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One of the most vital uses of a home power system is to power a water well pump. A pump can be a real power hog! Conventional pumps require a high surge of current in order to start. The entire circuit, from batteries to invert to pump, must be sized to handle the starting surge at the same time as other loads. Otherwise, the inverter will shut down. Use the following chart as a guide to inverter sizing.

Minimum continuous power rating of an inverter in watts to start an AC submersible well pump (with no additional loads)

Pump HP
Min. Rating of Modified-Sine Wave Inverter
Min. Rating of Sine Wave Inverter
Max. Running Power*

* Typical running power is 10-20% less, but add 10% for typical inverter loss. For "modified sine wave" add another 15%, due to loss of efficiency in the motor.

An inverter sized by these minimum guidelines will dip its voltage during the starting surge. This is not harmful, but it will cause lights to dim. Fluorescents may blink off, and computers are likely to crash. To eliminate voltage dips, oversize the inverter by an additional 50% minimum plus the watts capacity required to handle other household loads at the same time.

Minimum inverter sizing is based on field experience with Trace inverters, allowing ~25% voltage drop during startup. To eliminate noticeable voltage dip, add 50% to the minimum size. Other brands of inverters differ in their surge capacity relative to continuous rating. Exact starting capacity is difficult to predict and inverter manufacturers are hesitant to specify it. Dankoff Solar (now Conergy) welcomes your feedback and will publish more information as a result.

If a "modified sine wave" inverter is to be used and pump's control box is labeled "solid state", then it must be changed. Obtain a relay-type control box or a relay conversion kit, from any pump supplier.

If the pump is a "two-wire" type (having no control box), oversize the inverter by an additional 50%. A two-wire pump may not always work on a modified sine inverter.

Most well pumps require 230 VAC. Either two stacked inverters, or an inverter with 230V output, or a transformer must be used. (The Trace T-240 transformer will handle 2 HP max.). If all of this is too expensive for your situation, consider replacing it with a lower power pump, carefully selected for the best efficiency (watts per gallon). You can also consider an intermediary storage tank with a DC pressurizing pump. The use of a storage tank will relieve your well pump from the need to start every time the pressure runs low (many times per day). You can pump into the storage tank just once or twice per week, and then use a DC pump to supply the water pressure as needed (or use gravity flow, if feasible). See DC Pressurizing Pumps for Domestic Water Supply and Irrigation. You may also be able to change to a lower power well pump, even a DC well pump, after this step is taken because less pressure and less flow will be required from the well pump.

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