How the Generator Wattage Calculator works

Researched by Ilyas Alseitov · Last updated: September 21, 2026

Last data update: 2026-09-20. 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, 88 rows) was assembled from published manufacturer sizing charts: Honda Power Equipment's wattage estimation guide, Champion Power Equipment's generator wattage 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 have been worked through: the gas dryer and the portable humidifier went on 2026-09-20 and the two portable-AC classes on 2026-09-21, all four removed rather than re-estimated; the hot tub, the Level 1 EV charger and nine appliance rows were re-sourced to the manufacturer or to DOE. One is left — the electric furnace kW range — and it is 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. Four rows added on 2026-09-13 each come from one named model's manufacturer document rather than a sizing chart; they are listed with their sources below.

Eight appliances were removed on 2026-09-20, and that is worth explaining. Until then the table also drew on a Kohler residential sizing chart. Neither Kohler nor Rehlko, which now runs Kohler's generator business, publishes that chart: every copy we could find sits on a retailer's file server, and a search of the web archive across kohlerpower.com, kohler.com and rehlko.com returned nothing. This site does not take figures from retailer-hosted copies, so the chart had to go, and with it every row that rested on it alone. Where another manufacturer publishes the same appliance the row was simply re-sourced (refrigerator/freezer, chest freezer, 75 W bulb). Where none does, the row was removed rather than estimated: sump pump 3/4 HP, furnace blower 3/4 HP, central AC 5 ton, heat pump, window AC 24,000 BTU, attic fan, electric range 6-inch burner, electric oven. Two figures fell because Kohler had supplied the top of their range — toaster oven 1,400 W to 1,225 W, home security system 180 W to 40 W — and the 4-ton central AC range narrowed without its calculator value moving. If your load is one of the eight, read the nameplate: a central AC condenser prints RLA and LRA, a blower or pump motor prints full-load amps, and a heat pump in cooling mode draws what the central AC row of the same tonnage draws.

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. A 1,500 W space heater is the largest resistive load most households will put on a portable generator. A heater that keeps switching itself off has its own built-in protections to rule out before the generator is blamed — see space heater keeps shutting off. The 2026-09-13 rows take no inferred surge at all: where the manufacturer publishes no starting figure, starting = running, including for the washer and dishwasher motors (rated). Each row records which rule applied (starting_basis).

Presets from manufacturer ratings (added 2026-09-13)

An emergency-preparedness columnist asked for a front-load washer, a wheelchair charger, an electric kettle, and a dishwasher run with Heated Dry off. Each value below was read from a manufacturer document for a named model rather than from a generator-brand chart, no starting surge was estimated for any of them, and every value stays editable in the calculator.

Dishwasher — air-dry / Heated Dry off (Bosch rated maximum). Turning Heated Dry off does not guarantee lower peak demand. This preset uses the manufacturer's full rated load because the heater may still operate during wash/rinse cycles. GE's owner's manual 49-5916 says Heated Dry On "Turns the heating element on for fast drying" and Heated Dry Off means "Dishes air dry naturally and energy is saved", while the same machines' Hot Start option "Preheats wash water to the correct temperature." The U.S. Department of Energy notes that dishwashers "are usually equipped with booster heaters to raise incoming water temperature by 15°F to 20°F" and that "Air Dry" saves energy "during the drying cycle." Bosch dishwashers have no drying element at all — "All Bosch dishwashers use a condensation drying process," per Bosch — and are still rated 1,440 W for their concealed water heater. No manufacturer, ENERGY STAR, or DOE document we found publishes a lower wattage for Heated Dry off: it saves energy and time, not peak watts.

Two dishwasher presets, two sources. The presets differ by source, not by the Heated Dry setting. "Dishwasher — generic high-load estimate" (2,400 W running / 4,800 W starting) is the top of a generic generator-chart range (1,200–2,400 W) with the 2× starting rule applied; it is not any model's rating. "Dishwasher — air-dry / Heated Dry off (Bosch rated maximum)" (1,440 W) is one manufacturer's published rated load. If your dishwasher's nameplate lists a rating, enter it.

Wheelchair / powerchair charger (24 V, 10 A). 341 W is a conservative maximum estimate of AC input at full output, derived from Permobil's published 300 W maximum output and its minimum efficiency of 88 % (300 ÷ 0.88). It is not a published OEM input-watts number, and it does not apply to the 24 V / 3 A charger in the original feedback. A charger's AC draw is not its DC output volts × amps, so a "24 V / 3 A" label does not mean a 72 W load. The preset is a 10 A powerchair charger because Permobil publishes both its maximum output power and its efficiency at 120 V, which together give the AC input. The manufacturer documents we checked for smaller chargers — the 3 A Battery Maximizer EA24030-9C and UPG 24 V 2–5 A units in Pride Mobility's charger manual — state only an input voltage range, so the calculator offers no 3 A figure. An ACI 24 V / 5 A charger in the same manual prints "170W" on a line labelled both input and output, so it was not used. If your charger's label lists input watts, enter that. A label that lists only input volts and amps gives apparent power (volt-amperes), which is never less than the real watts, so entering it errs on the high side.

