| Scenario | Airflow (CFM) | Pressure (Pa) | Power (W) | CFM/W | Air-Power % |
|---|---|---|---|---|---|
| Small tent exhaust (filter) | 200 | 125 | 45 | 4.44 | 13.1% |
| Medium grow room (duct + bends) | 650 | 180 | 120 | 5.42 | 16.0% |
| Greenhouse circulation fan | 1500 | 40 | 120 | 12.50 | 9.4% |
- CFM per Watt = Airflow (CFM) / Power (W). Higher usually means better value.
- Air power (W) = Q (m3/s) * dP (Pa). Ideal aerodynamic power delivered.
- Air-power efficiency (%) = (Air power / Electrical power) * 100.
- Required airflow (m3/h) = Volume (m3) * ACH, when both are entered.
- Daily energy (kWh) = Power (W) * Hours / 1000.
- Enter airflow from the fan curve or test data.
- Add static pressure for filters, ducting, and restrictions.
- Enter electrical power draw; choose watts or horsepower.
- Optionally add room volume and ACH to check targets.
- Click Calculate Efficiency to show results above the form.
- Use export buttons to save a CSV or PDF report.
Why airflow-per-watt matters in horticulture
Ventilation fans run for long periods in tents, greenhouses, and propagation rooms. Airflow-per-watt (CFM/W) helps compare models on an energy basis, especially when electricity rates rise. Higher CFM/W typically means lower operating cost for the same air exchange, reducing heat buildup and improving CO2 distribution.
Static pressure is the hidden performance limiter
Filters, long duct runs, louvers, insect screens, and tight bends increase static pressure. As pressure rises, real airflow drops and fans draw more power. Entering pressure in Pa or inH2O lets the calculator estimate air power and efficiency at your operating point, not just the free-air rating printed on a box.
Air-power efficiency connects airflow and resistance
Air power is calculated from volumetric flow and pressure (Q × ΔP). This value represents the useful aerodynamic work your system demands. Air-power efficiency (%) compares that requirement to electrical input. Two fans can show similar CFM/W, but the one maintaining airflow under higher pressure usually delivers better real-world results.
Sizing with ACH reduces humidity and disease risk
Air changes per hour (ACH) is a planning shortcut that links room volume to required airflow. Many growers target higher ACH during lights-on and lower ACH overnight. Use volume and ACH fields to check if your fan meets the target. If it falls short, reduce restrictions or select a higher-capacity model.
Energy tracking supports budgeting and maintenance
When you add daily operating hours, the calculator estimates kWh and optional daily cost. Track these numbers as you change filters or duct layouts. A rising kWh for the same airflow can indicate clogging, belt issues, or worn bearings. Preventive maintenance protects crops, extends fan life, and limits downtime.
1) What airflow value should I enter?
Use airflow measured or specified at your expected pressure. Free-air ratings can be misleading when filters or ducting are installed.
2) Do I need static pressure to use the calculator?
No. You can still calculate CFM per watt. Adding pressure enables air-power and air-power efficiency for deeper comparisons.
3) Why can air-power efficiency look low?
Air power measures only useful airflow work. Motor, drive, and aerodynamic losses are not counted, so percentages commonly appear modest.
4) What is a good CFM per watt rating?
It depends on fan type and pressure. As a quick rule, 5–7 is good for many setups, while 10+ is excellent in low-resistance circulation.
5) How does ACH relate to plant health?
Higher ACH helps control humidity, heat, and stagnant zones that promote fungal pressure. Balance ACH with temperature management and CO2 strategy.
6) Will exports include my notes and optional fields?
Yes. CSV and PDF exports include entered notes, unit selections, and optional planning values when provided.