How to Size a Home Generator Without Overpaying

By BackupEngineer. Math review by a practicing power-electronics engineer (not a licensed PE) is pending. General education, not engineering or electrical advice. See the safety notes at the end.

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Most people buy a generator one of two ways. Either they guess, or a salesperson guesses for them. Guess too small and the lights dim and the breaker trips the moment the furnace kicks on. Guess too big and you've paid for capacity you'll never use, and you'll burn more fuel through every storm. The good news is that sizing comes down to three numbers and about ten minutes of arithmetic.

Step 1: Decide what actually has to run

Start with the loads that keep your family safe and your food cold, not with the whole house:

• Refrigerator and freezer
• Furnace blower (a gas furnace still needs electricity to run its fan, igniter and controls)
• Sump pump if you have a basement, or well pump if you're on a well
• A few lights, the Wi-Fi router, and phone chargers
• Medical devices like a CPAP

Then list the "nice to have" loads separately: microwave, TV, coffee maker, a window AC. You can usually run those one at a time instead of paying to run them all at once. That single decision is the biggest money saver in this whole guide.

Step 2: Write down running watts and starting watts

Every appliance has a running wattage, the steady power it draws once it's going. Anything with a motor or compressor also needs a short burst of extra starting watts for a second or two while the motor spins up. (The running vs. starting watts guide explains why.)

The most accurate number is the nameplate on your own appliance. If you can't find it, manufacturer charts give conservative planning figures. Here are a few from Generac's portable generator wattage chart (Generac RS-series brochure):

These chart numbers lean high on purpose. A modern refrigerator often runs well below 700 W. If you want to tighten your estimate, a plug-in watt meter or a clamp meter (used safely) tells you what your appliance draws.

Step 3: Do the math the right way

1. Add up the running watts of everything you want on at the same time.
2. Find the single biggest "additional starting watts" number on your list.
3. Add the two together. That's your peak surge.

Why only the biggest surge? Motors rarely start at the exact same instant, so the generator only has to cover one start at a time on top of what's already running. Both Generac and Champion use this method in their own sizing instructions (Generac brochure; Champion 201281 manual).

Worked example (essentials only): refrigerator 700 W + furnace blower 875 W + LED lights 100 W + router 25 W + chargers 75 W = 1,775 running watts. The biggest surge is the furnace blower's 2,300 W, so the peak is 1,775 + 2,300 = 4,075 watts.

Step 4: Add headroom and real-world derating

A generator's sticker rating assumes ideal conditions. Three things knock it down:

• Headroom. Running any engine or alternator flat out all night is hard on it. I add about 20% to the running load. That's a common design margin, not a code rule.
• Altitude. Engines lose about 3.5% of their output for every 1,000 feet above sea level (Champion manual). The Wichita area sits around 1,300 feet, so that's roughly a 4.5% loss.
• Fuel. Propane gives less power than gasoline. One popular dual-fuel model is rated 7,500 running watts on gasoline but 6,750 on propane, about 10% less (Champion 201281 spec). Standby units drop again on natural gas: Generac's 24 kW Guardian is rated 24,000 W on LP and 21,000 W on natural gas (Generac spec).

For the example above on gasoline near Wichita: 1,775 × 1.2 ÷ 0.955 ≈ 2,300 running watts, and 4,075 ÷ 0.955 ≈ 4,300 starting watts. That's a much smaller machine than many people assume they need for "essentials."

Step 5: Use the calculator, then sanity-check

Our free calculator does all of this and lets you edit every number. Treat its answer as a starting point. Then check three things before you buy:

1. Voltage. Well pumps, central AC and electric water heaters are often 240 V. You'll need a generator with a 120/240 V outlet and a proper transfer switch or interlock.
2. Fuel for the whole outage. Look up the run time at 50% load in the manual and plan your fuel storage safely.
3. Clean power. Inverter generators and most standby units produce a cleaner waveform, which matters for variable-speed furnaces and electronics. Check the manufacturer's total harmonic distortion spec.

Where people overpay

• Sizing for the whole house when they only need essentials. Load management (running the dryer or the AC, not both) can cut the size you need dramatically.
• Ignoring the one big motor. Sometimes a soft starter on a well pump or AC compressor costs far less than stepping up a generator size. Ask your electrician whether it fits your equipment.

• Buying features for a once-a-decade scenario. Look at how long outages actually last where you live (see our Kansas outage comparison) before paying for a week of whole-home backup.

• Generator comparison picks, with an affiliate disclosure: [AFFILIATE LINK] (paid link)

Safety: read this before you plug anything in

• Never backfeed. Plugging a generator into a wall outlet with a "suicide cord" can electrocute line workers and your neighbors, and it bypasses your home's protection (CPSC).
• Use a transfer switch or listed interlock installed by a licensed electrician, with a permit where required.
• Carbon monoxide kills fast. CPSC estimates about 85 people a year in the U.S. die from CO poisoning caused by portable generators. Run the generator outside only, at least 20 feet from the house with the exhaust pointed away, never in a garage, basement or porch, and install battery-backed CO alarms (CPSC).

Disclaimer: This guide is general education, not a design for your home. Wattages vary by model. Follow your equipment manuals, local code, and a licensed electrician's advice. Some links on this site may be affiliate links; we'll always disclose them, and they never change our math.