10.3: Calculating Power Costs

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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 kilowatt-hours (kW-Hr). 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 kW-Hr needed. Then, costs can be determined depending on what the local electric company charges per kW-Hr. 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 500,000-gallon 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
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)	Wire-to-Water Eff	Head Pressure (psi)
A	750	18	68%	110
B	1,800	13	61%	85
C	2,750	12	57%	95