10.3: Calculating Power Costs
 Page ID
 7188
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It is important for water managers to determine the potential costs in electricity for pumping water. Units used for measuring electrical usage are typically in kilowatthours (kWHr). In order to convert horsepower to kilowatts of power, the following conversion factor is used.
1 horsepower = 0.746 kilowatts of power
Once you know the hp that is needed you can then determine the amount of kWHr needed. Then, costs can be determined depending on what the local electric company charges per kWHr. Water utilities will calculate estimated budgets for pumping costs since these are typically the largest operating costs.
Exercises
Solve the following problems.

A well flows an estimated 3,200 gpm against a discharge head pressure of 95 psi. What is the corresponding hp and kW‐Hr if the pump has an efficiency of 70% and the motor 88%?

Based on the above question, how much would the electrical costs be if the rate is $0.12 per kW‐Hr and the pump runs for 10 hours a day?

A utility has 3 pumps that run at different flow rates and supply water to a 500,000gallon storage tank. The TDH for the pumps is 210 ft. The utility needs to fill the tank daily and power costs are to be calculated at a rate of $0.135 per kW‐Hr. Complete the table below.
Pump 
Flow Rate 
Hp 
Efficiency 
Run Time 
Total Cost 

1 
500 gpm 
50 

2 
1,000 gpm 
75 

4 
2,000 gpm 
250 
 A well draws water from an aquifer that has an average water level of 150 ft bgs and pumps to a tank 225 ft above it. Friction loss to the tank is approximately 22 psi. If the well pumps at a rate of 2,300 gpm and has a wire‐to‐water efficiency of 62% how much will it cost to run this well 14 hours per day. Assume the electrical rate is $0.13 per kW‐Hr.

A utility manager is trying to determine which hp motor to purchase for a pump station. A 400 hp motor with a wire‐to‐water efficiency of 65% can pump 3,000 gpm. Similarly, a 250 hp motor with a wire‐to‐water efficiency of 75% can pump 2,050 gpm. With an electrical rate of $0.155 per kW‐Hr, how much would it cost to run each motor to achieve a daily flow of 2 MG? Which one is less expensive to run?

Approximately 224 kW of power are needed to run a certain booster pump. If the booster has a wire‐to‐water efficiency of 67.5% and is pumping against 135 psi of head pressure, what is the corresponding flow in gpm?

It costs $88.77 in electricity to run a well for 7 hours a day. The well has a TDH of 100 psi and an overall efficiency of 58.3%. The cost per kW‐Hr is $0.17. What is the cost of the water per gallon?

Complete the table below based on the information provided.
Well 
Flow (gpm) 
Run Time (Hr/Day) 
WiretoWater Eff 
Head Pressure (psi) 
hp 
Cost/Year ($) @ $0.135/kWHr 

A 
750 
18 
68% 
110 

B 
1,800 
13 
61% 
85 

C 
2,750 
12 
57% 
95 