Comfort equipment is easy to mis-estimate: surge watts look scary, and “always on” hours inflate a month. Our heating and cooling worksheets keep one transparent rule—convert running watts and hours into kWh, then multiply by the $/kWh you supply.

kWh/day = (watts ÷ 1000) × hours/day. Cost = kWh × rate. Monthly and yearly lines scale by the days-per-month assumption on the page. That identity matches how DOE Energy Saver materials describe appliance energy use and how EIA conversion tools relate watts and kilowatt-hours.

Heater and space-heater calculators use the same engine with defaults aimed at those device classes. Prefer measured or Energy Guide average watts for cycling loads. Keep hours as a seasonal daily average so a heat-wave or cold-snap day does not silently become every day of the month.