Panel Size → Typical Generator Size
| Service Size | Typical Home | Standby Generator | Portable (Essentials) |
|---|---|---|---|
| 100 Amp | Small home, no central AC | 10–14 kW | 5,000–7,500 W |
| 150 Amp | Medium home with central AC | 16–20 kW | 7,500–10,000 W |
| 200 Amp | Large home, AC + electric appliances | 22–26 kW | 10,000–12,000 W |
| 400 Amp | Very large / commercial | 38–48 kW | N/A — use standby |
Transfer Switch Requirements
Always Required
NEC Article 702 requires a transfer switch for any generator connected to your home's wiring — it prevents dangerous backfeeding onto utility lines that can injure line workers.
Manual vs. Automatic
Manual transfer switches ($300–$800 installed) work with portable generators. Automatic transfer switches ($500–$2,000+) are standard with standby systems and switch over within seconds of an outage.
Never Use a "Suicide Cord"
Plugging a generator directly into a wall outlet with a double-male cord is illegal and can be lethal — it backfeeds power onto the grid. Always use a proper transfer switch.
Full Cost Breakdown
See exactly what a professional standby installation costs — pad, transfer switch, gas line, permits — with our Installation Cost Estimator.
How This Estimate Is Calculated
This calculator uses your electrical service size (the amperage of your main panel) as a proxy for overall home electrical load, since panel size is one of the few data points most homeowners know off the top of their head without pulling a permit history or an electrician's report. A 100-amp service is typically found in smaller or older homes with modest electrical demand; a 200-amp service is standard in most newer construction and larger homes; 400-amp services generally belong to very large homes or properties with heavy supplemental loads like a workshop or multiple outbuildings.
From that panel-size baseline, the calculator adds a load for central air conditioning (since AC is one of the largest single loads in most homes and isn't reflected in panel size alone) and applies an altitude derating factor if you're above 1,000 feet of elevation. If you select "essential circuits only" instead of whole house, the base estimate is reduced by roughly half, reflecting that you're backing up a subset of circuits rather than every circuit in the panel.
Whole-Home vs. Essential Circuits: Which Should You Choose?
This is one of the first real decisions in generator sizing, and it changes both the recommended size and the total cost significantly. Essential-circuit backup typically covers the refrigerator and freezer, a sump or well pump, furnace or boiler controls, some lighting circuits, and often a single AC zone or window unit — enough to keep a home safe and livable without full comfort. Whole-home backup covers every circuit in the panel, including full central air, electric kitchen appliances, and any EV charging equipment, and generally requires a generator roughly double the size (and cost) of an essential-circuits system.
A load-management module — offered by some standby generator manufacturers — can narrow this gap by temporarily shedding lower-priority circuits (like a second AC zone) whenever a higher-priority load like a well pump needs to start, letting a smaller generator safely cover more of the home than its raw wattage rating would suggest. If you're weighing this trade-off in dollar terms rather than wattage terms, the Installation Cost Estimator and Standby vs. Portable Calculator both let you compare scenarios side by side.
A Worked Example
A home with a 200-amp panel, a 3-ton central AC system, at 2,500 feet of elevation, wants whole-house coverage:
Base load estimate (200A, whole house): 24 kW
Central AC addition (3 ton × 1.2): +3.6 kW
Subtotal: 27.6 kW
Altitude derating (2,000 ft rounds to 2 × 3% = 6%): 27.6 × 1.06 ≈ 29 kW recommended standby size
Frequently Asked Questions
It's a reasonable starting proxy because larger services generally correlate with larger homes and heavier electrical loads, but it isn't a substitute for actually listing your loads. Two 200-amp homes can have very different generator needs depending on whether one has electric heat, a well pump, and an EV charger and the other doesn't. Use the Wattage Sizing Calculator for a more precise number once you have a ballpark from this tool.
That depends on budget and what you can't do without. Essential-circuit systems (refrigerator, some lighting, a sump or well pump, and often one AC zone) can be roughly half the size, and cost, of a true whole-house system that covers every circuit including full central air and an electric range.
Generators are combustion engines, and thinner air at higher elevation reduces how much fuel-air mixture the engine can burn per cycle, which reduces rated output. The generally accepted rule of thumb is roughly a 3% output loss per 1,000 feet of elevation above sea level, which is why this calculator applies a derating factor once you're above 1,000 ft.
Likely yes. Electric water heaters and electric ranges are large, mostly continuous loads that this quick panel-based estimate does not itemize. If either applies to your home, run the Wattage Sizing Calculator with those specific appliances checked for a more accurate number.
A dealer's tool is usually designed to route you toward a specific product line and a quote request, and it typically doesn't show its underlying math. This calculator shows the base load estimate, the AC addition, and any altitude derating as separate line items so you can see exactly how the final number was built.