Heating & Cooling generator calculators
Heating and cooling loads decide whether a portable generator is realistic at all. A gas furnace only needs its blower motor powered — a job a mid-size portable handles easily — while central air conditioning has the largest starting surge of any residential appliance, and electric resistance heat draws heavily for as long as it runs. These calculators separate those cases honestly, with sourced watt ranges by blower horsepower, AC tonnage and heater setting.
Pick your calculator
Central AC Generator Size Calculator (by Tonnage)
Generator sizing for central AC by tonnage: sourced running watts, the huge compressor starting surge (LRA), soft-start kits, and why 3+ tons usually means standby.
Open calculator →Window AC Generator Size Calculator (by BTU)
Window AC generator sizing by BTU: sourced running and starting watts for 5,000–12,000 BTU units, and why one cooled room on a portable beats whole-house dreams.
Open calculator →Furnace Blower Generator Size Calculator
Generator size to run a gas furnace: it only needs the blower motor — sourced watts by HP (~700W running, ~1,400W starting for 1/3 HP), plus the hardwiring caveat.
Open calculator →Electric Space Heater Generator Calculator
Space heater on a generator: 1,500W of pure resistive load, no surge — but the worst watts-per-warmth deal on generator fuel. Numbers and better alternatives inside.
Open calculator →
Which calculator should I use?
In winter, size for the furnace blower, not the furnace: gas and oil furnaces burn fuel for heat and only need electricity for the fan and controls. In summer, a window AC unit is the pragmatic outage choice — the central AC calculator will show you why whole-house cooling usually points to a standby generator with a soft-start kit and professional installation. The portable heater page covers electric space heaters, which have no surge but occupy 1,500W of capacity all by themselves.
Running several of these at once? Themulti-appliance builder adds them up with the staggered-start surge math, and the outage scenarios cover the common combinations pre-built.