What Size Generator Does a House Need?

1. Starting Watts vs Running Watts

Every generator is advertised with two numbers, and confusing them is the most common sizing error. Running watts is what the unit produces continuously. Starting watts, sometimes called surge or peak, is what it can deliver for a few seconds.

The gap exists because of electric motors. Anything with a compressor or a pump — refrigerator, well pump, air conditioner, sump pump, air compressor — pulls a large inrush current as the rotor comes up to speed, typically two to three times its running draw and occasionally more. That spike lasts a second or two, but if the generator cannot supply it, the motor stalls, the generator's breaker trips, or in the worst case the motor windings overheat.

The practical rule is that your generator's running capacity must cover everything that is on at once, and its surge capacity must cover that total plus the largest single motor start. Motors rarely start simultaneously, so you do not add every surge figure together — you add the biggest one to the running total.

Resistive loads have no surge at all. Incandescent lights, toasters, coffee makers, electric water heaters and space heaters draw the same watts the instant they turn on as they do an hour later. They are simple to add up and, in the case of anything with a heating element, they are enormous.

2. What Household Appliances Actually Draw

These are typical figures. Check the nameplate on your own equipment where you can, because a twenty-year-old refrigerator and a current model differ by a factor of two.

3. Doing the Calculation

Four steps, and a worked example that covers most houses.

The calculator applies the same method set out below, so you can either use it directly or work through the steps by hand to understand where the number comes from.

Step 1: List What Must Run

Be honest about the difference between must and want. Refrigerator, freezer, heating system, sump or well pump, a handful of lights, the internet, and phone charging is a genuinely comfortable outage for most families. Air conditioning, an electric range and a dryer are the loads that triple the number.

Step 2: Add the Running Watts

For a typical example: refrigerator 700, freezer 500, gas furnace blower 800, sump pump 1,000, lights and electronics 500. That comes to 3,500 running watts.

Step 3: Add the Largest Single Surge

The biggest starter in that list is the sump pump at roughly 2,600 starting watts, which is 1,600 above its running figure. Adding that to the 3,500 gives a peak requirement of about 5,100 watts. A 5,500 running watt generator with 7,000 starting watts handles this list comfortably.

Step 4: Add Headroom, But Not Too Much

Target running the generator at 40 to 80 percent of its rated capacity. That range is where fuel efficiency is reasonable, engine loading is healthy, and there is room for the load you forgot. Going far above that band wastes fuel; sitting at 95 percent all night leaves nothing for the microwave someone inevitably turns on.

4. Recommended Size by Home Type

One nuance on the standby rows: modern automatic standby systems include load management modules that shed the air conditioner for a few seconds while the well pump starts, or alternate between two AC units. Load management often lets a 22 kW unit do what would otherwise require 30 kW, and it is usually cheaper than the next size up. Our whole house generator guide covers which systems include it as standard.

5. Portable vs Standby at Each Size

Below about 12,000 watts, portables dominate on price. Above it, standby units take over, because a portable that large is a 300-pound machine that still has to be wheeled out and fueled by hand in the weather that caused the outage.

A portable in the 5,000 to 9,000 watt range with a manual transfer switch is the value sweet spot in backup power. Expect $700 to $1,800 for the generator and $500 to $1,500 for a professionally installed interlock or transfer switch. It requires you to be home, to have fuel stored, and to start it manually. It also costs a fifth of what a standby install does. Our roundup of the best home generators for power outages covers the models in this range.

A standby unit runs on natural gas or propane, starts itself within seconds of an outage, exercises weekly, and needs no attention beyond annual service. That convenience costs $4,500 to $15,000 installed depending on size and gas run. It is the right answer for medically necessary power, frequent multi-day outages, or a house nobody is home at. See our Generac generator cost breakdown for real installed pricing.

One warning that applies to both: never connect a generator to house wiring through a "suicide cord" plugged into a dryer outlet. It back-feeds the utility line, can kill a lineman working to restore your power, and it bypasses every protection in the panel. A transfer switch or interlock kit is the only correct connection, and it is not expensive.

6. The Loads That Complicate Sizing

Central Air Conditioning

A three-ton compressor's locked-rotor inrush is what forces people into much bigger generators than they expected. A soft start kit — a few hundred dollars, installed at the condenser — cuts that inrush by roughly 60 to 70 percent and frequently lets a 10,000 watt portable run an AC unit that would otherwise demand 14,000. It is the single highest-value accessory in residential backup power.

Well Pumps

Deep well submersibles surge hard and are non-negotiable in a rural outage, since without them there is no water and no toilet. Size for the surge, or fit a soft starter here too. Note also that many well pumps are 240 volt, which means the generator must have a 240 volt output rather than only 120 volt receptacles.

Heat Pumps and Electric Heat

Electric resistance heat is brutal on a generator — a 10 kW auxiliary heat strip is exactly 10,000 watts of pure resistive draw, and many air handlers have two. If the house heats electrically, either plan on a very large standby unit or plan on heating one room with a different source during an outage.

Sensitive Electronics

Computers, modern furnace control boards, variable-speed motors and LED lighting prefer clean power. Inverter generators produce a low-distortion sine wave suitable for anything in the house; conventional open-frame generators produce a rougher waveform that is fine for motors and heaters and less ideal for electronics. If a single generator has to do both, inverter technology is worth the premium — our guide to the best inverter generators covers the trade-offs.

7. Fuel, Runtime and the Cost of Being Wrong

Size determines fuel burn, and fuel burn determines whether a three-day outage is manageable. A 7,500 watt portable at half load burns roughly 0.6 to 0.8 gallons of gasoline per hour, which is 15 to 19 gallons a day — four or five full jerry cans, from stations that may have no power to pump. A 22 kW standby on natural gas has no such constraint, which is a large part of its appeal.

Dual fuel and tri fuel portables solve part of this by running on propane, which stores indefinitely and is easy to keep on hand in 20-pound tanks. Propane delivers slightly fewer watts than gasoline for the same engine, typically about 10 percent less, so size accordingly if propane will be your primary fuel. Our comparison of dual fuel generators covers how the derating works in practice.

The cost of undersizing is inconvenience: tripped breakers, a choice between the well pump and the freezer, and a machine running at its limit all night. The cost of oversizing is money spent up front plus worse fuel economy for the life of the unit, and on diesel units the risk of wet stacking from chronic light loading. Between the two, err slightly large on a portable — the price difference between 6,500 and 8,000 watts is small — and size a standby carefully, because there the difference between 18 kW and 26 kW is thousands of dollars.

8. Frequently Asked Questions