About GenSize

GenSize answers one question: how big a generator a given list of loads needs. The calculators all run the same math from the same data table, and this page documents both, so you can check our work.

The data

Appliance wattages are planning numbers assembled from the Champion Power Equipment wattage chart and the Lowe's and Honda portable-generator worksheets, taking the conservative end where charts give a range. Deliberately so: a chart has to size for the old fridge in the garage, not the new one in the showroom. Every appliance page shows the range by type next to the planning number, and everywhere we say it we mean it: your appliance's nameplate beats our table.

The sizing rule

Total running watts of everything on the generator at once, plus the single largest (starting minus running) surge in the list, equals peak demand. The recommended class is the smallest whose peak rating covers that demand and whose rated running watts cover the running total with 25% margin, keeping the engine at or under 80% continuous load per the manufacturer manuals. The bare-minimum figure shown beside every recommendation is the same math without the margin. Whole-house estimates use the NEC Article 220 conventions (3W per square foot demand-factored, appliance demand values), simplified; they are budgeting figures, and the installing electrician's load calculation is the one that counts.

What this site is not

Electrical design. Anything involving your panel (interlocks, transfer switches, standby units, well pump inlets) is licensed-electrician work, usually permitted work, and our guides say so at every turn. We also have opinions (soft starts are the best money in RV power; resistance heat on gasoline is a waste of both) and label them as opinions where they appear.

Who runs this

GenSize is one of a family of small single-topic calculators. It carries no sponsorships from generator makers; if affiliate links or lead referrals appear, they're marked as such and never change the math. Corrections and disagreements: contact page. If your nameplate disagrees with our chart, we want to hear about it.