Buying a generator on running watts alone is the most common way homeowners get burned. The fridge that pulls 700 watts pulls 1,800 watts for the half-second the compressor starts; the well pump that runs at 1,500 watts surges to 4,500. Add up nameplates without surge headroom and the generator buys you a stalled unit instead of cold beer.
Running watts vs surge watts
Every generator spec sheet lists two numbers:
- Running (rated) watts — what it can sustain continuously. Picture this as cruising horsepower.
- Surge (peak / starting) watts — what it can deliver for ~1-3 seconds before the engine stalls. Typically 10-20% higher than running.
Inductive loads (anything with a motor) are why surge watts matter:
- Refrigerator/freezer: 2-3× running
- Furnace blower (PSC motor): 2× running
- Window AC: 2-3× running
- Central AC compressor: 2.5× running (LRA on the nameplate)
- Well pump and sump pump: 3-4× running — the worst offender
Sizing math
The right way to size a generator:
- Add up running watts for everything you want to back up
- Find the largest single motor and compute its surge above its running
- Required surge = running total + that one biggest motor's startup spike
- Add 20% headroom on running so the engine isn't pegged
Why only the largest motor's surge? Because motors don't all start at the exact same instant. The fridge cycles on, the well kicks in two minutes later. The probability of two motors hitting peak surge simultaneously is low enough that generator manufacturers and electricians don't size for it. The generator size calculator handles this automatically.
Common essential-load builds
Bare-minimum essentials (3-4 kW running):fridge, furnace blower, lights + outlets, microwave on demand. A 4,000-5,000 W portable handles this.
Comfort essentials (5-7 kW running): add a sump pump, freezer, window AC for one bedroom, all the electronics. 7,500 W portable territory.
Well + essentials (6-8 kW running, 9-11 kW surge):add a ½ HP well pump, which is the surge driver. 9,000+ W portable or 10 kW standby.
Whole home (15-22 kW running, 30+ kW surge):central AC + electric range + dryer + everything else. 18-22 kW air-cooled standby (Generac Guardian, Kohler 20RESCL) is the sweet spot here.
Portable vs standby
Portable generators top out at about 12 kW and need a manual transfer switch or interlock kit. Cheap, flexible, gas-powered (or dual-fuel propane). Have to be rolled out, started, and refueled every 6-10 hours.
Standby generators are permanent installs: pad-mounted, hardwired through an automatic transfer switch (ATS), fueled from natural gas or a propane tank. They start themselves within seconds of an outage and run until utility power returns. 14 kW air-cooled units start around $4,500 installed; 22 kW around $7,500; liquid-cooled 30 kW+ units go to $15,000+.
NEC 702 — the legal bits
Anything wired into the home panel needs:
- A transfer switch (manual or automatic) or a UL-listed interlock kit on the panel
- Properly sized generator-input wiringfrom the inlet box to the transfer means
- A generator inlet box on the exterior for portables (no extension-cord-through-window)
Backfeeding through a dryer outlet without an interlock is the worst common DIY mistake — the generator energizes the utility lines, and a lineman working on the de-energized service gets electrocuted. Don't do this. Ever.
Fuel sizing — runs out faster than you think
A 7,500 W portable at 50% load burns ~0.7 gal of gasoline per hour — 17 gal in 24 hours. Five-gallon cans get old fast. Standby on natural gas runs as long as utility gas pressure holds. Standby on propane: a 250-gal tank at 50% load on a 22 kW unit lasts about 3-4 days.
Common generator-sizing mistakes
Sizing on running watts only. The surge-watts trap. A 5,000 W generator backs up loads that total 4,500 W running and 9,000 W surge — the well pump starts, the gen stalls, the lights drop.
Backing up the central AC with a 7,500 W portable. A 3-ton AC pulls 9,000+ W on startup. Not enough peak. Get the AC contractor to spec a soft-start kit (Micro-Air EasyStart) that cuts surge by ~70%, or step up to a 12 kW unit.
Skipping the transfer switch on a "temporary" install. Every "temporary" lasts longer than planned. Spend the $300-500 on an interlock kit and do it right the first time.
Estimate only. The generator size calculator uses typical motor surge multipliers (2.5-3×) and 20% running-watts headroom. Specific motors can surge higher — always check nameplate LRA (locked rotor amps) on critical loads. Permanent installs require a licensed electrician, transfer switch / interlock per NEC 702, and AHJ approval.