Sizing Solar Systems for Off-Grid Water Supply — Key Considerations?

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Calculating Power Needs for Well Pumps When sizing a solar system for off-grid water supply, the first step is to calculate the power needs of your well pump. The required power depends on the pump type, flow rate, and pressure. For example, a submersible pump with a flow rate of 10 gallons per minute (gpm) and a pressure of 100 feet may require a power rating of 1.5 horsepower (hp). To calculate the required solar panel array size, multiply the pump’s power rating by the duty cycle, which represents the percentage of time the pump operates. Assuming a duty cycle of 50%, a 1.5 hp pump would require a solar panel array with a minimum power rating of 1.5 hp * 0.5 = 0.75 hp or approximately 540-600 watts.
Determining System Capacity To determine the required solar panel array capacity, consider the following factors: the number of users, water requirements, and desired water pressure. A general rule of thumb is to allocate 10-15 gallons of water per person per day. Based on this, a family of four may require a solar panel array with a capacity of 40-60 gallons per hour. To calculate the required system capacity, multiply the required daily water volume by the duty cycle. For a family of four with a daily water requirement of 40 gallons per person, and assuming a duty cycle of 50%, the required system capacity would be 40 * 4 * 0.5 = 80 gallons per hour.
Battery Backup and Energy Storage Requirements To ensure a stable and reliable water supply, a solar system with battery backup is recommended. The required battery capacity depends on the system’s duty cycle and the desired backup time. A general rule of thumb is to allocate 10-20% of the system’s total capacity for battery backup. Based on this, a 100 Ah battery bank would require a 10-20 Ah deep discharge battery. To calculate the required battery capacity, multiply the required backup time by the system’s power rating. For example, a system with a power rating of 5 kW and a desired backup time of 6 hours would require a 30 Ah deep discharge battery.