2026-09-03
Attic Fan Electricity Cost Per Ventilation Hour Used: Measure Duty Cycle
Quick answer: Estimate attic-fan electricity cost per ventilation hour from measured kWh, thermostat duty cycle, weather window, and delivered electricity rate.
Inputs that change the result
Define a ventilation hour as one clock hour in which the fan is allowed to operate, not necessarily one hour at full power. Record outdoor and attic temperature, thermostat setting, fan mode, start and stop times, measured kWh, roof or gable configuration, and whether another fan or controller shares the circuit. Keep the home’s cooling system outside the fan measurement unless it is intentionally part of the question.
Formula and measured example
cost per ventilation hour = measured fan-system kWh during the defined window ÷ ventilation hours × delivered electricity rate. If the fan ran for only 38 minutes in a one-hour window, retain both values: one describes the hourly allowance and the other describes powered runtime. A fan-system meter reads 0.18 kWh across a six-hour hot-afternoon window, but the thermostat allowed only 4.5 powered hours. At 0.22/kWh, the window costs 0.0396; the allowance-hour view is 0.0066, while the powered-hour view is 0.0088. Label the denominator instead of presenting one as the only answer.
Compare real-use windows
Compare a fixed-speed fan on a warm afternoon, a thermostat-controlled fan during a cooler evening, and a solar-attic setup with a separate controller. Keep airflow settings, circuit boundary, weather, and house cooling behavior visible. Ventilation runtime and indoor comfort are different outcomes and need separate measurements.
Continue with the cooling guide, filtration guide, humidity guide, and daily-cost guide. Each link answers a different measured unit; do not merge their denominators.
Limits and common mistakes
Roof insulation, intake ventilation, attic volume, outdoor weather, thermostat hysteresis, motor condition, fan speed, controller draw, wiring boundary, voltage, and maintenance affect energy. A lower fan bill does not prove lower indoor temperature, moisture control, roof life, or energy savings for the whole home. Do not add HVAC electricity unless you measured the combined system.
Frequently asked questions
Is the fan’s rated wattage enough to calculate the hourly cost?
It can provide a rough upper-bound scenario, but it is not evidence of duty cycle or controller draw. A wall meter over a named weather window captures cycling and standby; report the rate, window, and denominator with the result.
Sources
Read EIA’s electricity price explanation, ENERGY STAR’s product information, and CFPB’s home spending guidance. They provide context, not a substitute for measuring your fan circuit and local tariff.