Answer a few questions about your home and the equipment you want to keep running during an outage. You'll get an estimated generator size in kW, the transfer switch you need and how much natural gas or propane it will burn.
How standby generator size is determined
An automatic standby generator is permanently installed outside your home, connected to natural gas or a propane tank and wired to an automatic transfer switch (ATS). When the grid fails it starts within seconds and powers the circuits you chose. Sizing it correctly matters: an undersized unit trips its breaker when the air conditioner starts; an oversized one costs thousands more and burns extra fuel for decades.
Running load
Running load is the total power of everything that operates at the same time. The calculator adds a base load for lighting, outlets and electronics (scaled by your living area), then each major appliance you selected. Heating loads such as an electric range or dryer are counted at realistic operating levels rather than nameplate maximums.
Motor starting surge
Central air conditioners, heat pumps and well pumps draw a locked-rotor current of three to six times their running current for a split second. A 4-ton air conditioner that runs at about 4.6 kW can need over 13 kW to start. Because generators handle a single starting surge at a time, we add only the largest motor's extra starting demand to your running load. Installing a soft-start kit on the compressor is the cheapest way to fit a larger air conditioner on a smaller generator.
The 80 % rule
Generators should not run continuously above about 80 % of their rating. The minimum size shown is therefore the larger of your running load ÷ 0.8 and your peak demand. Natural gas also contains less energy than propane, so most air-cooled generators produce about 8 – 12 % less power on natural gas – the calculator accounts for this when choosing a model size.
Typical generator size by home
Home
Typical coverage
Generator size
Small home, gas heat, no central AC
Essentials, 100 – 150 A
10 – 14 kW
2,000 sq ft, 3-ton AC, gas appliances
Whole house, 200 A
18 – 22 kW
3,000 sq ft, 4 – 5 ton AC, well pump
Whole house, 200 A
22 – 26 kW
All-electric home, heat pump + strips
Whole house, 200 – 400 A
26 – 48 kW (liquid-cooled)
Large home, two AC units, pool
Whole house, 400 A
32 – 48 kW or load management
Transfer switches explained
A whole-house (service-entrance rated) ATS sits between the utility meter and your main panel and is sized to your service – 100, 150, 200 or 400 A. It lets the generator feed every circuit, usually combined with load-shedding modules that temporarily switch off large appliances to protect the generator. An essential-circuit ATS (typically 50 or 100 A) feeds a small sub-panel with 8 – 16 chosen circuits such as the refrigerator, furnace, sump pump, lights and internet. It allows a smaller, less expensive generator.
Natural gas or propane?
Natural gas is delivered continuously by the utility, so runtime is unlimited – but your gas meter and supply line must be large enough for the generator (often 250 – 400 cubic feet per hour for a 22 kW unit). An undersized meter is one of the most common installation problems. Propane is stored on site, works anywhere and produces slightly more power, but runtime is limited by tank size and refills can be slow during regional emergencies. Underground or above-ground tanks of 250 – 500 gallons are typical for residential standby use.
Safety and code: standby generators must be installed by a licensed electrician and, where required, a licensed gas fitter, with permits and inspection. Manufacturers and NFPA 37 specify clearances from windows, doors, vents and combustible walls (often 5 ft from openings). Plan a yearly maintenance visit and let the unit run its weekly self-test.
Questions to ask installers
Did you perform a load calculation or measure peak demand on my service?
Is my gas meter and line sized for the generator at full load?
Is load management included, and which appliances will be shed?
What is the warranty (5 – 10 years) and who performs service?
What is the sound level at 23 ft, and does placement satisfy local setbacks and HOA rules?
Is a concrete pad, battery heater or cold-weather kit needed in my climate?
Frequently asked questions
What size generator do I need to run a whole house?
Most 1,500 – 3,000 sq ft homes with gas heat and a 3 – 4 ton air conditioner need an 18 – 24 kW standby generator. All-electric homes with heat pumps or electric furnaces often need 26 – 48 kW or load management.
Will a 22 kW generator run my central air?
Usually yes, for units up to about 5 tons, provided other large loads are not starting at the same time. A soft-start kit makes it much more reliable.
How much propane does a 22 kW generator use?
Roughly 2 – 2.5 gallons per hour at half load and 3.3 – 3.6 gallons per hour at full load. A 500 gallon tank filled to 80 % lasts about 6 – 8 days at average loads.
Can I install a standby generator myself?
No – the gas and electrical connections and the transfer switch require licensed professionals and permits in nearly all jurisdictions. Incorrect installation can back-feed the grid and endanger utility workers.
Air-cooled or liquid-cooled?
Air-cooled units cover roughly 8 – 26 kW and are the most common for homes. Liquid-cooled generators (typically 25 kW and above) run slower and quieter, last longer under heavy loads and suit large or all-electric homes.
About StandbyScale
StandbyScale is an independent, free calculator that estimates the kW size, transfer switch rating and fuel consumption of an automatic home standby generator. It is designed to help homeowners have a better-informed conversation with a licensed installer. We are not affiliated with any generator manufacturer or dealer.
Every formula on this page runs entirely in your web browser. The numbers you type are never uploaded, stored or shared. Our goal is simple: give you an honest starting point so you can compare product specifications with confidence and avoid paying for capacity you do not need – or buying something too small to do the job.
The methodology combines widely published engineering rules of thumb, manufacturer data sheets and the electrical code practices that installers use every day. We review the assumptions regularly and update the calculator when typical product specifications change.
Contact
Found a bug, have a suggestion or want to report an outdated assumption? We read every message. Write to us at contact@electricity-and-accessories.shop and we will usually reply within two business days. Please do not send personal, financial or sensitive information by email.
Privacy Policy
Last updated: October 28, 2026
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Terms of Use & Disclaimer
The calculator and articles on this website are provided free of charge for general information and educational purposes only. Results are estimates based on typical values and simplifying assumptions; real-world performance depends on the specific product, installation, climate, wiring and how you use your equipment.
Nothing on this site is professional engineering, electrical or financial advice. Always follow the manufacturer's instructions and local electrical codes, and have permanent installations, transfer switches and grid-connected systems designed and installed by a licensed electrician or qualified installer. Never operate fuel-burning generators indoors or in enclosed spaces – carbon monoxide can kill within minutes.
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