How to use the motor energy cost calculator
Electric motors driving pumps, fans and compressors account for a large share of an industrial power bill. Because a motor usually costs far less than the electricity it consumes over its life, a difference of one or two efficiency points adds up to real money. Enter nameplate horsepower, load factor, efficiency and annual run hours to see actual draw and yearly cost.
The sequence is shaft power = rated output ร load factor, then input power = shaft power รท efficiency. Horsepower is converted at 0.746 kW per HP. Multiplying by annual run hours gives kilowatt-hours, and the electricity rate turns that into an annual cost. Entering a NEMA Premium efficiency in the last field shows the yearly savings from an upgrade.
NEMA Premium levels defined in NEMA MG 1 Table 12-12 run roughly from 89 percent on small motors to about 95 percent on 100 HP and larger four-pole TEFC machines, so a 40 HP replacement is typically near 94 percent. The average US industrial retail rate has recently run in the range of 8 to 9 cents per kilowatt-hour, but check your own tariff before making a decision.
If the load factor stays below 50 percent the motor is probably oversized, and both efficiency and power factor suffer there. A variable frequency drive on a pump or fan often saves more than a straight motor swap. These figures exclude demand charges and time-of-use rates, so verify against actual billing data.
Frequently asked questions
Measure running current with a clamp meter and divide by the nameplate full-load amps for a rough figure. A power meter reading input kilowatts compared with rated input power is more accurate.
Divide the extra purchase cost by the annual savings shown here for a simple payback period. Machines running more than about 4,000 hours a year usually pay back quickly.