Purdue University
Cooperative Extension Service
West Lafayette, IN 47907

Comparable Costs of Various Energy Sources for Irrigation Pumping

Rolland Z. Wheaton and Edwin J. Monke
Department of Agricultural Engineering, Purdue University

Because of continued rising prices of petroleum products, many irrigators are looking more closely at the comparative costs of various energy sources for irrigation pumping. The purpose of this publication is not only to provide such a comparison, but also to alert you to other factors that must be considered in selecting a particular energy source.

Performance Standards for Various Energy Forms

From Nebraska Test data, standards have been developed for the horsepower-hours that engines or motors should produce using four different energy forms--gasoline, diesel fuel, propane gas and electricity. (A horsepower-hour is 1 horsepower produced continuously for 1 hour.) The results are shown in Table 1.

These standards are the best that can be expected for engines operating at 85 percent and electric motors operating at 100 percent of their horsepower ratings. If loadings are reduced below these percentages, the standards must also be reduced. For continuous operation, recommended loadings are 80 percent of maximum horsepower ratings for water-cooled engines, 65 percent for small air-cooled engines and 100 percent for electric motors.

Table 1. Horsepower-Hours of Work Output per Unit of Energy Input for Various Power Units.

                  Unit of
   Power Unit   energy input     Work output
 Gasoline engine  Gallon      11.3 horsepower-hour
 Diesel engine    Gallon      14.7 horsepower-hour
 Propane engine   Gallon      8.92 horsepower-hour
 Electric motor Kilowatt-hour 1.18 horsepower-hour

Determining Comparable Costs

The above standards were used to calculate the amount that can be paid for each of the energy sources to obtain an equal number of horsepower-hours per dollar spent. The price figures are presented in Table 2. However, they are only for the energy purchased, and do not take into account such factors as initial cost, maintenance requirements, convenience, energy availability and dependability of service.

Table 2. Comparative Costs of Various Energy Sources to Obtain Equal Work Output per Dollar Spent.

Gasoline  Diesel fuel  Propane gas    Electricity
(gallon)   (gallon)     (gallon)    (kilowatt-hour)
   .60       .78         .47            6.3
   .70       .9l         .55            7.3
   .80     $1.04         .63            8.4
   .90      1.18         .71            9.4
 $1.00      1.31         .79           10.4
  1.10      1.44         .87           11.4
  1.20      1.57         .95           12.5
  1.30      1.70       $1.03           13.5
  1.40      1.83        1.11           14.6
  1.50      1.97        1.19           15.6
  1.60      2.10        1.26           16.7
  1.70      2.23        1.34           17.7
  1.80      2.36        1.42           18.7
  1.90      2.49        1.50           19.8
  2.00      2.62        1.58           20.8

To compare costs between the different sources, find the applicable price for one source and read horizontally across the table. For example, at $1.00 per gallon for gasoline, an irrigator could afford to pay $1.31 per gallon for diesel, 79 cents per gallon for propane, or 10.4 cents per kilowatt-hour for electricity to get equal work output. Again, these `comparable' prices are for the purchase of energy only.

Special Considerations in Evaluating Electricity

To more accurately evaluate electric power, three important factors must not be overlooked--availability of an adequate source, line extension charges and minimum monthly or annual charges.

For instance, since most irrigation pumping only occurs over a 3-4 month period in Indiana, such short-term use is often subject to extra charges that can greatly influence the total cost. In fact, some irrigators have found that stand-by charges nearly equal the total charge for kilowatt-hours purchased, in effect doubling the unit price.

Each case will vary, so it is important that, initially, pricing structure and other arrangements be worked out with the electrical supplier.


Comparable cost--now and in the future--is a major consideration in selecting an energy source for irrigation pumping. But questions of availability, dependability and required maintenance must also be answered before finalizing your decision.

New 9/80

Cooperative Extension work in Agriculture and Home Economics, State of Indiana, Purdue University and U.S. Department of Agriculture cooperating: H.A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. The Cooperative Extension Service of Purdue University is an equal opportunity/equal access institution.