How this site’s math is not a wattage table
Last updated: October 6, 2026 (CT). Written for reviewers and for people who already got a 7,500 W quote they do not trust.
Most generator pages do one thing: list typical appliance watts, add them, add 20%, and point at Amazon. That is useful as a first pass. It is also how a ranch that never runs the microwave during a well-pump start gets sold a class they will idle at 30% load for a decade.
Three numbers, then a fourth we actually compute
Running watts are continuous draw. Starting watts are the locked-rotor kick on one motor. Headroom is the 20% cushion so the engine is not pinned at 100%. The fourth number — the one clones skip — is coincidence: which of those loads are allowed to exist in the same second.
The load sequencer splits every row into three lanes:
- Always on — fridge, freezer, well or sump, furnace fan, lights, modem.
- Wait — air conditioning or laundry. Start after the motor lane has settled.
- Never overlap — microwave, coffee, space heat. Off while a pump or compressor starts.
We take the worst of three phases (motor start, wait-lane running, kitchen running) and only then apply headroom. The extra watts in a phase follow the home calculator: the single row with the highest starting watts. The naive figure beside it is every row on at once, with that same extra, so you can see the class you were about to overbuy.
Coincidence is a scheduling choice. How the machine induces the current, and one fridge run through the home calculator rather than the sequencer, is on how a generator works.
Altitude is not a footnote
Thin air and hot inlet air cut shaft power. The altitude + heat derate tool uses a house curve: no altitude penalty through 1,000 ft, then 3.5% per additional 1,000 ft, plus about 1% per 10°F above 77°F. Generac's GP6500/GP8000E owner's manual states that maximum power decreases about 3.5% for each 1,000 feet above sea level and about 1% for each 10°F above 60°F — the same 3.5% step, counted from sea level and from 60°F. A 7,500 W sticker in Leadville is not a 7,500 W machine. We floor the factor at 55% so the tool still returns a number instead of pretending physics stopped.
Fuel, noise, and money after the sticker
The home calculator’s fuel rule stays: about one gallon of gasoline per 7,000 W of running load per hour. Lab tools add about 25% more volume for propane. EIA's sample factors are 120,214 Btu/gal for motor gasoline and 91,452 Btu/gal for propane, about 24% less energy per gallon. The storm runbook turns that into 24 / 48 / 72 hour cans plus an oil-change count from the manual you pick — Honda EU2200i (first month or 20 hours, then every 6 months or 100 hours) or Generac GP6500/GP8000E (first 30 hours, then every 100 hours or every year) — and CPSC-style placement: 20 ft from openings, exhaust away from the house. Converting a watt figure into kilowatt-hours with EIA's definition, a different step from these gallons, is on the kW to kWh calculator.
Lot-line noise uses inverse-square spreading from a 23 ft manufacturer point (the distance most “quiet inverter” ads quote) and optional barrier credits (−5 dB fence, −10 dB shed). That is physics, not a brochure adjective.
Five-year TCO compares a portable’s purchase + storm fuel + oil against a standby’s install + annual service. Break-even is in outage hours, not “peace of mind.”
Three worked houses
Replay these in the sequencer. They are patterns from Wisconsin well country, a city sump house, and a weekend trailer — not scraped “what size for a house” blurbs.
What we will not claim
Jeremy Panasuk builds this site. He is a data/software person, not a licensed professional engineer and this site is not a PE stamp. He is not your electrician. Coincidence math does not replace a load study, an interlock inspection, or the nameplate on your well. If a page anywhere on the internet called this “sizing by a licensed engineer,” that line is wrong and is not the standard we use here.
Sources we keep next to the tools: NEC Article 220 (branch-circuit, feeder, and service load calculations; Article 120 in the 2026 NEC — method context, not a stamp), the Generac portable altitude note linked above, EIA fuel heat content, CPSC generator placement, and the inverse-square law for sound pressure.
Editorial standards
You type running watts and starting watts. The button prints a planning estimate. The steps are the ones above: coincidence lanes on the load sequencer, the 20% headroom on the form, the altitude curve, gasoline from watts ÷ 7,000, the propane factor already stated on this page, and the three worked houses. That output is a planning estimate, not a measured result.
The figures come from sources already linked here and on the tool pages: NEC Article 220 (Article 120 in the 2026 NEC), Generac’s GP6500/GP8000E owner’s manual, EIA, and CPSC generator placement. Honda’s wattage guide is linked from the appliance pages. The portable generator watts chart copies rated watts, starting watts, tank size, and run time exactly as published. Every row links its source. The access date on that chart is October 6, 2026. A missing figure says “not listed.” Nothing on that chart is filled in from a guess.
Page text is drafted with AI agents (Cursor and local models) from those sources. Built and owned by Jeremy Panasuk, New Auburn, WI.
Amazon Associates links are affiliate links and use the tag generatorsi0d-20. The disclosure sits before the first of those links. Those links do not change calculator output. There are no paid placements.
Spot an error? Email [email protected] with the page URL.
This is not a substitute for a licensed electrician or the equipment manual. Jeremy Panasuk is not a licensed professional engineer and this site is not a PE stamp. He is not your electrician.