Outage Sizer

Methodology and sources

Every figure on this site comes from a named table, spec sheet or government page, read on 2026-10-20. Nothing is estimated; where sources disagree, both figures are shown.

The formula

Starting watts needed = the sum of every device's running watts × quantity + the single largest additional starting watts in the list. This is the method printed in all three appliance tables and on Generac's standby sizing page: "Total kW Required = (Sum of all running wattages) + (Highest single starting wattage burst)." It assumes motors start one at a time. A portable generator fits when its running rating covers the running total and its peak (or "max") rating covers the starting total. A standby generator must cover the starting total with its rating on the chosen fuel, after the derate.

Derate: "Maximum power decreases approximately 3.5% for each 1,000 ft (304.8 m) above sea level and approximately 1% for each 10 °F (6 °C) above 60 °F (16 °C)." (Generac Guardian spec sheet). The calculator multiplies the rating by 1 − (3.5% × feet ÷ 1,000 + 1% × (°F − 60) ÷ 10), counting only elevation above sea level and temperatures above 60 °F.

Fuel run time = fuel quantity ÷ the hourly burn at half or full load from Generac's table. No interpolation between the two loads is shown as a single figure.

Appliance watts

Every startW below is ADDITIONAL starting watts: the surge a device draws on top of its running watts while its motor starts. All three tables used here print it that way and add the single highest figure to the running total. Each figure was read from the source PDF by column position (pdfplumber) and compared with the rendered page. Where two sources disagree, the primary figure is the one listed first under sources and the others are kept in alt.

Primary source per device: Generac's 2023 brochure where it has a row, then EWEB, then Champion, then the device maker. Champion's flyer prints starting figures for some resistive loads (a desk lamp, a work light, a space heater) that look shifted across columns; those rows are not used except as alternatives on the pages where they disagree. Electric clothes dryers are not included because none of the three tables lists a usable figure.

Generators

Portable ratings from each maker's product page. Standby ratings from Generac Guardian spec sheets (10-24 kW, read from the PDFs) and the 26 kW product page; fuel use from Generac's support article (Guardian Series table, page dated Feb 12, 2026). Generac's 20/22/24 kW spec sheet prints four sets of fuel figures for five model columns, so the site uses the support article's table, which lists one row per size. Kohler and Briggs & Stratton standby ratings were checked but are not used, because this site could not read a fuel table for every size from either maker's own site.

Editorial choices

Safety sources

Refresh

Tables and ratings are rechecked every 365 days or when a maker changes a model. Corrections: hello@outagesizer.com.