Terminology Used in Assessing Farm Equipment Efficiency

Assessing Farm Equipment Efficiency involves the following terms:

  • Load and Proper Loading of Equipment
  • Efficiency
  • Energy Use
  • Electrical Demand Rate

Load and Proper Loading of Equipment

Determining if a motor is properly loaded enables you to make informed decisions about when to replace it and which replacement to choose.

Load is the burden imposed on a motor by the driven machine; the torque required to overcome the resistance of the machine it drives.

Most electric motors are designed to run at 50% to 100% of rated load. Maximum efficiency is usually near 75% of rated load. Efficiency drops if a motor is underloaded and, overloaded motors can overheat, lose efficiency, and have a shortened life span. Most motors are designed with a “service factor” that allow them to handle overloading of a certain amount for a short period of time.


Intuitively, we know that the more efficient equipment is, the less it will cost to operate. Simply put, efficiency is the comparison of what we put in to what we get out. To make smart decisions about which particular piece of electrical equipment to buy, we need to calculate or measure efficiency in a comparable way. The Federal government has standard methods for estimating efficiency that apply to cooling, refrigeration and heating equipment, intended to provide estimates of efficiency that reflect real world conditions. These ratings are analogous to vehicle MPG ratings – they are intended to provide guidance in comparing products.

Efficiency is the ratio of input power to usable output power.
Efficiency is defined slightly differently for different types of equipment:

  • Motor Efficiency is the ratio of electrical power input to mechanical power output.

Energy Efficiency (%) = [0.746 x HP x Load] / Power Input
where HP = horsepower;
1 HP = 746 Watts = 0.746 kW

  • Pump Efficiency is Water Horse Power divided by Pump Horse Power for specific operating conditions. Water horse power (WHP) is determined by water flow rate (Q) and the operating pressure (P) of the pump.

WHP = Q x P /C
where C is a constant

For Q in gallons per minute (GPM) and P (TDH or total discharge head) in feet of water, WHP = Q x P / 3,960.
For example, a pump that delivers 265 gallons per minute with a discharge pressure of 30 feet of water, water horse power is 265 x 30 / 3960, or about 2 horsepower. If the electrical requirement for the pump under those conditions is 3 horsepower, then the pump efficiency is 66%.

Remember the actual efficiency of a pump depends on the specific conditions that may not correspond to information listed on the nameplate. That is, pump efficiency listed on a nameplate may not be a good indication of performance under actual conditions. Manufacturer product literature typically provides performance curves for pumps which indicate the expected efficiency over a range of conditions.

  • Fan Efficiency is Volumetric Air Flow Rate (usually CFM) divided by power input (Watts) for specific operating conditions. Fan comparisons should be based on fans operating at the same static pressure (static pressure is the air pressure exerted on the fan when it is at rest). Fan nameplates do not usually have enough information to estimate efficiency. The manufacturer’s product literature will provide more detailed performance information.
  • Air Cooler Efficiency is the output cooling divided by the input power for specific operating conditions over a season.

The Federal government standards for cooling with air conditioners or heat pumps, is EER, or energy efficiency ratio. This is a number that attempts to quantify operating performance over an entire cooling season.
SEER, or seasonal energy efficiency ratio, represents the performance of cooling equipment for specific indoor and outdoor temperatures, and is the output cooling (in BTUs per hour) divided by the input electrical power (in watts).
Older air conditioners may have SEER ratings of 9 or less, but since 2005 central air conditioners must have SEER ratings of 13 or higher. Equipment with higher efficiencies (SEER 20 or better) is available. EER ratings are about 88% of SEER ratings.

  • Heat Pump Efficiency is the output heating divided by the input power for specific operating conditions over a season. Heat pump heating ratings are expressed as Heating Seasonal Performance Factors (HSPF). HSPF ratings reflect average performance during the heating season. Older units may have HSPF ratings as low as 6. HSPF ratings of 7 or greater meet Energy Star requirements, and highly efficient units can have ratings as high as 10.
  • Combustion Heating Equipment Efficiency is most simply expressed as the energy content of the input fuel divided by heat output under specific conditions. The Federal standard for rating heaters is annual fuel energy efficiency (AFUE). Nameplates on older equipment may simply list thermal efficiency at specific conditions, but newer equipment is more likely to list AFUE.

In practice, there are many ways to determine efficiency based on the information you have available: stamped on a nameplate, communication from the manufacturer, found on nominal efficiency value tables, and direct-read
power measurements. In addition, there are three categories of sophisticated methods of determining efficiency: special devices, software methods, and analytical methods. The National Electrical Manufacturers Association (NEMA) provides a free software tool to help make comparisons between equipment efficiencies.