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Annual savings of 90% vs 85% efficient motor

Short answer

Annual cost saving ≈ Rs 73,137, found from output 18.65 kW, input powers 21.94 kW (85%) and 20.72 kW (90%), giving 1.219 kW saving and 9752 kWh/year at Rs 7.5/kWh.

Calculate the annual cost savings of a 25 hp motor operating at 90% efficiency compared to 85% efficiency, if the operating hours is 8000 hrs/year and the energy cost is Rs 7.5 per kWh.

Energy saving comes from the lower input power of the more efficient motor for the same shaft output. Assume the motor runs at full rated output (25 hp) for all 8000 h.

Output power:

Po = 25 hp × 0.746 kW/hp = 18.65 kW

Input power at each efficiency (Pin = Po / η):

At 85 %: Pin1 = 18.65 / 0.85 = 21.941 kW
At 90 %: Pin2 = 18.65 / 0.90 = 20.722 kW

Power saved:

ΔP = 21.941 − 20.722 = 1.219 kW

(Formula form: ΔP = hp × 0.746 × (1/η1 − 1/η2).)

Annual energy saved:

ΔE = 1.219 kW × 8000 h = 9751.6 kWh/year

Annual cost saving:

Saving = 9751.6 × 7.5 = Rs 73 137 per year
Item85 % motor90 % motor
Output18.65 kW18.65 kW
Input21.94 kW20.72 kW
Energy/year175 529 kWh165 778 kWh
Cost/yearRs 1 316 471Rs 1 243 333

If the high-efficiency motor costs, say, Rs 50 000 more, the simple payback is 50 000 / 73 137 ≈ 0.7 year, which shows why energy-efficient motors are recommended in energy audits.

Answer: Annual saving ≈ 9752 kWh, i.e. about Rs 73 140 per year (at full load).

✓ Answer checked by Claude Opus 5.5 (high effort)

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