Solar Water Pump Inverter: correct sizing essentials

The correct sizing of a solar water pump inverter is crucial for the optimal, stable operation of the pump system, especially when running a submersible water pump.

Inverter sizing for submersible solar water pumps

Depending on the application, solar water pumps, particularly submersible (deep-well) pumps need a high amount of power. For the pump system start up, a high current surge must be handled, requiring a fit and correctly sized inverter for the solar water pump system. The complete circuit, including the batteries, should be adjusted to sustain the start up surge concurrently with other system loads. If improperly sized, the solar water pump inverter in the circuit can shut down, offsetting the correct operation of the pump systems which can be a very serious problem in applications where continuous water supply, such as for feed-stock irrigation and drinking water supply, is crucial. The minimum continuous watt rating of a solar water pump inverter is required to start off a submersible water pump without additional loads. Reference calculation values: [caption id="attachment_4986" align="aligncenter" width="300"] Solar water pump inverter sizing table[/caption] The standard average running power is usually between 10-20% with typically about 10% of power losses in the inverter. The modified sine-wave inverter will take the losses to 15% because the motor efficiency is affected negatively.

Inverter oversizing pros and cons

Sizing of an inverter using the minimum guideline above will dip inverter voltage during the start up surge. This is acceptable, however lights will be dim and may also cause crashing of computers or make fluorescents to blink off. Voltage dips can be eliminated by over-sizing the inverter with an extra 50% minimum in addition to the watts capacity needed to sustain other loads concurrently. However, this is based on trace inverter field experience which allows a -25% drop in voltage when starting. An additional 50% of the minimum size (oversizing)should be made to eliminate a significant drop in voltage. Some inverter brands vary in surge capacity in relation to continuous rating. The precise inverter startup capacity is hard to tell and some manufacturers are unwilling to disclose it. Should a modified sine-wave inverter be applied and the control box of the pump be labeled “solid state”, the latter should be replaced. A relay-type control box can be used as a replacement or a relay conversion kit from a reliable pump supplier.

Cost/efficiency ratio essentials

For a 2-wire pump without a control box, the solar water pump inverter should be oversized with an extra 50%. The efficiency of this pump can be affected negatively with a corrected sine-wave inverter. Water pumps mainly need 230VAC. They can use 2 inverters stacked together or a single inverter. The most important thing is to realize an output of 230V. Otherwise, a transformer should be used instead. There are cost implications while sizing inverters and when looking for the best choice to fit your application. Cost cutting can be avoided by selecting a low power-high efficiency pump based on watts/ gallon. A pressurizing pump drawing a DC current installed in an intermediary storage tank can also be useful. It helps in relieving the water pump from starting on low pressure runs and the number of times it will need to do that in a day. Maximum output is achieved both for power and water pumping.