How the Generator Wattage Calculator works
Last data update: 2026-08-24. This page is the citable record of where every number comes from and how the result is computed.
Data
The appliance table (appliance-watts.csv, 96 rows) was assembled from published manufacturer sizing charts: Honda Power Equipment's wattage estimation guide, Champion Power Equipment's generator wattage chart, Kohler's residential sizing chart, Generac's wattage worksheet, and the Westinghouse WGen12000 owner's manual. Where charts disagree, the table stores the range and the note names each source; the calculator uses the upper end of the range for running watts (conservative for sizing). Rows sourced only from third parties (gas dryer, humidifier, hot tub, electric furnace, portable-AC classes, Level 1 EV charging) are flagged in the notes column. Mini-split, garbage disposal, and corded mower figures are derived from manufacturer ratings (BTU ÷ EER2; amps × volts) and say so.
Starting watts: where the source chart lists a figure, it's used. Where it doesn't and the appliance is a motor or compressor load, the calculator applies Generac's worksheet convention that starting watts are "typically 2X running watts." Resistive loads (heaters, ranges, bulbs, electronics) are assigned starting = running. Each row records which rule applied (starting_basis).
Formula
running_total = Σ (running_i × qty_i)
largest_surge = max_i ( max(0, starting_i − running_i) ) // one motor starts at a time
peak_need = running_total + largest_surge
sized_running = running_total × (1 + headroom) // headroom default 0.20
sized_peak = peak_need × (1 + headroom)
class = smallest tier with rated_running ≥ sized_running AND rated_peak ≥ sized_peak
The "add the single largest starting requirement" rule is stated by Generac's wattage worksheet and by Briggs & Stratton's sizing guide. Because manufacturer charts list starting watts as the total momentary draw (not the increment), the surge is computed as starting minus running.
Generator classes
Tiers are generic and named by peak watts: inverter 2,000 (1,600 running / 2,000 peak), inverter 3,500 (3,000 / 3,500), conventional 5,500 (4,500 / 5,500), 7,500 (6,000 / 7,500), 9,500 (7,500 / 9,500), 12,000 (9,500 / 12,000); home standby 10, 14, 18, 22, 26 kW (rated running = peak for standby tiers). They represent common retail classes, not specific models. Kohler's published sizing bands (10–14 / 14–20 / 20–26+ kW) are used for the standby wording.
Connection guidance
The transfer-switch/interlock note follows Generac's backfeeding article and the NEC Article 702 summaries cited above: with manual transfer equipment the source must handle the loads the user chooses to run at once; with automatic transfer it must carry the full transferred load. Interlock-vs-switch descriptions are general; listing of an interlock kit for a specific panel is an inspector's question.
Assumptions and limits
- One motor starts at a time.
- 20 % headroom is a convention, not a code requirement; the slider lets you change it.
- Altitude/temperature derating is not applied.
- Duty cycle is not applied (the fridge is counted at full running watts even though it cycles).
Change log
- 2026-08-24 — first release; 96 appliances; sources as listed.
Cite this page
GridComfort, "Generator Wattage Calculator — methodology," https://gridcomfort.com/tools/generator-wattage-calculator/methodology/, data updated 2026-08-24.