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 NEC Article 702, which requires transfer or interconnection equipment that keeps the normal and standby supplies from ever being connected to each other: 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.
• 2026-09-13 — added four presets from manufacturer ratings after feedback from an emergency-preparedness columnist: front-load washer (1,300 W), electric tea kettle (1,500 W), dishwasher with Heated Dry off at its full rated load (1,440 W), and a wheelchair / powerchair charger (341 W) in a new Emergency & Accessibility category; 100 appliances. No 24 V / 3 A charger figure was added because no manufacturer document we found states its AC input watts. Existing rows and the formula are unchanged.
• 2026-09-21 — two portable air-conditioner presets were removed: the 8,000 BTU ASHRAE (940 W) and the 10,000 BTU ASHRAE (1,180 W) classes. Both were a content site's estimates, worked out by dividing BTU by an assumed EER, with no model behind either one. The 12,000 BTU class stays on GE's APHS08WWF and the 14,000 BTU class on Whynter's ARC-14S, both named models with published figures. For a portable in between, read the watts or amps on the unit's own plate.
• 2026-09-21 — nine appliance rows were moved off a retailer's listings and onto the manufacturers' own specification documents. Eight figures were confirmed unchanged by the documents: the 35-pint (370 W) and 50-pint (470 W) Frigidaire dehumidifiers, the 5,000 (450 W), 8,000 (730 W), 15,000 (1,280 W) and 18,000 BTU (1,620 / 1,640 W at 208 / 230 V) window units, and the Whynter ARC-14S portable at 1,300 W — which the manufacturer publishes as a maximum, the most useful kind of figure for sizing. One moved: the 12,000 BTU window unit is now 1,070 W rather than 1,030 W, because LG publishes rated amps and no watts for it (9.3 A × 115 V = 1,070 W, an upper bound), and the 1,030 W it replaces came from a retailer listing LG does not confirm. The 10,000 BTU row now cites the generator makers' charts it actually takes its 1,500 / 2,400 W from, with LG's 870 W for a current model named in the note. The documents themselves are kept in research/specs/ with the URL and date each was obtained, because these manufacturers refuse automated requests and a link alone would not be checkable.
• 2026-09-21 — the oxygen concentrator preset was re-sourced and now names its model. It had read 350 W from a retailer's spec page for the Philips EverFlo; it now reads 310 W from DeVilbiss's own instruction guide for the 5-Liter Series (catalog 525DS), which specifies "Power Consumption 310 Watts Average, 275 Watts @ 2.5 LPM & below". The label says which machine that is, because a figure for one manufacturer's concentrator is not a figure for the class. DeVilbiss's own site no longer resolves, so the guide is cited through an archived capture of the manufacturer's URL. The preset id is unchanged.
• 2026-09-20 — two presets were removed because they stood entirely on a third-party generator chart and no manufacturer publishes a replacement: the portable evaporative humidifier (175 W) and the gas clothes dryer (700 / 1,800 W). AIRCARE publishes coverage, tank capacity and run time for its console humidifiers and no electrical rating at all. For gas dryers GE publishes a 120 V, 15–20 A circuit requirement and no wattage — a breaker size is not an appliance load, and multiplying it out would have produced a number no one published. An electric dryer, which GE does publish a full wattage table for, is still in the list; for a gas one, read the nameplate.
• 2026-09-20 — the garage door opener row was relabelled from "Garage door opener 1/2 HP" to "Garage door opener" and now carries Honda's figures alone, 720 W running / 1,420 W starting, down from 875 / 2,350. Honda's chart states no horsepower for that row, so the label claimed one it had no source for; the higher figures came from Generatorist, which is not a manufacturer. The 875 / 2,350 figures turned out to be Honda's too, from the row four lines above the opener: 'Furnace Fan, gas or fuel oil', 1/2 HP, 2,350 starting / 875 running. Both the horsepower and the higher numbers had migrated from the furnace fan onto the opener. The preset id is unchanged, so shared links still work.
• 2026-09-20 — the Kohler residential sizing chart was removed as a source: neither Kohler nor Rehlko publishes it, and the only copies available are retailer-hosted. Three rows were re-sourced to Westinghouse and Champion (refrigerator/freezer, now 700 / 2,200 W; chest freezer and 75 W bulb unchanged), eight rows that rested on the chart alone were removed (sump pump 3/4 HP, furnace blower 3/4 HP, central AC 5 ton, heat pump, window AC 24,000 BTU, attic fan, electric range 6-inch burner, electric oven), and two values fell to what the remaining sources support (toaster oven 1,225 W, home security system 40 W). The wheelchair charger preset now cites the copy of the Permobil VoltPro manual that Permobil publishes itself rather than a dealer's copy; the 341 W value is unchanged and was re-read from that document. 92 appliances.
• 2026-09-14 — wording only: the two dishwasher presets are now labelled "Dishwasher — generic high-load estimate" and "Dishwasher — air-dry / Heated Dry off (Bosch rated maximum)", so their difference reads as a difference in source rather than a Heated Dry power reduction; the charger is labelled "Wheelchair / powerchair charger (24 V, 10 A)". Ids, wattages, and the formula are unchanged.

Cite this page

GridComfort, "Generator Wattage Calculator — methodology," https://gridcomfort.com/tools/generator-wattage-calculator/methodology/, data updated 2026-09-20.

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