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Plant Engineering Calculator

Updated Sep 18, 2026

Cooling Tower Fan Power Savings Calculator — Speed, Fan Laws & Annual kWh

Estimate cooling-tower fan power at reduced speed using the cube law, then calculate kW, annual kWh and cost savings against measured baseline power.

Live calculationWorking shownPrivate in browser

Inputs

Results

Speed ratio 0.800
Ideal cube-law power 15.36 kW
Power incl. drive/control loss 15.82 kW
Power saved 14.18 kW (47.3%)
Annual energy saved 85075 kWh
Annual cost saved $8508
Simple payback 1.41 years
More actions

Shared links reopen these inputs. Recent and saved calculations stay in this browser unless you clear site data.

Formula

P2=P1×(N2/N1)^3; saved=P1−P2.

Your calculation, step by step

These steps use the values currently entered above so the result can be checked independently.

  1. 1.Speed ratio r = 80.00% ÷ 100 = 0.80000.
  2. 2.Ideal fan-law power = 30.000 × 0.80000³ = 15.3600 kW.
  3. 3.Add entered drive/control loss: 15.3600 × (1 + 3.000%/100) = 15.8208 kW.
  4. 4.Screened power saving = max(0, 30.000 − 15.8208) = 14.1792 kW (47.26%).
  5. 5.Annual energy = 14.1792 × 6000 = 85075.2 kWh; annual value = $8507.52; simple payback = 1.411 years.

How to use this calculator

  1. Enter measured or design values using the units shown beside each field.
  2. Review the live results and the substituted calculation steps before using the estimate.
  3. Check the assumptions below, then continue to a related calculator when the next engineering decision needs another model.

Worked example

Reducing a 30 kW fan from 100% to 80% speed gives 15.36 kW ideal cube-law power before the entered drive/control loss is applied.

Assumptions and limits

  • Fan laws apply to dynamically similar operation; actual VFD, motor and system efficiencies vary.
  • Tower thermal performance and approach constraints determine whether reduced speed is feasible.
  • Static-pressure/system effects can make real power depart from the pure cube law; validate against measured fan/VFD data.

How to interpret the result

Treat the primary result as an engineering screening value, then use the secondary outputs to understand scale, energy or operating impact. Confirm site measurements, equipment limits and applicable standards before making a design or control change.

Questions engineers ask

Why does fan power use the cube of speed?

For dynamically similar fan operation at unchanged density/system geometry, airflow scales with speed, pressure with speed squared and power with speed cubed. That makes modest speed reductions potentially valuable.

When is the pure cube-law saving too optimistic?

Static-pressure/system effects, fan efficiency, motor/VFD efficiency and operating-point changes can weaken the ideal relationship. Natural Resources Canada specifically cautions that many real fan/pump applications behave less ideally; cooling-tower fans are among the cases more likely to approach the cube law.

Why is tower thermal performance not included in the saving calculation?

Fan speed cannot be reduced independently of approach, wet bulb and heat-rejection duty. This owner values the electrical change at an entered average speed; verify that the tower still meets thermal/process requirements across the load profile.

Continue the calculation

Use the next calculator in the same engineering workflow instead of restarting from search.

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Methodology. MAXScanner shows the governing formula, entered values and calculation outputs so the estimate is auditable. The calculator is a screening aid; assumptions and equipment-specific limits remain part of the engineering review.