How to Choose the Right Size Generator for Your Home

August 14, 2026 · Family-owned electricians in North Idaho

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This is for anyone shopping for a generator before winter storms or wildfire-season outages hit North Idaho. You don't need an engineering degree, just a list of what you want to run and about 20 minutes with a calculator. Get the math wrong and you'll either overpay for capacity you don't need or buy a unit that trips out the moment your well pump kicks on.

What you need

• A notepad or spreadsheet
• The nameplate or owner's manual for each appliance you want to power (look for watts or amps and volts)
• A basic calculator, or phone calculator app
• Rough knowledge of your home's key loads: well pump, furnace blower, refrigerator, sump pump, range, and any shop or cabin equipment
• A generator sizing chart from a manufacturer (Honda, Generac, and Champion all publish free ones online) if you don't have nameplate data for something

Step by step

1. List everything you actually need during an outage. Not everything you own, just what matters. For most North Idaho homes that's the furnace or boiler circulator, well pump, refrigerator and freezer, some lighting, a few outlets for phones and a modem, and maybe a sump pump if you're in a low spot. Cabin owners often add a propane wall heater or a mini-split. Businesses might need a walk-in cooler or point-of-sale system.
2. Find the running watts and starting watts for each item. Running watts is what it draws once it's on. Starting watts (also called surge watts) is the short spike it needs to start, and for motors this can be two to three times the running number. A well pump might run at 1,000 watts but need 2,000 to 3,000 watts to start. A furnace blower motor is often 800 to 1,200 running watts with a similar surge multiplier. Refrigerators typically run at 100 to 250 watts with a starting surge of 800 to 1,200 watts. If you can't find the number on the nameplate, generator manufacturer sizing charts give reasonable ranges for common appliances.
3. Add up all the running watts, then add the single largest starting watt figure on top. This is the key trick. You don't add up every appliance's starting watts, because they don't all start at the same instant. You add the running totals for everything, then add just the biggest surge number from whichever single appliance has the highest starting draw, usually the well pump or the AC compressor. That combined number is your minimum required generator capacity.
4. Add a buffer and decide portable or standby. Add 15 to 20 percent to your total for headroom and for accessories you might forget, like a coffee maker or a shop light. Portable generators in the 5,000 to 10,000 watt range cover most single-family homes running essential circuits. Whole-house standby units, sized from 14 kW up to 22 kW or more, run everything at once and switch on automatically, but they cost more and need a transfer switch and, usually, a propane or natural gas line sized to match.

Where this goes wrong

The most common mistake is sizing off running watts alone and ignoring the starting surge. A generator that looks fine on paper for running loads will stall or trip its breaker the moment a well pump or air conditioner compressor tries to start, because the surge briefly overloads it. This is especially common with older well pumps and single-phase shop equipment like table saws and air compressors, which can have surprisingly high starting draws for their size.

The second common mistake is guessing instead of checking nameplates. People assume a refrigerator draws almost nothing and skip it in the math, then wonder why the generator bogs down. Every motor-driven appliance deserves an actual number, not a guess.

Oversizing is a real cost too, just a quieter one. A 22 kW standby unit sitting there powering a house that only needed 10 kW burns more propane per hour at partial load efficiency losses, costs more upfront, and needs a bigger gas line and pad. Right-sizing saves money on both ends of the ledger.

Another failure mode specific to this region: people size for summer loads and forget winter. A furnace blower motor and an electric baseboard backup, or a well pump working harder against frozen ground pressure, can push winter demand well above what the same house needs in July. If your outage risk is mostly winter storms, size against your winter loads, not your air conditioner.

Finally, people forget load management. Cheaper portable generators don't manage which circuits get power when. If your well pump, furnace, and refrigerator all happen to call for power in the same few seconds, even a generator that's technically sized right on paper can trip. Higher-end setups use load management modules or simply require you to stagger use manually, which is worth knowing before you're standing in the dark trying to remember which breaker is which.

When to stop and call someone

Sizing the generator yourself is fine. Connecting it to your home's electrical system is not a DIY step, and this isn't a liability disclaimer, it's a practical one. Backfeeding a generator into your panel without a proper transfer switch or interlock kit can energize the utility lines outside your house, which is a real hazard to any lineworker restoring power in your area during a storm. It also voids most homeowner's insurance if a fire or injury results.

Call a licensed electrician for the transfer switch installation, for any whole-house standby system, and for load calculations on larger or mixed-use properties like a home with a detached shop or a rental unit on the same meter. A standby generator installation also usually needs a permit and inspection in Bonner and Kootenai counties, and the gas or propane line sizing needs to match the generator's fuel demand at full load, which is its own calculation.

If you're comparing quotes or just want a second set of eyes on your wattage math before you buy, Sageland Electric has been doing this kind of sizing and installation work across North Idaho for over 21 years and can tell you honestly whether a portable setup will cover your needs or whether a standby system makes more sense for your situation.