Motor Efficiency and Energy Cost Calculator
Over a motor's life, electricity costs many times its purchase price, so a few points of efficiency matter. This calculator shows the annual running cost, the losses and the payback of a higher efficiency replacement.
Your numbers
Measured with a power meter if possible.
Net of any rewind or scrap value avoided.
Your results
Efficiency changes with load, and many motors are least efficient below about 40 percent load. Use the efficiency value at the actual operating load from the data sheet or a measurement. Savings ignore demand charges and maintenance differences.
Annual energy cost versus load
Sweeps Average load from 20 to 100, holding your other inputs fixed.
Rank all your motors by savings potential
Optional expert review of pulling asset and runtime data to build a motor replacement priority list.
Optional. The tool and every export work without this. No spam.
How to use the calculator
Enter the motor rated power, the average load as a percentage of rated and the efficiency at that load. Add the annual running hours and your electricity price. Then enter the efficiency of the candidate replacement and the extra cost over a like for like alternative. The output gives shaft power, electrical input, losses in kW, annual cost, the annual saving and the simple payback in months. The chart shows how cost scales with load so you can see the effect of a lightly loaded motor.
- Use a clamp meter or logger to find the true average load
- Take efficiency from the data sheet at the real load point
- Count only the hours the motor actually runs
- Include the cost difference, not the full motor price, when repairing is the alternative
The formulas
Shaft power equals rated power times load fraction. Electrical input equals shaft power divided by efficiency. Losses are input minus shaft power. Annual energy is input power times hours, and annual cost is energy times price. Savings from a new motor are the difference in input power times hours times price. Payback is the extra cost divided by annual savings. These are steady state estimates and ignore starting current and harmonics.
Interpreting results
A payback under one or two years on a continuously running motor usually justifies replacing instead of rewinding. If the motor is oversized and runs below half load, fixing the sizing or adding a variable speed drive on a fan or pump often saves far more than a better efficiency class. Also note that rewinding can lower efficiency by a small amount if not done to a good standard.
Common mistakes
People often use the nameplate efficiency at full load while the motor runs at 50 percent. Another mistake is assuming 8760 hours when the motor is idle part of the year. Forgetting that power factor and demand charges affect the real bill, or ignoring the carbon cost, can also skew the case. Verify with a short power logging study before approving capital.
Frequently Asked Questions
How much does it cost to run a motor for a year?
Multiply electrical input power in kW by running hours and the tariff. A 75 kW motor at 75 percent load and 92 percent efficiency draws roughly 61 kW, so at 6,000 hours and 0.10 per kWh it costs about 36,700 per year. Use the calculator for your numbers.
Is it worth replacing a working motor with a premium efficiency one?
Often yes for large motors running most of the year, because the energy saving pays for the premium in a short time. For small motors with low hours the payback is long, so replace on failure instead. Run your own numbers before deciding.
What is IE3 and IE4?
They are efficiency classes defined in IEC 60034-30-1. IE3 is premium efficiency and IE4 is super premium. The classes set minimum efficiency by power rating and poles, so compare actual data sheet values for your motor size rather than the class label alone.
Why does low load reduce efficiency?
Fixed losses such as core and friction losses stay roughly constant while output falls, so they take a larger share of input. Below about 40 percent load, efficiency and power factor drop noticeably. Right sizing or speed control helps.
If you want motor running hours and loads pulled from your historian or ERP to rank every motor by savings potential, Netray can scope that analysis.
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