whole house coverage without a generator the size of a small car.

6. What actually determines your number

Five things drive the final amp rating for your house:

• Heating fuel. Gas or oil heat needs a fraction of the power that electric heat needs.
• Cooling load. Number of AC units and their tonnage, plus whether they start together or on a stagger.
• Water heating and cooking. Electric versions add 20 amps each and run for long stretches.
• Well and sump pumps. Small running load, large starting load.
• How much of the house you want live. A few essential circuits is a very different project from every circuit in the panel.

Two homes on the same street with the same square footage can need very different generators because of these five factors. That is why a number pulled from a chart online is a starting point, not a final answer.

If you want a real number for your home, the next step is a load calculation. That means reading the nameplates on your major appliances, checking your panel, and confirming what you want to run during an outage. We do that as part of a free estimate, and we will tell you honestly whether you need a 100 amp, 150 amp, or 200 amp generator, or whether load management lets you drop a size. Call (718) 555-0142 to schedule a free estimate or an inspection, and we will walk your panel and your appliances with you before you spend anything.