Updated Jan 31, 2026
Whole House Generator Calculator
Size a whole house generator with a fast quiz or a detailed load list. See running watts, surge demand, and a recommended kW range with buffer.
Reviewed by SuperCalc Engineering
Sizing inputs
Extra load (optional)
Recommended size
Quick quizSend your request or a correction and we'll review it within 24 hours.
You can also use the Feedback button in the bottom-right corner.
How this calculator works
We estimate your running load, add surge allowance for motor starts, then apply a safety buffer to suggest a practical generator size range.
The quick quiz is fast for planning, while the load list is best for accurate sizing.
Whole house generator sizing tips
- High-surge loads (AC, well pumps) drive generator size more than small appliances.
- Electric heat and EV charging can push sizing into 20-30 kW ranges.
- Load management can reduce required kW by staggering startup loads.
Example result snapshot
FAQ
What size whole house generator do I need?
Most homes fall in the 12-24 kW range, but the right size depends on your HVAC, electric appliances, and whether you back up the whole home or just essentials.
How does this sizing calculator work?
It estimates running load, adds a surge allowance for motor starts, then applies a safety buffer to recommend a practical generator size.
Is a load list more accurate than a quick quiz?
Yes. A full load list is the most accurate way to size a generator. The quick quiz is a fast estimate to narrow the kW range.
Do I need a load management module?
If your surge demand is high or you want to keep generator size smaller, load management can stagger large appliances like AC or electric dryers.
About This Calculator
Calculate the right whole-house generator size for your home based on appliance wattage, starting surge requirements, and backup priorities. Compare natural gas, propane, and diesel options.
Frequently Asked Questions
What size whole house generator do I need?
Most homes land in the 12-24 kW range, but the right size depends on HVAC loads, electric appliances, and whether you back up the whole home.
Is a load list more accurate than a quick quiz?
Yes. A load list reflects your actual appliances and yields a tighter sizing estimate.
Do I need load management?
Load management can reduce required kW by staggering large motor starts like AC, dryers, or pumps.
How do I calculate what size whole house generator I need?
List critical appliances and their wattage: central AC (2,000-5,000W), electric water heater (4,000-5,500W), refrigerator (150-400W), furnace blower (500-1,000W), sump pump (750-1,500W), lighting/outlets (1,000-3,000W). Add these for running wattage. Then account for starting surge — electric motors draw 2-3× running watts at startup. A 3-ton AC that runs at 3,500W may surge to 9,000-10,500W. For a typical 2,000 sq ft home with central AC in 2025, a 20-22kW standby generator is commonly recommended. For essentials only (without AC), 7.5-12kW suffices. Add 20-25% headroom. Natural gas standby generators range from $3,000-$12,000 plus $3,000-$7,000 for professional installation including the transfer switch.
What is the operating cost of a whole house generator during an outage?
A natural gas generator at 50% load consumes roughly 100-150 cubic feet per hour, costing approximately $1.20-$2.25/hour at 2025 rates. Propane-fueled generators consume 2-3 gallons per hour at 50% load — at $2.50-$3.50/gallon, that's $5-$10.50/hour. For a 3-day outage on natural gas at moderate load, expect $85-$160 in fuel. Generators should run a 12-minute exercise cycle weekly, costing $5-$15/year in fuel. Annual professional servicing costs $150-$300. Total 10-year cost of ownership for a 22kW standby generator including installation is typically $20,000-$30,000, spread over actual outage events depending on your region's grid reliability.