Most people shop for a generator the same way they shop for a microwave: they look at one number on the box and assume bigger is safer. Then the power goes out, the fridge and a window AC unit turn on at the same moment, and the unit that was supposed to be "more than enough" trips and shuts down. The wattage label wasn't wrong. It just wasn't answering the question the shopper thought it was answering.
Sizing a generator correctly means separating two different kinds of load, running watts and starting watts, and adding up the specific appliances you actually plan to power at once. Skip that step and the sticker number on the box is close to meaningless.
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Running Watts and Starting Watts Are Not the Same Number
Every generator has two ratings, and the gap between them is where most sizing mistakes happen. Running watts, sometimes called rated watts, is the continuous power the unit can supply for as long as it has fuel. Starting watts, sometimes called surge or peak watts, is a much higher number the generator can supply for only a second or two.
That second number exists because motors, the kind found in refrigerators, well pumps, window air conditioners, and sump pumps, draw a short spike of current when they first switch on, often two to three times their running draw. A refrigerator that runs at 150 watts might briefly demand 600 watts or more the instant its compressor kicks on. A generator sized only for the running total of everything in the house will handle steady operation fine right up until two motors happen to start within the same second, at which point it overloads and shuts off.
Add Up What You Actually Need to Run
Start with a real list, not a guess. Walk through the circuits you'd want powered during an outage and note the running wattage for each, usually printed on an appliance's nameplate or in its manual.
A typical short list for a partial-home setup includes a refrigerator, a sump pump, a handful of lights, a router and modem, and phone charging. A window air conditioner or well pump changes the math considerably, since both are motor-driven loads with a meaningful starting spike layered on top of their running draw.
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Add the running watts of everything you expect to use simultaneously. Then, separately, identify the single largest starting-watt spike among your motor-driven appliances, since in practice only one large motor tends to start at the exact same instant as another in a well-managed setup. Add that one largest starting spike on top of your running total, and that combined number is closer to the real capacity you need than either the running total or the running-plus-all-starting-watts sum would be.
Why Amperage and Voltage Both Matter
Wattage is what most shoppers look at, but a generator's output is really a relationship between voltage and amperage, and mismatches show up in unexpected places. Most portable generators supply both 120V and 240V circuits, and a well pump or electric dryer wired for 240V draws differently than a lamp wired for 120V even at a comparable wattage.
Amperage matters just as much for the extension cords and inlet wiring you use to connect the generator to your home. A cord rated for less amperage than your load draws will heat up, which is both a fire risk and a source of voltage drop that can damage sensitive electronics like a refrigerator's compressor control board.
Amps = Watts / Volts is the formula worth keeping in your head. A 1,500 watt load on a 120V circuit draws 12.5 amps, which matters when you're deciding whether a 15 amp or 20 amp rated cord and outlet are appropriate for the connection.
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Three-Phase and Whole-House Considerations
Most residential backup generators are single-phase, and that's the right choice for a typical house. Three-phase power shows up almost exclusively in commercial buildings and some larger equipment, so unless a well pump or HVAC compressor specifically calls for three-phase, single-phase sizing math is what applies.
Whole-house standby units, the kind permanently installed outside and wired through a transfer switch, are sized differently than portable units because they're expected to carry the entire home's running load, central air conditioning included, rather than a curated subset of circuits. That's a bigger, more expensive calculation, and it usually involves an electrician confirming the home's total service amperage before a unit gets selected.
A Simple Worked Example
Say a household wants to run a refrigerator (150 running watts, 600 starting watts), a sump pump (800 running watts, 1,300 starting watts), five LED lights (50 watts total), a router and modem (30 watts), and phone charging (60 watts).
Running watts add up to 1,090. The largest single starting spike, the sump pump's 1,300 watts, gets added on top of the running total for everything else running at the moment it kicks on, which brings the peak requirement to roughly 2,240 watts. A generator rated for at least 2,500 to 3,000 running watts gives enough headroom for that peak moment without running the unit flat out under normal conditions, which also tends to extend its usable life and reduce fuel consumption per hour.
