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PUMPS By Bill Green R Where


kWh = total kilowatt hours acre foot = volume of water pumped; 1 acre foot in this example 1.0241 = constant TDH = total dynamic head or total lift (pump pressure) OPE = overall pumping plant efficiency (wire to water)


In order to use this equation, we have to define the variables.


Kilowatt hours is the amount of kilowatts coming through the electric meter to the pump for a one-hour duration. Most farmers think in terms of horsepower, and converting it is simple. HP × 0.746 = kW or kW/0.746 = HP. If the meter says that the pump is using 74.6 kW while the pump is running, that would equal exactly 100 HP (0.746/74.6 kW).


Pumps generally don’t use the exact nameplate horsepower, depending on conditions, water, pressure, etc. Your pump may say it’s 100 HP on the nameplate, but it may be using 92 HP, 104 HP or some other number. But the meter shows how many kW, and that’s how the electric company bills you.


unning pumps for irrigation is becoming an expensive part of a farmer’s budget every year. Moving water takes energy, and energy always has a cost (with the exception of gravity-fed irrigation systems). That cost can vary greatly from one state to another and one region to another. Costs can vary seasonally and even daily in many cases. Installation and upkeep costs


on a single pump can range from a few thousand dollars to hundreds of thousands, depending on horsepower and equipment needed.


As a grower, it is valuable to be able to estimate the cost to move water with an installed pump and established irrigation system. This type of information can be useful for planning and budgeting purposes.


The basic energy unit depends on the energy source; in this example we will use electricity, which is billed in increments of kilowatt hours — kWh. You can also convert cost based on other energy sources such as diesel, gasoline, propane and natural gas. In these examples, kWh is the baseline.


Figuring pumping costs


The easiest way to figure pumping costs is to use the following equation for kWh to pump 1 acre foot of water. An acre foot is approximately 325,851 gallons of water, or it can be described as pumping 1 acre 1 foot deep. Most crops require anywhere from 1 to 5 feet of water per acre annually, so this example makes sense. The following equation helps clarify this point:


kWh/acre foot = 1.0241 × TDH/OPE


Electric meter showing kW Spring 2020 | Irrigation TODAY 19


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