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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 (£).

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.

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.

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.

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.

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