What Size Generator Do I Need?
Sizing a generator is the single most important decision you'll make. Too small and it'll trip or run flat-out under load; too big and you'll pay for capacity you'll never use. Here's how generator pros think about it.
Step 1: Decide what you want to back up
Generator sizing starts with a simple question: whole-home backup or essential circuits only? Whole-home means everything keeps working the same way it does on utility power — every outlet, every breaker, including HVAC. Essential circuits means you pick a handful of must-have loads (fridge, well pump, furnace, a few outlets, maybe one AC) and back up only those.
Whole-home is more convenient and dramatically more expensive. Essential-circuits is cheaper, smaller, and forces you to think about what really matters when the power's out.
Step 2: Air-cooled vs liquid-cooled
Home standby generators come in two engine platforms:
- Air-cooled (typically 10–26kW): Smaller, less expensive, simpler. Great for essential-circuit and many whole-home setups on average single-family homes.
- Liquid-cooled (25kW and up): Larger displacement, designed for sustained heavy load. Required for larger homes, multiple HVAC systems, or when you want true whole-home with no load shedding.
The transition happens around 22–26kW. If you're being quoted a 26kW air-cooled and a 27kW liquid-cooled, they're very different machines under the hood, even though the labels look similar.
Step 3: Estimate the load
Two numbers matter: running watts (steady draw while operating) and surge watts (the brief spike when a motor starts). For HVAC, well pumps, refrigerators, and freezers, the surge can be two to three times the running wattage. Your generator must cover the worst-case combined surge — not just the average load.
You can do this two ways:
- Bottom-up: Add up each appliance you want to power. Nameplates list watts (or amps × volts). Add a surge buffer for the largest motor.
- Top-down: Use the home's main breaker size as a ceiling. A 200A service is up to 48kW continuous, but you almost never use that much at once.
If that sounds tedious, it is. Run our free 60-second sizing calculator — it asks for the basics (square footage, HVAC, key appliances) and gives you a recommended range.
Rough rules of thumb (not a substitute for actual sizing)
- Essential circuits only: an air-cooled unit in the 10–14kW range is often enough.
- Average single-family home, one HVAC system: 18–24kW air-cooled is a common pick.
- Larger home, two HVAC systems, electric appliances: 26kW air-cooled or step up to liquid-cooled.
- Large or multi-zone homes, all-electric, or commercial: liquid-cooled 30kW and up.
These are starting points, not specs — your home, climate, and loads will move the answer.
Step 4: Match fuel to your situation
Most home standby units run on natural gas or propane (LP). Natural gas is the default if your house has utility gas — runtime is effectively unlimited. LP works from a buried tank when you don't have NG. For commercial or long-runtime applications with no gas service, diesel is the right tool. For mobile or jobsite use, look at portable generators instead.
Step 5: Don't forget the transfer switch and load management
A standby generator needs a transfer switch — either a full service-entrance ATS or a load center / load shed setup that prioritizes circuits. Load shedding lets a smaller (cheaper) generator handle a bigger home by automatically dropping low-priority loads when a big motor starts. Your installer should spec this with the generator.
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FAQ
- Is a bigger generator always better?
- No. An oversized generator costs more up front, burns more fuel, and can short-cycle on light loads. Right-sizing for your actual loads is what you want — use our sizing calculator or talk to us before buying.
- What's the difference between running watts and surge watts?
- Running watts is the steady power an appliance draws while operating. Surge (starting) watts is the brief spike when a motor or compressor starts — for ACs, well pumps, and refrigerators it can be 2–3× the running wattage. Your generator must cover both.
- Can a portable generator power my whole house?
- Usually not. Portables top out around 12–13kW and don't run automatically. They're great for essential circuits with a manual transfer switch. For automatic whole-home backup, you want a standby generator.
Not sure where to start?
Run our free 60-second sizing calculator or talk to a generator pro for a no-pressure quote.