What size inverter is recommended for AC well pumps?
One of the most vital uses of a home power system is to power a water well pump. A regular AC 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 inverter 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 to start an AC submersible well pump (with no additional loads)
|
HP of Pump
|
Min. Rating of Modified-Sine Wave Inverter (KW)
|
Min. Rating of Sine Wave Inverter (KW)
|
KW Max. Running Power (Watts)
|
|
0.5
|
2.0
|
1.5
|
970
|
|
0.75
|
2.4
|
2.0
|
1325
|
|
1.0
|
2.8
|
2.5
|
1600
|
|
1.5
|
4.5
|
4.0
|
2150
|
|
2.0
|
0
|
4.5
|
2650
|
An inverter sized by these minimum guidelines will lower its voltage during the initial starting surge. This is not harmful, but it may cause lights to dim. 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.
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.
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. Another method to avoid the typical surge of an AC pump is to have a "soft start control box" this will allow a much smaller inverter to be used since the surge is reduced by increasing the amps to the pump gradually during initial start up.
If all of this is too expensive for your situation, consider replacing it with a lower power solar pump, which is much more efficient and 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 or cistern 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.
At the Solar Store, we will be happy to help find the products that are right for you. For pricing and product information give us a call. We'll also answer any questions you might have about your system. 1 (541) 388 3637