How to Size a Whole-Home Standby Generator: The Complete 2026 Guide

Why Sizing Is the Most Important Decision You'll Make

Undersizing a standby generator means it will struggle under load, run hot, and potentially fail during the outage you bought it to survive. Oversizing means you've spent $2,000–$5,000 more than necessary for capacity you'll never use. Getting the size right is the single most important step in the buying process — and it's more straightforward than most homeowners expect.

The Two Approaches to Generator Sizing

There are two ways to size a whole-home generator: essential circuit coverage and whole-home coverage.

Essential circuit coverage means powering only the circuits you can't live without: HVAC, refrigerator, lights, well pump, and a few outlets. This approach typically requires 10–16 kW and costs less upfront. The trade-off is that you'll need to manually manage what's running during an outage.

Whole-home coverage means the generator can handle your home's full electrical load simultaneously — everything runs as if the grid is on. This requires 20–38 kW for most homes and costs more, but eliminates load management entirely.

Most homeowners shopping for a standby generator want whole-home coverage. The rest of this guide focuses on that approach.

Generator Size by Home Square Footage

Home Size	Recommended Size	Kohler Model	Generac Model
Under 1,200 sq ft	10–14 kW	Kohler 14RESAL	Generac Guardian 14kW
1,200–2,000 sq ft	14–18 kW	Kohler 14RESAL / 20RESCL	Generac Guardian 18kW
2,000–3,000 sq ft	20–22 kW	Kohler 20RESCL	Generac Guardian 22kW
2,500–3,500 sq ft	22–26 kW	Kohler 24RCL	Generac Guardian 24kW
3,500–5,000 sq ft	36–38 kW	Kohler 38RCLB	Generac Protector 36kW
5,000+ sq ft	48–60 kW	Kohler 48RCL	Generac Protector 48kW

Note: Square footage is a starting point, not a definitive answer. HVAC system size, electric vehicle charging, and hot tubs can significantly increase requirements.

The Appliance Load Method: More Accurate Sizing

For a more precise calculation, add up the wattage of the appliances you want to run simultaneously. Use these typical values:

Appliance	Running Watts	Starting Watts
Central AC (3-ton)	3,500	7,000
Central AC (4-ton)	5,000	9,800
Refrigerator	700	2,200
Well Pump (1 HP)	1,000	3,000
Electric Range	3,500	3,500
Clothes Dryer (gas)	700	700
Lighting (whole home)	1,200	1,200
Sump Pump	800	2,000
EV Charger (Level 2)	7,200	7,200

Add up your running watts for the appliances you want to run simultaneously, then add the highest single starting watt value (motors draw 2–3× running watts on startup). The result is your minimum generator requirement. Add 20% headroom for safety.

Key Factors That Increase Your Size Requirement

Electric vehicle charging: A Level 2 EV charger draws 7,200 watts continuously. If you want to charge your EV during an outage, add this to your calculation — it often bumps a 20 kW requirement to 26+ kW.

Large central AC systems: A 5-ton central AC system draws 6,000+ running watts and can spike to 12,000 watts on startup. Homes with large HVAC systems often need 26–38 kW generators.

Electric water heater: A standard 4,500-watt electric water heater adds significant load. Gas water heaters require only a small ignition draw.

Hot tubs and pool pumps: These are high-draw appliances that are often overlooked. A hot tub heater can draw 5,500 watts; a pool pump 1,500–3,000 watts.

Air-Cooled vs Liquid-Cooled: Does It Matter for Sizing?

All residential Kohler and Generac models up to approximately 22 kW are air-cooled. Units above 22 kW are typically liquid-cooled. Liquid-cooled generators are quieter, more durable under continuous load, and better suited for large homes. If your calculation puts you above 22 kW, you're likely looking at a liquid-cooled unit — which also means a higher price point.

Get a Professional Load Calculation

The most accurate sizing method is a professional electrical load calculation performed by a licensed electrician or generator installer. This involves reviewing your electrical panel, identifying all circuits, and calculating actual load based on your specific appliances and usage patterns.

Most certified Kohler and Generac installers will perform this calculation as part of the quoting process at no charge. Use our free quote tool to connect with local installers, or review our cost guide for pricing by kW size.