30 November 2000

Avoiding Efficiency Miscalculation

by Chuck Yung

With so much emphasis on energy efficiency, many industrial users are rushing to reduce operating costs by replacing standard electric motors with energy-efficient models. Properly applied, these motors do save energy, although it's sometimes far less than the amount used to justify the purchase. No one enjoys unpleasant surprises, so it's wise to avoid common mistakes when weighing the benefits of a new, energy-efficient motor.

If It Sounds Too Good To Be True…

While sales literature can sometimes be misleading, erroneous reports predicting huge savings also appear occasionally in publications. One recent article, for example, projected annual savings that actually exceeded the annual operating cost of the original motor. Once in print, inaccurate information can easily lead unsuspecting readers astray. In most companies, any justification for spending money will be verified. When actual savings are only a fraction of those projected, the result can be uncomfortable.

Calculating Energy Costs

It's fairly easy to calculate the energy cost for a three-phase motor using basic equations:

Equation 1

Even so, mistakes can be made, as demonstrated in the following examples.

Because power is billed per kilowatt hour (kWh), one place to err is in forgetting to convert watts to kilowatts. Since 1 kilowatt = 1,000 watts, the resulting calculated costs can vary greatly.

Another common error is to look at total energy cost, instead of the difference in energy costs for the motors being compared. A simple formula for calculating the difference in energy costs for a standard motor and an energy efficient replacement is:

Equation 2


Sa Savings (annual)
hp Horsepower
C Cost (dollars/kilowatt hours)
N Running time (hours per year)
Ea Efficiency of replacement motor
Eb Efficiency of original motor

Lastly, it's important to use realistic numbers when evaluating the benefits of energy-efficient motors. Savings are often calculated based on the assumption the motor will run fully loaded, 24 hours per day, 365 days per year. Then, when the motor is installed and operates only three or four hours per day, Monday through Friday, the anticipated savings don't materialize.

To avoid the fallout from overstating potential savings, refer to Table 1, which shows annual savings for energy-efficient motors. Based on average values for standard motors replaced by energy-efficient models, it shows the anticipated savings for typical horsepower ratings based on various operating duties and energy costs.

As an example, a 100-hp, 3,600-rpm, energy-efficient motor running continuously at seven cents per kWh would cost about $1,772 less to operate each year than a standard model. If, instead, the motor actually operates 40 hours per week, the savings would be only $424. This is less than one-fourth of the projected savings if the wrong figures are used to justify the replacement motor.

Table 1 shows the estimated annual savings (U.S. dollars) for energy-efficient motors operating at 90% of full load. This table assumes average values for the efficiency of standard motors replaced by energy efficient motors.

Effect of Load on Efficiency

From Table 1, it's easy to see that the actual savings for an energy-efficient motor depend on the hours of operation and electricity rates. Another factor to consider is load. Electric motors tend to operate most efficiently at 75-80% of full load. A motor operating below 50% of full load is very inefficient.

Given the tendency for industry to oversize motors, a great deal of energy could be saved simply by sizing motors correctly for the specific application. Often, to avoid specifying equipment and then later realizing that the motor needed to be just a little bigger, the specs are "padded" by one rating. The trouble is, by the time the design engineer raises the horsepower requirement and the project engineer does the same, the motor selected may be significantly oversized.

Free Software Support

When considering upgrading to energy-efficient motors, it's safest to double-check your figures. The U.S. Department of Energy (DOE) offers a free computer program called MotorMaster+ 3.0 that compares energy use and costs for a variety of electric motors. By inputting the actual electricity rate, hp rating, percent load, and operating hours, it's possible to calculate (and print) the projected savings for applications where an energy efficient motor is being considered. Users then have one printout per motor that lists hp, rpm, actual energy cost per kWh, annual operating time, and percent load-a sure way to lend credibility to data and motor selection.

To get a free copy of MotorMaster+ software, call the DOE's Office of Industrial Technologies Clearinghouse at (800) 862-2086, or visit http://mm3.energy.wsu.edu/mmplus/.

Properly applied, energy-efficient motors can and do save electricity and money. Savvy users will determine which motors in the plant are underutilized, consider their annual operating costs, and examine the efficiency of each application before replacing operating motors. Equally important is determining the actual horsepower required for each application. Not only are energy-efficient motors considerably more expensive than the ones they replace, but like all motors, they run less efficiently when lightly loaded. If an energy-efficient, 50-hp model can replace a 75-hp motor, the savings will be even greater.  MC

Additional Information

Figures and Graphics

Table 1
Table 1. Estimated annual savings for energy-efficient motors (source: Electrical Apparatus Service Association).

Author Information 
Chuck Yung is a technical support specialist at the Electrical Apparatus Service Association (EASA), an international trade organization consisting of more than 2,500 electromechanical sales and services firms in 59 countries. Through its many engineering and education programs, EASA keeps members up to date on materials, equipment, and state-of-the-art technology related to the sale, service, and maintenance of motors, generators, drives, controls, and other electromechanical equipment. Contact him at 1331 Baur Boulevard, St. Louis, MO 63132; tel: (314) 993-2220; fax: (314) 993-1269; www.easa.com