How every number here is worked out

The worksheet adds the running watts of everything you select, then adds the single largest starting surge on top. That surge method is the correct one: motors draw a brief spike when they start, but they do not all start at the same instant, so the peak is the running total plus the one biggest surge, never the sum of every surge. We add about 25% headroom so the generator is not pinned at full output, then round to a standard size. Generac's own sizing guide suggests 10 to 20 percent for residential loads (its best practice, not a code rule); we default to 25 to stay on the safe side.

The appliance running and starting watts come from the wattage worksheets that portable-generator makers publish in near-identical form (Generac, Honda, Champion, Westinghouse), with motor surge behavior from Generac's power-design guidance. These are typical figures; your own appliance's data plate, which lists its running amps and locked-rotor amps, is the final word for a specific unit.

The propane runtime uses the industry energy content of propane, about 91,500 BTU per gallon and 21,548 BTU per pound (PERC and EIA figures). Gallon-rated tanks are figured at the safe 80% maximum fill that leaves room for expansion, following NFPA 58, the Liquefied Petroleum Gas Code. Generator fuel use is estimated from Generac's air-cooled fuel-consumption tables, at the high end of the range so the runtime is conservative rather than optimistic.

The non-dismissable safety note on the tool reflects standard guidance: never backfeed a generator into a wall outlet, use a transfer switch installed to NFPA 70 (the National Electrical Code) by a licensed electrician, and never run a generator indoors or in a garage because its exhaust is carbon monoxide.

The AC and furnace calculators share one engine. Cooling starts from a load of roughly 20 to 25 BTU per hour per square foot. That figure is a common HVAC rule of thumb, corroborated across the Carrier, Bryant, and Trane sizing guides, not an official DOE table; we scale it by climate zone using the DOE and IECC zone map. It is then adjusted for ceiling height (more air to condition), sun exposure (about 10% either way), each occupant over two (about 600 BTU each), and a kitchen (about 4,000 BTU). Those room adjustments are the official ENERGY STAR room air-conditioner figures. Central systems round to the nearest half-ton nominal size, where one ton is 12,000 BTU per hour.

Window and portable units use the ENERGY STAR room air-conditioner sizing chart directly, a lookup by floor area with the same sun, occupancy, and kitchen adjustments; that chart is the best-sourced number on the site. Heating uses the same structure with a climate-zone load of roughly 30 to 60 BTU per square foot of output, which is an HVAC-industry rule of thumb, not a DOE standard, since no official BTU-per-square-foot-by-zone table exists. Because furnaces are sold by their input rating, we divide the needed output by the AFUE efficiency to show the input rating to shop for.

None of this is an ACCA Manual J. A Manual J accounts for insulation, window area and orientation, air leakage, and duct design, which a rule of thumb cannot. ACCA's own analysis of real load calculations found they land anywhere from about 600 to over 3,000 square feet per ton, so a flat per-square-foot rule tends to oversize. That is the other reason a contractor's lower Manual J number is usually the right one, and it is why we warn when a size rounds up hard: an oversized AC or furnace short-cycles, cooling or heating in bursts too short to work properly.

Every calculation runs on your device in plain JavaScript. Nothing you type, check, or select is sent to a server, because there is no server doing the math. There is no account, no email, and no lead form. That is a technical fact about how the site is built, not a marketing line.

Standards and manufacturer figures drift, so the underlying tables are reviewed on a schedule: the appliance wattages against current manufacturer worksheets, and the heating and cooling rules against current DOE and ENERGY STAR guidance. We do not restate a date without rechecking the numbers behind it. If you spot a figure that looks off, that feedback is genuinely useful.

Primary sources worth reading yourself: DOE and ENERGY STAR for heating and cooling (energy.gov and energystar.gov), ACCA for the Manual J standard (acca.org), and NFPA for propane and electrical code (nfpa.org).

• Any generator install that feeds house circuits: a licensed electrician and a transfer switch, always.
• Replacing or adding central AC or a furnace: an HVAC contractor's Manual J, before you buy.
• Gas and propane connections, venting, and gas-line sizing: a licensed plumber or gas fitter. We never give connection instructions.
• Anything that smells like gas or trips a carbon monoxide alarm: leave, ventilate, and call your supplier or the fire department.