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Variable Speed Drive inverter

Energy Calculations​


The effective monthly energy savings and resultant operational cost savings when using spa pump motor with a VSD can be calculated using the following formulas:


Approximate Monthly energy savings (kWh) = number of operational hours per day × 30 × motor rated power (kW) × (1/rated motor efficiency) × (1-(reduced speed/rated speed)3).
Monthly cost savings (£) = Monthly energy savings (kWh) × average kWh unit price (£).
Payback time (months) = Price of VSD (£) / Monthly cost savings (£).

Example 1:
Domestic spa system is used for 4 hours every day. The typical rated power of pump motor is 1.5kW with efficiency of 80%.

Calculate the monthly energy and cost savings with a variable speed drive enabled speed reduction of 20%, and average kWh unit price of £0.15. Also, calculate the payback time for a VSD that is priced at £400.

For 20% speed reduction:
Monthly energy savings (kWh) =  4 × 30 × 1.5 × (1/0.8) × (1-0.83) = 109.8.
Monthly cost savings (£) = 109.8 × 0.15 = 16.47.
Payback time (months) = 400/16.47 = 24.

Example 2:

Commercial spa system is used for 12 hours every day. The typical rated power of pump motor is 1.5kW with efficiency of 80%.

Calculate the monthly energy and cost savings with a VSD enabled speed reduction of 10% and 20%, and average kWh unit price of £0.15. Also, calculate the payback time for a VSD that is priced at £400.

For 10% speed reduction:
Monthly energy savings (kWh) =12 × 30 × 1.5 × (1/0.8) × (1-0.93) = 182.93.
Monthly cost savings (£) = 182.93 × 0.15 = 27.44.
Payback time (months) = 400/27.44 = 14.5.


For 20% speed reduction:
Monthly energy savings (kWh) =4 × 30 × 1.5 × (1/0.8) × (1-0.83) = 329.40.
Monthly cost savings (£) = 329.4 × 0.15 = 49.41.
Payback time (months) = 400/49.41 = 8.