GeneratorMath

Methodology: how the sizing math works

Data last reviewed · Maintained by GeneratorMath Editorial Team

Every one of the 29 appliance calculators and6 scenarios on this site runs the same published model over the same sourced dataset. This page documents the formula, its assumptions, where the numbers come from, and — just as importantly — what the model does not claim. The goal is that you can check our work, and know exactly how far to trust each number.

The sizing formula

  1. Total running watts = the sum of every selected appliance's running watts (× quantity). Running watts add plainly — this is the load the generator carries continuously.
  2. Peak requirement = total running watts + the single largest(starting − running) delta among the selected motor loads. Motors (compressors, pumps, blowers) demand 2–4× their running draw for a fraction of a second at startup; resistive and electronic loads don't surge at all.
  3. Recommended minimum = peak × 1.25 (25% headroom), rounded up. Generators run continuously at their limit overheat, drink fuel, and brown-out motors; headroom is standard sizing practice, not padding.
  4. Size class = the smallest standard market class whose continuous rating covers the recommendation: 2,000W inverter generator, 3,500–4,500W portable generator, 5,500–7,500W portable generator, 9,000–12,000W portable generator, 14–22kW home standby generator. Class boundaries follow the common retail lineup — they are market conventions, not physical constants.

The staggered-start assumption — read this part

Adding only the single largest surge assumes you start loads one at a time, largest motor last, so no two startup surges overlap. This is the standard convention of manufacturer sizing guides, and it is stated on every page here because it is an assumption about your behavior, not about physics. It has a real limit: once compressors are cycling on their own thermostats, two can occasionally restart together. The 25% headroom plus the surge margin generators carry above their continuous rating absorb this in practice — but if you want simultaneous worst-case starts covered on paper, size from the top of the recommended class.

120V vs 240V

Wattage never overrides voltage. Appliances that run on 240V split-phase power — most well pumps, central AC, electric water heaters, larger welders and Level 2 EV charging — need a generator with a 240V outlet, full stop; a 120V-only generator cannot run them at any wattage. Every dataset entry carries its voltage, and any 240V load in a selection triggers a prominent flag. Where a requirement exceeds the largest portable class, the calculators say so and point to standby equipment and a licensed electrician rather than pretending.

Where the watt figures come from

Every appliance's running and starting figures are published as ranges with a typical value, and every range carries a named source in the "data behind this calculator" table on its page. We use, in order of preference:

  1. Manufacturer specifications — e.g. Dometic and Coleman RV air conditioner spec sheets, Franklin Electric pump motor data, Rheem/A.O. Smith water heater element ratings, Keurig and Panasonic input-power figures.
  2. Government energy data — DOE Energy.gov appliance energy estimates and the Alternative Fuels Data Center for EV charging levels.
  3. Established generator sizing charts (Generac, Honda, Champion) — used as cross-checks and for starting-watt conventions, since manufacturers publish these figures for exactly this purpose.
  4. Arithmetic — anything that is pure math (amps × volts, kWh ÷ kW) is labelled as such.

Two honesty notes. First, appliances genuinely vary — a "refrigerator" spans a 3× wattage range across ages and sizes — which is why we publish ranges, compute on typicals, and tell you on every page to check your nameplate first. Second, the full dataset with its sources is versioned in the site's repository, and figures we could not tie to a primary source at review time are flagged for verification in the project's decision log rather than silently presented as certain.

Consistency is enforced, not hoped for

Every page quotes a worked example in its FAQ. Those numbers are not typed twice: an automated test suite recomputes each page's example through the same calculation engine the on-page calculator uses, and the build fails if the prose and the math ever disagree. The same gate excludes any page missing sourced data, substantive FAQs or method steps.

What this site will not do

All figures here are planning estimates to get you to the right size class and the right questions. Final sizing belongs to your appliances' nameplates and, where home circuits are involved, to professionals. We publish no wiring, installation or code-compliance instructions — connecting a generator to household circuits requires a transfer switch or interlock installed by a licensed electrician, and standby generator sizing should be confirmed by the installer against your actual equipment (NEC load calculation). And the safety rules on every page — outdoors only, 20+ feet from the house, never backfeed an outlet, heavy outdoor-rated cords — are not style; carbon monoxide is how generator misuse kills.

Review cadence & corrections

The dataset carries a "last reviewed" date, shown on every page. When manufacturers change published figures, or a reader flags an error athello@generatormath.com, we correct the dataset and the date. Every change is traceable in version control.

Not professional advice: these calculators estimate generator size classes for planning. They are not electrical engineering, and they don't replace an electrician's load calculation for permanent installations or a physician's guidance for medical equipment.