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Proper standby sizing means working from the appliances you want covered and their starting surges, not from square footage. Here’s the method installers use, simplified so you can do a first pass yourself and check the quotes you receive.
Gather these before you start
- Photos of your main panel and its directory (breaker labels).
- Data plates for the AC or heat pump outdoor unit, the furnace or air handler, the well pump (if any) and the water heater.
- Your fuel situation: natural gas meter size, or propane tank size.
- A year of electric bills. Some utilities also show your peak demand.
The sizing method
- Decide the scope. Whole house, or essential circuits only?
- List the loads. Write the running watts for each. For 240 volt motors, multiply amps by 240. For 120 volt items, multiply by 120.
- Find the biggest starting surge. Usually the AC compressor. Its LRA is on the data plate. Note whether a soft starter is fitted or planned.
- Add the running loads you expect to run together. In summer, that’s AC, fridge, freezer, lights, electronics and maybe a well pump. In winter, furnace or heat pump instead of AC.
- Add the largest surge on top. The generator must handle this combined peak.
- Apply the fuel rating. Use the natural gas rating if you’re on NG. It’s lower than the LP figure.
- Add headroom. Many installers aim to keep the normal load comfortably below the generator’s rating.
- Consider load management. Modules that shed the AC, water heater or other big loads let a smaller generator cover more.
Running the method on a real house
A gas-heated home with a 3 ton AC (about 3,500 W running), a fridge and freezer (about 1,000 W), a furnace blower (about 700 W), lights and electronics (about 1,500 W), and a well pump (about 1,000 W) adds up to roughly 7,700 W running. Add a large compressor start surge without a soft starter and the peak can reach the low to mid teens in kilowatts. A 14 to 18 kW unit on natural gas, with load management for the well pump, is a reasonable starting point to discuss with installers. Your own numbers will differ.
Then confirm the fuel supply
A generator needs its full-load fuel flow at the right pressure. Generac’s Guardian spec sheets list 3.5 to 7 inches of water column for natural gas and 10 to 12 inches for LP at the generator inlet. Your gas utility may need to upgrade the meter. Propane tanks must be large enough to vaporize enough gas in cold weather, so your propane supplier will advise on that.
Checking an installer’s quote
- A size picked from square footage with no load list attached.
- No mention of the AC’s start-up surge or of soft starters.
- A natural gas home quoted against the LP rating.
- No gas pipe sizing shown.
And when you do your own first pass, avoid these shortcuts:
- Adding up every breaker’s rating. Breakers aren’t loads.
- Ignoring the surge from motors.
- Using the LP rating when you’ll run on natural gas.
- Forgetting a well pump or sump pump that has to run during storms.
Use your numbers to check quotes, but let a licensed installer do the final calculation. Ask how they sized it and whether a soft starter or load management was considered.
Keep your worksheet. When quotes arrive, compare each installer’s size and load list with yours. A big gap in either direction is worth a polite question before you sign.
Standby sizing questions
Can I measure my real load?
Some smart meters or whole-home energy monitors show peak demand. That’s a useful reality check.
How much headroom should I leave?
Many installers avoid planning for continuous loads above roughly 80 percent of the rating, but practices vary. Ask how they handled it.
Do I need to include my electric car charger?
Only if it will charge during outages. Many owners lock it out, or let load management shed it.
What if the installer’s size differs from mine?
Ask to see their load list and assumptions. Differences usually come from AC surge handling, load management or the fuel rating used.
See how big a standby to buy, the 2,400 sq ft example and the standby hub. Do a first pass in the wattage calculator.
