Variable speed drive saving calculator
Fan and pump power goes as the cube of speed. Slowing to 80% of flow needs only about half the power, which is why drives pay back so fast.
kW
%
per kWh
%
%
Payback period
0.50years
- Annual saving
- 8940.78
- Power on a drive
- 5.299 kW
- Power when throttled
- 12.750 kW
- Power reduction
- 58.4 %
- Energy saved per year
- 44 704 kWh
- Net saving over ten years
- 84 907.80
- Head as a fraction of full speed
- 0.4900
Show the working
- Motor power = 15
- Average flow required = 70
- Running hours per year = 6000
- Electricity price = 0.2
- Power when throttled = 85
- Drive and installation cost = 4500
- Drive losses = 3
- speed_ratio = 70 / 100 = 0.7
- Power on a drive = 15 * 0.7^3 * (1 + 3/100) = 5.299 35
- Power when throttled = 15 * 85/100 = 12.75
- saving_kw = 12.75 - 5.299 35 = 7.450 65
- Energy saved per year = 7.450 65 * 6000 = 44 703.9
- Annual saving = 44 703.9 * 0.2 = 8940.78
- Payback period = 8940.78 > 0 ? 4500 / 8940.78 : 0 = 0.503 312
- Power reduction = 12.75 > 0 ? 7.450 65/12.75*100 : 0 = 58.4365
- Net saving over ten years = 8940.78 * 10 - 4500 = 84 907.8
- Head as a fraction of full speed = 0.7^2 = 0.49
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- affinity laws: flow scales with speed, head with speed squared, power with speed cubed
- saving = throttled power - drive power
The cube law only holds for a purely friction system. A system with static head — pumping uphill — saves much less.
Drives also cut motor wear and let you soft start, which are worth something the arithmetic does not show.