What Size Generator Do I Need? A Plain-English Sizing Guide
Last updated: March 2026
You've decided you need a standby generator. Now the first question every homeowner hits: what size?
Get it wrong in either direction and you've got a problem. Too small and the generator trips its own breaker the moment your AC kicks on: right in the middle of the outage you bought it for. Too large and you've overspent by thousands on capacity you'll never use, burning fuel inefficiently the whole time.
This guide walks you through exactly how to figure out the right size for your home. No jargon without an explanation. No guessing.
The Quick Answer
If you're not here for the deep dive, here's the cheat sheet. These are general recommendations: your actual number depends on what appliances you need to run and whether your heating is gas or electric.
| Home Size | Generator Size | What It Keeps Running |
|---|---|---|
| Under 1,500 sq ft | 10–15 kW | Essentials: lights, fridge, freezer, furnace blower, sump pump |
| 1,500–2,000 sq ft | 12–17 kW | Essentials plus smaller central AC or window units, well pump |
| 2,000–3,000 sq ft | 15–22 kW | Most of the house: central AC, kitchen, home office, laundry |
| 3,000–5,000 sq ft | 22–32 kW | Multiple AC zones, full kitchen with electric range, water heater |
| 5,000+ sq ft | 36+ kW | Everything: including pool pump, hot tub, EV charger |
A word of caution on these ranges: square footage is a rough proxy. A 2,000 sq ft home with a gas furnace and gas stove might get by with 12 kW. That same 2,000 sq ft home with electric heat, an electric range, and a 4-ton AC could need 22 kW or more. The appliances matter more than the floor plan.
Two Questions That Decide Your Generator Size
Before you look at a single spec sheet, answer these two questions. They'll get you 80% of the way to the right answer.
1. Do you need to run central air conditioning?
Central AC is the single largest electrical load in most homes. A 3-ton system — common in a 2,000 sq ft home — draws 2,500 to 3,500 watts while running. But when the compressor first kicks on, it surges to 7,000 to 10,500 watts for a few seconds. Your generator doesn't just need to handle the AC running — it needs to handle the AC starting.
If central AC needs to stay on during an outage, you're looking at 14 kW minimum. If you have a 4- or 5-ton system, you're closer to 18–22 kW. If you don't need central AC, you can drop to the 10–14 kW range and save several thousand dollars.
2. Is your heating gas or electric?
A gas furnace uses electricity only for the blower fan, about 700 to 875 watts. That's nothing. Your generator barely notices it. Electric heat or a heat pump is a different story: 5,000 to 15,000 watts depending on the system.
Gas furnace? Size for your AC load. Electric heat? Size for your heating load. It's usually bigger.
Running Watts vs Starting Watts
Every appliance in your home has two power numbers, and confusing them is the most common sizing mistake.
Running watts are the steady power draw to keep something operating once it's already on. Your fridge hums along at about 200 watts.
Starting watts are the brief surge of power needed to start a motor. When your fridge compressor kicks on, it needs 1,200 to 2,200 watts for a few seconds before settling back to 200.
This is the part that trips people up: your generator must be sized to handle starting watts, not just running watts. If everything in your house draws 12,000 running watts but your AC compressor needs a 10,000-watt surge to start, a 12 kW generator will trip the moment the AC cycles on.
Climate Zone Impact
Hot climates (Gulf, South, Southwest): AC is your biggest load. The startup surge from a central AC compressor often dictates your minimum generator size. 14–22 kW+ for whole-house.
Cold climates (Upper Midwest, NE, Mountain): If gas furnace, your heating load is tiny. If electric heat, 5,000 to 15,000 watts. Can double your requirement.
Mixed climates (Mid-Atlantic, PNW): Size for worst-case season: usually summer AC.
Load Management Saves Thousands
Most homeowners have never heard of load management. It can save you thousands of dollars. A load management system uses a smart controller to decide which major appliances can run at the same time: so you can get whole-house coverage from a smaller (and cheaper) unit.
Example: 2,500 sq ft home with two 5-ton ACs. Without load management: 30 kW ($12,000–$18,000). With load management running one AC at a time: 20 kW ($7,000–$10,000). That's $5,000+ saved.
Cummins includes load management on every QuietConnect model at no extra cost. Generac and Kohler offer it as an add-on or through specific transfer switch models.
Common Sizing Mistakes
"My panel is 200 amps, so I need a 48 kW generator." No. Panel amperage is the maximum capacity of your electrical system, not what you're actually using. Sizing off the panel label is the most expensive mistake you can make.
Ignoring starting watts. If you add up running watts and buy that size, your generator will work great: right up until the AC compressor cycles on.
Not applying demand factors. You won't run the oven, dryer, AC, hot tub, and EV charger all at the same time. NEC Article 220 demand factors reduce the theoretical maximum to a realistic number.
Forgetting about the future. Planning to add an EV charger? A pool? Size for what you'll need in 5 years, not just today.
What Happens If You Get the Size Wrong
Too small: Circuit breaker trips or the unit shuts down. Engine overheats, voltage drops, electronics can be damaged. Chronic overloading shortens the generator's life and creates fire risk in extreme cases.
Too big: Wasted money upfront. Fuel inefficiency: generators run best at 50–75% capacity. An oversized unit running at 20–30% wastes fuel for years.
Too small is dangerous. Too big is expensive.