Calculating surge wattage for a well pump — how to do it?

Understanding Well Pump Surge Wattage

When sizing a solar inverter for a well pump, calculating the surge wattage is crucial. The well pump motor is designed to handle a specific load during its startup phase, which differs from the nominal power it operates at under normal conditions. This surge wattage is essential to ensure the inverter can provide the necessary power without failing.

Basic Calculation

A typical well pump motor consumes around 2-10 horsepower, depending on the application. To calculate the surge wattage, first, you need to convert the horsepower to watts. Since 1 horsepower equals 746 watts, a 2 horsepower motor would consume 1492 watts under normal operating conditions. When calculating surge wattage, multiply the motor’s horsepower by 3.5 to get an initial estimate, which in this case is 5233 watts. Then, multiply this result by 1.25 for a conservative estimate, yielding 6546.25 watts.

Efficiency Considerations

However, these calculations are simplified and don’t account for the motor’s efficiency, which typically ranges from 70 to 90 percent. To get a more accurate assessment, divide the calculated surge wattage by 0.8, which is a commonly used efficiency factor for well pump motors. This brings the surge wattage to 8166 watts, considering a 75 percent efficiency in this scenario. When sizing the solar inverter, you should use this calculated value to ensure a reliable power supply to the well pump during its startup phase.