Guessing energy savings from a brochure percentage is how good projects get turned down. Finance teams have seen too many of them.
A week of logged data and four lines of sums give you a number you can defend. The whole job takes less than a day.
Log fan power for a week, sort the hours into load bands, then apply the cube law to estimate power in each band after the upgrade. Correct for motor efficiency at each load, multiply by tariff, and add avoided maintenance. Finally, verify the result six months after installation by logging again.
Step 1: build a load profile
You cannot work out energy savings without knowing how the fan really runs, and nobody knows that from memory.
- Fit a logger on the fan supply for at least seven days.
- Include a weekend, since occupancy patterns change the answer.
- Record power rather than current, because power factor varies with load.
- Note the control method in use, whether damper, speed or fixed.
- Sort the hours into bands: full, three-quarter, half and off.
- Finally, scale the week to a year using the operating schedule.
The output is a simple table of hours per band. That table then drives every later step in the energy savings calculation. Our guide to real EC fan savings explains why the profile matters so much.
Step 2: estimate the new power in each band
Fan power follows the cube of speed, so each band gets its own factor.
| Airflow band | Speed | Power factor | Example from 4 kW |
|---|---|---|---|
| 100% | 100% | 1.00 | 4.00 kW |
| 80% | 80% | 0.51 | 2.05 kW |
| 60% | 60% | 0.22 | 0.86 kW |
| 40% | 40% | 0.06 | 0.26 kW |
If the existing fan uses damper control, its power barely falls between bands. That difference is where most of the saving comes from.
Step 3: correct for motor efficiency
An AC induction motor loses efficiency at part load. An EC motor holds it. That gap is a second, separate saving.
Add roughly 5 points at full load, rising to 10 or 15 points at half load, in favour of the EC motor. Meanwhile take off 2 to 4 percent if you are comparing against an AC motor on a drive rather than a fixed-speed one.
The cube law covers the speed reduction. Motor efficiency covers the conversion loss. Keep them as separate lines, because combining them into a single percentage is exactly how estimates become unbelievable.
Step 4: turn energy savings into money
Multiply the annual saving in kilowatt hours by your actual tariff, including any capacity or demand charge that changes with load.
- Use the tariff you actually pay, not a national average.
- Include maximum demand charges where peak load falls as well.
- Add avoided maintenance, particularly belt replacement and tensioning labour.
- Add reduced cooling load in conditioned spaces, since fan heat disappears too.
- Subtract any increase in monitoring or control costs.
- Finally, present the result as a range rather than a single figure.
Our payback guide covers the cost side of the same calculation.
A worked example
A 4 kW supply fan on damper control logs 3.6 kW average across 4,500 annual hours.
The load profile shows 20 percent of hours at full airflow, 45 percent at 80 percent, and 35 percent at 60 percent. Applying the cube law gives 4.0, 2.05 and 0.86 kW respectively, weighted to about 1.9 kW average.
Annual use therefore drops from 16,200 kWh to roughly 8,600 kWh. That is 7,600 kWh saved, or about 1,140 a year at 0.15 per kWh, before any maintenance saving. Our AC to EC retrofit guide covers what it takes to deliver it.
Step 5: verify afterwards
Almost everybody skips this step, yet verified energy savings are what make the next project an easy approval.
Log the fan again six months after installation, under comparable weather and occupancy. Then compare against the original profile rather than against the estimate.
For measurement and verification protocols, the US Department of Energy building technologies resources provide the standard frameworks used in commercial projects.
Savings calculation FAQ
How long should I log before calculating?
A week including a weekend is usually enough for a building with a regular pattern. Seasonal plant needs logging in more than one season, or a documented assumption instead.
Can I use current instead of power?
It is a rough guide only, since power factor changes with load on an induction motor. Where the number has to be defended, log true power.
What if I cannot fit a logger?
Use the fan curve and measured static pressure to estimate absorbed power at several operating points. State the assumption clearly, because it is an estimate rather than a measurement.
Should I include reduced cooling load?
Yes in conditioned spaces, since every watt of fan power becomes heat that something must remove. In refrigeration the effect is large enough to change the payback.
Measured load profile, cube law, motor efficiency, tariff. Calculate energy savings that way and the number survives scrutiny instead of collapsing under it.