Using a Calculator Instead of Doing It by Hand
Running this math manually for every appliance combination gets tedious fast, especially once amperage and voltage drop enter the picture alongside starting and running watts. The Electrical Power Calculator handles the running-versus-starting math, converts between watts, amps, and volts for both single and three-phase setups, and includes a dedicated motor-starting mode so you're not manually padding your totals with a rough guess.
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Fuel Type Changes the Sizing Conversation Too
Gasoline generators are the cheapest option upfront and widely available, but stored gasoline degrades within a few months unless treated with stabilizer, which matters if the unit sits unused between outages. Propane burns cleaner and stores indefinitely in a sealed tank, but propane generators typically produce slightly less output than the same engine running on gasoline, so a unit rated at 3,000 watts on gasoline might only deliver around 2,700 watts on propane. That gap is worth building into your safety margin rather than discovering it mid-outage.
Dual-fuel units split the difference, letting you switch between gasoline and propane depending on what's available, which is useful in a prolonged outage where gas station lines are long but a spare propane tank is sitting in the garage. Whichever fuel you choose, the running-watt and starting-watt math from earlier doesn't change. What changes is how much margin you should add on top of your calculated peak to account for fuel-specific output loss.
Extension Cords vs a Transfer Switch
How you actually connect a generator to your home's circuits matters as much as sizing the unit correctly. Running individual extension cords from the generator to specific appliances works for a handful of essentials, but each cord needs to be rated for the amperage of what's plugged into it, and heavy-gauge outdoor-rated cords are not optional once you're pulling more than a few amps over any real distance.
A transfer switch, installed by an electrician, lets the generator feed a subset of your home's existing circuits directly through the breaker panel, which is safer and more convenient for anything beyond a couple of appliances. It also eliminates the risk of backfeeding power into utility lines, a genuine hazard to utility workers that extension-cord setups avoid simply by not being wired into the panel at all. If your peak-watt calculation is pushing toward whole-circuit coverage rather than a handful of individual devices, a transfer switch is worth the added cost.
Common Mistakes That Undersize a Generator
Buying based on running watts alone, without checking the starting spike of the biggest motor in the house, is the single most common error. A close second is ignoring simultaneous use, assuming appliances cycle on and off politely instead of overlapping the way refrigerators and sump pumps actually do during a storm, when both tend to run more than usual.
Skipping the extension cord and inlet amperage check is a quieter mistake that shows up later as flickering lights or a tripped breaker rather than an obvious failure at purchase time. And treating a generator's continuous rating as its peak rating, rather than the other way around, leads directly back to the sticker-number trap this whole calculation exists to avoid.
What to Do Before the Next Storm
Make the appliance list now, while the power is on and nameplates are easy to read, rather than during an active outage when you're working by flashlight. Confirm which circuits actually matter to you, since a generator sized for comfort during a two-day outage looks different than one sized for bare survival during a two-hour blip.
If you're shopping for a new unit, buy based on your calculated peak number plus a reasonable safety margin, not the biggest number on the shelf tag. If you already own a generator, run the same math against your actual appliance list so you know its real limits before you're depending on it in the dark.
For background on service ratings and safe wiring practices, the NFPA publishes the National Electrical Code that generator inlets and transfer switches are built around, and Ready.gov has practical guidance on generator safety and carbon monoxide risk during outages. OSHA covers portable generator hazards from a workplace safety angle that applies just as well at home, and the U.S. Department of Energy has consumer guidance on backup power options and efficiency.
For more calculators like this one, browse the EvvyTools tools directory, or check the EvvyTools blog for more guides built around numbers you can actually calculate for your own situation rather than generic advice. EvvyTools builds these tools specifically so the math gets done once, correctly, instead of guessed at every time your setup changes.
