Estimate the 30-day heating and electricity cost of your bathroom fan, then compare its timer setting with 20 minutes after each shower.
Table of contents
How to use our Bathroom Exhaust Fan Heat Loss Cost Calculator
- Choose the Post-shower timer plan, then enter your Showers or fan uses per day and fan minutes during each shower.
- Enter the rated Fan airflow from the fan label and the positive Indoor minus outdoor temperature for the heating conditions you want to estimate.
- Choose Heating type, then enter its Heating efficiency and usage-based Heating energy price. Enter Fan power and Electricity price separately.
- Click Calculate. Check Fan run time per day and Airflow used in the estimate first; if either does not match your routine or installation, correct the inputs and calculate again.
Definitions
CFM: Cubic feet per minute, a measure of how much air the fan moves.
BTU: British thermal unit, a unit of heat energy. This calculator uses BTU for heat carried out by exhausted indoor air. [1]
Sensible heat: Heat linked to air temperature. This estimate does not include moisture or humidity energy.
COP: Coefficient of performance. For a heat pump, it is heat delivered divided by electricity used. A COP of 3 means one unit of electricity supplies about three units of heat.
Heating efficiency: For electric resistance heat and a furnace, the percent of purchased fuel that becomes delivered heat. For a heat pump, this field uses COP instead.
Therm: A natural-gas energy unit equal to 100,000 BTU. [1]
Estimated installed airflow: The percent of rated Fan airflow expected to reach the duct system after duct restrictions are considered.
Common mistakes and quick fixes
Mistake: Entering the fan's wattage as Fan airflow.
Fix: Enter airflow in CFM from the fan label. Put electrical input in Fan power.
Mistake: Treating Indoor minus outdoor temperature as the outdoor temperature alone.
Fix: Subtract the outdoor temperature from the indoor temperature and enter the positive difference for heating weather.
Mistake: Counting the post-shower timer in both Fan time during each shower and Fan time after each shower.
Fix: Enter only shower-time minutes in the first field and only extra run time in the second field when using Custom time.
Mistake: Using a furnace percent as the Heating efficiency after choosing Electric heat pump.
Fix: For a heat pump, enter COP from equipment information. For resistance heat or a furnace, enter a percent efficiency.
Mistake: Including fixed account charges in Electricity price or Heating energy price.
Fix: Use the usage-based rate from the bill when possible. A total-bill average is only a planning estimate.
Mistake: Assuming rated Fan airflow is always the airflow installed in the bathroom.
Fix: Leave Estimated installed airflow at 100 percent if unknown, or enter a measured or professionally estimated lower percent for restrictive ducts.
Limitations & Key Assumptions / Boundary Conditions
- This is a sensible winter heat-loss estimate. It does not calculate summer cooling cost, humidity removal, or condensation risk.
- The calculation assumes the exhausted air is replaced by outdoor air that must be heated by the selected equipment.
- The 1.08 sensible-air factor is an approximation for typical air conditions. Air density, elevation, and indoor conditions can change the actual load.
- Rated fan airflow can differ from installed airflow because duct length, bends, backdraft dampers, and restrictions affect real performance.
- The entered indoor-outdoor temperature difference is treated as constant for every fan run and, if used, every day in Days in your heating season.
- Costs include only usage-based heating energy and direct fan electricity. Fixed utility charges, equipment costs, repairs, and timer installation are excluded.
- A lower-cost timer plan does not prove that the bathroom is dry enough. Keep ventilation long enough to manage moisture according to your fan, bathroom, and local guidance.
Methodology
Calculation method
The calculator first finds post-shower minutes from the selected Post-shower timer plan. The 20-minute and 30-minute plans ignore Fan time after each shower; Custom time uses that field.
run_minutes_day = uses_per_day * (minutes_during_use + post_minutes)
Rated airflow is adjusted only when Estimated installed airflow is changed from 100 percent.
effective_cfm = fan_airflow_cfm * delivered_airflow_percent / 100
The active heat-loss rate uses the common 1.08 approximation for sensible heat moved by air at typical conditions.
heat_loss_rate_btu_hr = 1.08 * effective_cfm * temperature_difference_f
daily_heat_loss_btu = heat_loss_rate_btu_hr * run_minutes_day / 60
Heating cost depends on Heating type. Resistance heating and furnaces use percent efficiency. Heat pumps use COP. Electricity is converted using 3,412.142 BTU per kWh, and natural gas is converted using 100,000 BTU per therm. [1]
daily_fan_cost = fan_power_watts / 1000 * run_minutes_day / 60 * electricity_price / 100
monthly_total_added_cost = (daily_heating_cost + daily_fan_cost) * 30
Timer comparison
The 20-minute comparison holds every other input constant and recalculates the cost with 20 post-shower minutes. A negative 30-day cost change means the selected plan costs less than that baseline.
timer_monthly_cost_change_vs_20 = selected_30_day_cost - baseline_20_minute_30_day_cost
Worked example
With 80 CFM airflow, 100 percent installed airflow, a 30 deg F indoor-outdoor difference, two daily uses, and 10 shower minutes plus 20 post-shower minutes, the fan runs 60 minutes per day. The heat-loss rate is 2,592 BTU per hour and daily sensible heat loss is 2,592 BTU. With 100 percent resistance heat, 18.29 cents per kWh, and a 20-watt fan, the 30-day estimate is about $4.28: about $4.17 for replacement-air heating and $0.11 for fan electricity.