Whole House Generator Sizing Guide

Getting whole house generator sizing right is the difference between a standby unit that quietly carries your home through a blackout and one that either trips constantly or costs thousands more than you needed. Size it too small and it overloads; too big and you overpay on the unit and your gas bill. The trick is to add up the watts your home actually draws, account for startup surges, and lean on load management to right-size the generator.

Here’s how a pro sizes a standby generator, with the wattage math laid out so you can sanity-check any installer’s recommendation.

Generators Are Sized in Kilowatts

Standby generators are rated in kilowatts (kW), where 1 kW equals 1,000 watts. Residential whole-house units typically run from 10 kW to 26 kW. Your job is to figure out how many watts your home needs at once, convert to kW, and add a safety margin. The two numbers that matter are running watts (steady draw) and starting or surge watts (the brief spike when motors kick on).

The Surge Watt Problem

Anything with a motor, air conditioners, well pumps, refrigerators, furnace blowers, draws far more power for the first second or two of startup than it does running. A well pump that runs at 1,000 watts might surge to 3,000 watts starting. If several motors try to start at the same moment, the combined surge can trip an undersized generator. Good sizing accounts for the largest surge landing on top of everything already running.

Step 1: List Your Loads and Their Watts

Write down what you want to power and the running and surge watts of each. Typical figures:

• Central AC (3 ton / 36,000 BTU): 3,500 running, 7,000 surge
• Electric furnace: 5,000 to 20,000 running (large)
• Furnace blower (gas heat): 800 running, 2,350 surge
• Well pump (1 HP): 1,000 running, 3,000 surge
• Refrigerator: 700 running, 2,200 surge
• Sump pump: 800 running, 2,000 surge
• Water heater (electric): 4,500 running
• Electric range: 5,000 to 8,000 running
• Lights, TV, electronics, outlets: 1,000 to 2,000 running

Electric heat, electric water heaters, and electric ranges are the big power hogs. Homes with all-electric appliances need much larger generators than homes with gas heat, gas water heaters, and gas cooking.

Step 2: Add Running Watts, Then the Largest Surge

The core formula is straightforward:

1. Add up the running watts of everything you want on at once.
2. Find the single largest surge among your motor loads.
3. Add that one surge on top of the running total. That’s your peak demand.
4. Add 20 to 25 percent headroom so the generator isn’t maxed out.
5. Divide by 1,000 to get kilowatts, and round up to the next available size.

You only add the single largest surge, not every surge, because motors rarely all start at the exact same instant, and modern generators handle brief momentary spikes.

A Worked Example: Gas-Heat Home

Say you want to power the essentials in a home with gas heat:

• Furnace blower: 800 running
• Refrigerator: 700 running
• Well pump: 1,000 running
• Sump pump: 800 running
• Lights and electronics: 1,500 running
• Microwave: 1,200 running

Running total: about 6,000 watts. The largest surge is the well pump at 3,000, so 2,000 watts of surge above its running draw. Peak demand: about 8,000 watts. Add 25 percent headroom: about 10,000 watts, or 10 kW. A 10 to 13 kW generator covers this home comfortably.

A Worked Example: All-Electric Home With Central AC

Now a larger home wanting central air and electric appliances running:

• Central AC: 3,500 running, 7,000 surge
• Electric water heater: 4,500 running
• Refrigerator: 700 running
• Well pump: 1,000 running
• Lights, electronics, outlets: 2,000 running
• Electric range: 6,000 running (intermittent)

Running total without the range: about 11,700 watts. Add the AC surge margin of roughly 3,500 watts and you’re near 15,000 watts. Add headroom and cooking, and this home needs a 20 to 24 kW generator to run comfortably. Whole-house coverage for an all-electric home is why the big units exist.

Load Management: The Smarter, Cheaper Path

You don’t always have to buy the biggest generator. Load management (also called load shedding) uses smart modules or a smart transfer switch to prevent your two biggest loads from running at once. For example, the system might pause the electric water heater while the AC is running, then let it resume. This lets a smaller generator, say 18 kW instead of 24 kW, power the same home by staggering the large loads.

Managed loads can shave several kilowatts off your sizing, cutting both the unit price and the fuel consumption. A good installer will design load management into the system rather than just selling you the largest generator.

Whole-Home vs. Essential-Circuit Sizing

Decide what “whole house” means to you:

• Essential circuits only: heat, fridge, well/sump, some lights and outlets. A 10 to 14 kW unit usually covers it. Cheaper to buy and run.
• Managed whole home: everything with load management juggling the big loads. Typically 18 to 22 kW.
• True whole home, all electric, no shedding: 24 to 26 kW-plus.

What It Costs

Whole-house standby generators run roughly $5,000 to $15,000 installed for most homes, including the automatic transfer switch, gas hookup, pad, and electrical work. A 10 to 14 kW unit lands at the lower end; 22 to 26 kW units with load management push toward the top. Skimping on size to save money often backfires when the generator overloads during the first real outage.

Sizing by Home Square Footage: A Rough Guide

When you don’t yet have an appliance list, square footage gives a ballpark, though it’s no substitute for a real load calculation:

• Under 1,500 sq ft, gas heat: often 10 to 14 kW covers essentials or a managed whole home.
• 1,500 to 2,500 sq ft: typically 16 to 20 kW with load management.
• 2,500 to 3,500 sq ft: usually 20 to 24 kW.
• Over 3,500 sq ft or all-electric: 24 to 26 kW-plus.

Treat these as starting points. A 1,400-square-foot all-electric home with central air can need more generator than a 3,000-square-foot home with gas heat, gas water heating, and gas cooking. Always confirm with the actual wattage math above before buying.

Why Oversizing Is a Real Mistake

People assume bigger is always safer, but an oversized generator has real downsides. It costs more up front, often thousands more between a 16 kW and a 26 kW unit. It burns more fuel, since a large engine lightly loaded is inefficient, which matters on propane you’re paying for by the gallon. And running a big generator at very light loads for long periods can cause “wet stacking,” where unburned fuel fouls the engine. The goal is right-sizing: a generator that comfortably carries your real load with 20 to 25 percent headroom, not one that idles at a fraction of its capacity. Load management lets you hit that target with a smaller, cheaper, more efficient unit.

Fuel and Practical Notes

Most whole-house units run on natural gas or propane. Natural gas offers unlimited runtime from the utility line; propane needs a large tank sized for days of use. Have a licensed electrician and, where required, a plumber handle the install with permits. And always site the generator outdoors on a pad well away from windows and doors, because like any generator its exhaust contains carbon monoxide and it must never run in an enclosed space.

The Bottom Line

Sound whole house generator sizing comes down to adding your running watts, stacking your single largest motor surge on top, adding 20 to 25 percent headroom, and converting to kilowatts. Gas-heat homes often need only 10 to 14 kW; all-electric homes with central air can need 20 to 26 kW. Use load management to right-size down and save money on both the unit and fuel. Run the numbers yourself, then let a licensed installer confirm the load calculation before you buy.