Pump and Fan VFD Savings Screener
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A VFD quote can promise a big saving, but pumps with static head do not obey a brochure. Use the cube law as an upper-bound screen, then test the duty before buying.
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The formula we use
newPower = kW * (1 - r/100)^3
savings kWh = (kW - newPower) * hours
cost saved = savings kWh * rate
- The cube law assumes friction-dominated systems.
- Static head, throttling style and duty cycle change it a lot.
- Treat as an upper-bound screening.
Typical numbers
Reference figures to sanity-check your inputs. Your own bill and equipment data always win.
For centrifugal pumps and fans, a 20% speed reduction cuts ideal shaft power by about 49%.
Source: Affinity law
For centrifugal pumps and fans, a 10% speed reduction cuts ideal shaft power by about 27%.
Source: Affinity law
At 50 Hz, a 4-pole induction motor has 1,500 rpm synchronous speed before slip.
Source: Synchronous-speed identity
BEE's MSME guideline table shows a 30 kW, 4-pole IE3 motor at 93.6% efficiency versus 92.3% for IE2.
Source: BEE MSME Guidelines, 2019
BillTrends' screening default treats motor-terminal fixed capacitors as capped at 90% of motor no-load kVAr.
Source: BillTrends default model
Common questions
Why does the calculator use a cube?
For friction-dominated pumps and fans, power roughly follows speed cubed. It is a screen, not a guarantee.
When can VFD savings be much lower?
High static head, already efficient control, low running hours, minimum flow needs or poor duty cycle can reduce savings sharply.
Should I install a VFD only from this result?
No. Measure flow, head, throttling, hours and motor loading before approving capex.