Spot Exhaust Energy Cost Calculator
Annual conditioned-air cost of a spot exhaust fan.
Enter your values
Enter the exhaust airflow in CFM.
Enter the runtime in hours/day.
Enter the avg indoor-outdoor temp diff in °F.
Enter the heating cost in $/kWh equiv.
Enter the fan power draw in W.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Annual makeup-air heating cost
- $140
- Annual fan electrical cost
- $4
- Total annual cost
- $144
- Note
- Cooling-season costs would use similar math with the AC system's efficiency
Quick answer
The energy cost of a spot exhaust fan is driven mainly by the conditioned makeup air it pulls in, not the fan's own small electrical draw.
Cost = CFM x 1.08 x deltaT x hours/yr / heating efficiency x fuel cost, plus fan electrical cost
At a glance
| What it does | Annual conditioned-air cost of a spot exhaust fan. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Exhaust airflow, Runtime, Avg indoor-outdoor temp diff, Heating cost, Fan power draw |
| Main output | Annual makeup-air heating cost |
| Formula | Cost = CFM x 1.08 x deltaT x hours/yr / heating efficiency x fuel cost, plus fan electrical cost |
| Cost | Free — no sign-up, no download |
How to use the Spot Exhaust Energy Cost Calculator
- 1Enter the fan's exhaust airflow and daily runtime.
- 2Enter the typical indoor-outdoor temperature difference during operation.
- 3Enter energy cost and fan wattage to see conditioned makeup air cost versus the fan's own electricity use.
Inputs explained
- Exhaust airflow(CFM)
- Enter the exhaust airflow in CFM.
- Runtime(hours/day)
- Enter the runtime in hours/day.
- Avg indoor-outdoor temp diff(°F)
- Enter the avg indoor-outdoor temp diff in °F.
- Heating cost($/kWh equiv)
- Enter the heating cost in $/kWh equiv.
- Fan power draw(W)
- Enter the fan power draw in W.
Worked example
Using the values the calculator loads with:
Inputs
- Exhaust airflow150 CFM
- Runtime3 hours/day
- Avg indoor-outdoor temp diff30 °F
- Heating cost0.09 $/kWh equiv
- Fan power draw40 W
Results
- Annual makeup-air heating cost$140
- Annual fan electrical cost$4
- Total annual cost$144
- NoteCooling-season costs would use similar math with the AC system's efficiency
Frequently asked questions
Why is makeup air cost so much bigger than fan electricity?
A small fan draws very little power, but the heated or cooled air it exhausts must be replaced by outside air that needs conditioning.
How can I reduce this cost?
Use timers/occupancy sensors so exhaust fans run only as long as needed, and consider heat-recovery ventilation for continuously-running exhaust.
What do I need to enter into the Spot Exhaust Energy Cost Calculator?
Just 5 values: exhaust airflow, runtime, avg indoor-outdoor temp diff, heating cost, fan power draw. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Spot Exhaust Energy Cost Calculator work out the answer?
The energy cost of a spot exhaust fan is driven mainly by the conditioned makeup air it pulls in, not the fan's own small electrical draw. It applies the formula Cost = CFM x 1.08 x deltaT x hours/yr / heating efficiency x fuel cost, plus fan electrical cost and shows the working so you can check each step by hand.
Is the Spot Exhaust Energy Cost Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.