Compare the extra electricity cost of leaving your HVAC blower fan ON instead of using AUTO for a bill period.
Table of contents
How to use our HVAC Fan On vs Auto Cost Calculator
- Enter Blower fan power in watts. If you only know amps and volts, multiply amps by volts for a rough estimate.
- Enter AUTO fan run time and ON fan run time in hours per day. Use 24 for ON only if the fan runs all day.
- Enter your Electricity price in $ per kWh and the Days in bill period from your electric bill.
- Select Calculate, then compare Extra cost for ON instead of AUTO, Extra electricity for ON instead of AUTO, and the two separate fan cost cards.
- Sanity-check the result: if ON hours are much higher than AUTO hours, the extra cost should be positive; if ON hours are lower, the extra cost can be negative.
Definitions
Blower fan power: The electric power used by the indoor HVAC fan motor, measured in watts.
Watt: A unit of electric power. The calculator divides watts by 1000 to convert to kilowatts.
kWh: Kilowatt-hour, a unit of electricity use. One kilowatt used for one hour is one kWh [1].
AUTO fan run time: The hours per day the blower fan runs while the thermostat calls for heating or cooling in AUTO mode.
ON fan run time: The hours per day the blower fan runs while the thermostat fan setting is ON.
Electricity price: The cost of one kWh of electricity, usually shown on a bill as $ per kWh.
Extra cost for ON instead of AUTO: The ON fan cost minus the AUTO fan cost for the bill period.
Common mistakes and quick fixes
Mistake: Entering whole-system HVAC power in Blower fan power.
Fix: Use blower fan watts only, not the compressor, heat pump, furnace burner, or full HVAC nameplate load.
Mistake: Putting 24 in AUTO fan run time because the thermostat is powered all day.
Fix: Enter the hours the fan actually runs during heating or cooling calls while set to AUTO.
Mistake: Leaving ON fan run time at 24 when you only use ON for part of the day.
Fix: Enter the daily hours the fan is set to ON, such as 8 or 12.
Mistake: Using only the supply charge for Electricity price when your bill also has delivery charges per kWh.
Fix: Use the total $ per kWh you pay if your bill gives one, or add per-kWh supply and delivery charges.
Mistake: Typing a month name or date range into Days in bill period.
Fix: Enter the number of days, such as 30, 31, or the exact count printed on your bill.
Mistake: Reading a negative Extra cost for ON instead of AUTO as an error.
Fix: Check the hours. A negative result means your entered ON schedule is shorter than your entered AUTO fan runtime.
Limitations & Key Assumptions / Boundary Conditions
- The calculator estimates blower fan electricity only. It does not include compressor, heat pump, furnace burner, humidifier, or air cleaner power.
- Results depend on the Blower fan power you enter. Variable-speed and electronically commutated motors can use different watts at different speeds.
- AUTO fan run time and ON fan run time are treated as daily averages for the whole period. Weather, thermostat settings, and occupancy can change the real runtime.
- Estimated extra cost per year assumes the same daily hour pattern and same electricity price for 365 days. Seasonal heating and cooling patterns may make the real yearly cost different.
- The calculator uses one Electricity price. It does not model time-of-use prices, tiered rates, demand charges, taxes, fixed monthly fees, or solar export credits.
- Entering ON fan run time below AUTO fan run time is allowed for part-day schedules. In that case, the extra cost and extra kWh can be negative.
Methodology
Energy use
The calculator converts Blower fan power from watts to kilowatts, then multiplies by hours and days. A kilowatt-hour is power used over time [1].
fan_on_kwh_period = (fan_watts / 1000) * on_hours_per_day * days_in_period
fan_auto_kwh_period = (fan_watts / 1000) * auto_hours_per_day * days_in_period
Cost comparison
Each mode uses the same fan watts and electricity price. The only difference is the daily fan runtime entered for that mode.
fan_on_cost_period = fan_on_kwh_period * electric_rate
fan_auto_cost_period = fan_auto_kwh_period * electric_rate
extra_cost_period = fan_on_cost_period - fan_auto_cost_period
extra_kwh_period = fan_on_kwh_period - fan_auto_kwh_period
annual_extra_cost = (fan_watts / 1000) * (on_hours_per_day - auto_hours_per_day) * 365 * electric_rate
Mini-example
If Blower fan power is 500 watts, AUTO fan run time is 6 hours per day, ON fan run time is 24 hours per day, Electricity price is $0.16 per kWh, and Days in bill period is 30, ON use is 360 kWh and AUTO use is 90 kWh. The extra electricity is 270 kWh, so the extra cost for the period is 270 * $0.16 = $43.20. The annualized extra cost is $525.60 using the same daily runtimes.