What Size Generator Do I Need for My Whole House

If you've started shopping for a whole-house standby generator, you've probably run into the same wall every homeowner hits: manufacturers list dozens of kW sizes, and nobody tells you which one actually fits your house. Buy too small, and your generator trips offline the moment the AC and water heater kick on together. Buy too big, and you overpay by thousands for capacity you'll never use.

The good news: sizing a generator isn't guesswork. It's basic math you can do in about ten minutes. Here's exactly how to do it.

The Quick Answer

Most homes need a generator sized between 14 kW and 26 kW, depending on square footage and how much you want to run at once:

• 1,500–2,500 sq ft homes: 14kW–17kW typically covers HVAC, well pump, fridge, and lighting circuits.
• 2,500–4,000 sq ft homes: 20kW–26kW covers whole-home coverage including larger HVAC systems and multiple large appliances.
• 4,000+ sq ft homes or homes with well pumps, pools, and multiple AC units: 30kW and above is usually required for true whole-house coverage.

That's the shortcut. If you want an installer-accurate number (or you're comparing quotes and want to know if a contractor is oversizing or undersizing your system), walk through the three steps below.

Step 1: Add Up Your Running Watts

"Running watts" is the continuous power an appliance draws once it's already on. Walk through your home and total the running watts for everything you want powered during an outage.

Add these together for a realistic "everything running normally" total. For a typical 2,500 sq ft home with central AC, a well pump, and standard appliances, this usually lands between 10,000W and 16,000W.

Step 2: Add Your Highest Single Starting Watts

Motors — AC compressors, well pumps, sump pumps — draw a short surge of extra power the instant they switch on, called "starting watts" or "locked rotor amps." This surge can be 2–3x the running wattage for a split second.

You don't add up every appliance's starting watts — only the single largest one, since it's unlikely every motor in your house starts at the exact same millisecond. A 5-ton AC unit, for example, might have a running load of 6,000W but a starting surge of 15,000W+.

Formula: Total Generator Size Needed = Sum of Running Watts + (Largest Single Starting Watts − its own Running Watts)

This accounts for the surge without double-counting the appliance's baseline draw.

Step 3: Convert to kW and Add a Buffer

Divide your total wattage by 1,000 to get kW, then add a 20% buffer. This buffer isn't optional — it protects the generator from running at full redline during heat waves (when AC draws more) and gives you room to add appliances later without re-sizing your whole system.

Example: If your running + surge total is 18,500W, that's 18.5kW. Add 20% buffer → ~22kW, which rounds up to a 26kW standby generator — the next available Kohler size and the safer choice.

Real-World Example Calculation

Let's size a generator for a 2,800 sq ft home with central AC, a well pump, and standard appliances:

• Central AC (4-ton): 4,800W running
• Refrigerator: 700W
• Well pump: 1,500W
• Sump pump: 1,000W
• Water heater: 4,000W
• Lighting: 1,500W
• Misc. outlets/electronics: 1,500W
• Running watts total: ~15,000W

Largest starting surge (AC compressor, ~12,000W starting minus its 4,800W running) adds ~7,200W on top.

Total: 15,000W + 7,200W = 22,200W → 22.2kW → with 20% buffer → ~26.6kW

This home needs a 30kW+ standby generator to run everything simultaneously without load-shedding, or a 26kW unit paired with a load management module if the homeowner is comfortable staggering high-draw appliances.

What Size Generator by Home Size (Quick Reference)

These are starting points, not final answers — a home with a well pump and These are starting points, not final answers — a home with a well pump and electric everything will always need more capacity than a similarly-sized home on natural gas with municipal water.

Don't Forget Fuel Type

Sizing is only half the decision — fuel availability matters just as much. Homes on a natural gas line typically run standby units directly off that supply for unlimited runtime. Homes without gas service usually run on propane, sized to a 250- or 500-gallon tank. If you want flexibility to run on gasoline, propane, or natural gas depending on what's available during a prolonged outage, a tri-fuel generator gives you that redundancy — though most true whole-house standby units are gas/propane only, with tri-fuel more common in portable and backup setups.

When to Call a Licensed Electrician

Sizing math gets you close, but a licensed electrician or generator installer should always confirm your final number with a proper load calculation per NEC Article 220, especially if you're installing an automatic transfer switch. This is required for code-compliant installations in most U.S. states and ensures your system won't overload during peak demand — no online calculator, including this one, replaces a site-specific electrical load assessment.

Generator Vault

Phone: 9042068588
Email: sales@generatorvault.com
Address: 6501 Arlington Expressway, Jacksonville, Florida 32211
Website: https://generatorvault.com/

FAQs

Q: Can a 22kW generator run a whole house?

For most homes under 3,000 sq ft with standard appliances, yes — a 22kW generator comfortably covers central AC, a refrigerator, lighting, and typical household loads. Larger homes with multiple AC units or electric water heaters may need to stagger usage or size up.

Q: Is it better to oversize or undersize a generator?

Oversize within reason. A generator running below 30% of its rated load for long periods can experience "wet stacking" (incomplete fuel combustion), while an undersized unit trips breakers and shortens its lifespan under constant overload. A 20% buffer above your calculated need is the sweet spot.

Q: Do I need a whole-house generator or will a portable one work?

Portable generators (typically 3,000W–10,000W) can cover essentials — fridge, some lights, a window AC unit — but can't run central HVAC or your whole electrical panel. If you want your house to function normally during an outage, a standby generator sized using the steps above is the right category.

Q: How much does a whole-house generator cost installed?

Equipment costs generally range from $4,899 for a 14kW unit up to $20,000+ for 30kW and above, before installation, which typically adds $3,000–$6,000 depending on electrical work, concrete pad, and permitting in your area.

Q: What size generator do I need for a 2,000 sq ft house?

Most 2,000 sq ft homes with central AC and standard appliances fall in the 14kW–18kW range. Homes with a well pump or electric water heater should size toward the higher end of that range.

Conclusion

Sizing a whole-house generator comes down to three numbers: your running watts, your largest starting surge, and a 20% safety buffer. Run through the appliance chart above, do the math, and you'll land within one size tier of what a professional installer would recommend — giving you a real number to compare quotes against instead of guessing based on price alone.

Ready to shop by size? Browse 14kW, 26kW, and 30kW+ Kohler standby generators, or see the full standby generator lineup to compare every size side by side.