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Fan Power Calculator: What It Is, How It Works & Sample Project
Quickly estimate air power (Q×ΔP), brake horsepower, motor input (kW), and monthly energy cost with standards-inspired presets.
What this calculator is used for
It estimates the power and energy requirements of ventilation, exhaust, and process fans. You’ll get:
Presets are inspired by AMCA, ASHRAE 90.1, and SMACNA good practices. Always verify values for your project.
Inputs you provide
Per Fan (segment)
- Flow (m³/s or CFM)
- Total pressure ΔP (Pa or in.wg)
- Leakage % (adds to flow) & Fouling % (adds to ΔP)
- Efficiencies: fan η, drive η, motor η, VFD η
- Air temperature & Altitude or manual density
- Duty % (if a fan runs fewer hours)
Global (energy & cost)
- Country (IN/US/UK) → currency & default tariff
- Tariff (per kWh) & load factor (%)
- Hours/day and days/month
- Global safety (%) on motor sizing
- Capex rate per kW or HP + markup (%)
How to use it (quick steps)
- Click + Add Fan and pick a Preset (e.g., AHU Supply).
- Enter/confirm Flow, ΔP, and efficiencies.
- Set leakage, fouling, and global safety allowances.
- Fill in tariff, hours, days, and load factor.
- Click Calculate to see kW, HP, kWh/month, and cost.
- Use Reverse Mode for “motor limit → max flow” or “target duty → motor”.
- Generate the HTML Report for KPIs, breakdown table, and assumptions.
Methodology & unit math (click to expand)
Air power: P_air = Q · ΔP (Q in m³/s, ΔP in Pa → W). With allowances: Q_eff = Q · (1+leak%), ΔP_eff = ΔP · (1+fouling%).
Motor input: P_in = P_air ÷ (η_fan · η_drive · η_motor · η_vfd), then apply global safety: P_in,final = P_in · (1 + safety%).
Brake HP (AMCA form): BHP = (CFM · ΔP_in.wg · ρ/0.075) ÷ (6356 · η_fan).
Energy per month: E = P_in,final · LoadFactor · hours/day · days/month · duty%.
Capex estimate: Capex = Rate × (kW or HP) × (1+markup%).
Sample project: AHU Supply fan (complete calculation)
Assume the AHU Supply preset with SI units and India tariffs. Verify against your project criteria.
Given / Inputs
- Flow Q = 2.50 m³/s, Leakage = 3% → Qeff = 2.575 m³/s
- ΔP = 650 Pa, Fouling = 5% → ΔPeff = 682.5 Pa
- Efficiencies: ηfan=0.68, ηdrive=0.95, ηmotor=0.93, ηvfd=0.98
- Global safety = 10%
- Tariff = ₹7/kWh, Hours/day=10, Days/month=30, Load factor=80%
- Air 24 °C, sea level (auto density)
Step-by-step
- Air power: Pair = 2.575 × 682.5 = 1757.44 W = 1.757 kW
- Shaft power: 1.757 ÷ 0.68 = 2.584 kW
- Motor input (core): 2.584 ÷ (0.95×0.93×0.98 = 0.86583) = 2.985 kW
- Motor input (with safety): 2.985 × 1.10 = 3.283 kW
- Brake HP (AMCA): Qeff=5456 CFM, ΔPeff=2.743 in.wg → BHP ≈ 3.46 HP
- Energy/month: 3.283 × 0.8 × 10 × 30 = 788 kWh
- Cost/month: 788 × ₹7 ≈ ₹5,516
| KPI | Value |
|---|---|
| Total air power | 1.757 kW |
| Brake horsepower | 3.46 HP |
| Motor input (final) | 3.283 kW |
| Energy / month | ~788 kWh |
| Cost / month | ~₹5,516 |
| Indicative Capex* | At ₹8,000 per kW + 10% markup → ~₹28,892 |
*Capex uses the global rate/markup in the calculator and scales with total input kW or HP.
Tips to improve accuracy
- Use actual system ΔP from your duct design (see Duct Design Calculator).
- Confirm air density from site altitude & temperature, or enter manual density.
- Pick realistic efficiency chain from manufacturer data (fan + drive + motor + VFD).
- Set load factor from BMS trends to avoid overestimating energy.
Related reading
Make readers stay longer
- Add a short quiz (e.g., “Which parameter increases BHP the most?”) with instant feedback.
- Embed a before/after slider showing energy impact of better efficiencies.
- Offer a downloadable report from the calculator (already supported) and a CSV export.
- Place a preset gallery (“AHU, Cleanroom, Parking, Kitchen Hood…”) above the fold.
