Pumps vs Pumps... I'm So Lost!

We are going to look at how to make the most of the energy that you are producing with your solar system and/or gas powered generator focusing on water pumping. Even though the water pump is probably not your largest overall power consumer for the month, it is quite possibly the single largest amp draw item on your house. This is especially true if you do not have an air conditioning unit, which most of us do not.

There are two very common types of off-grid pumps used to bring water from the storage

tank to the house. One is a small 12 or 24V RV style diaphragm pump which can move around 3gallons per minute at 30 PSI. This equates to running one faucet or shower head at a time. It has a limited draw ability, which means if it is not very close to the tank or in a gravity feed situation, these little pumps will either burn out very quickly or not work at all. They are, however, very energy efficient, only drawing about 90 watts. Note that they do have to be wired directly to the batteries with a safety breaker (or fuse), and they are they not known for their long life (average is one year if using it full time).

The second, and more common, type of pump is a shallow-well jet pump. These cast iron

pumps can draw water from 25’ below grade and are capable of moving over 20 gallons per

minute at 30 PSI depending on the pump size. They do draw considerably more power, however, with a ½ hp 110V pump drawing approximately 12 amps or 1440 watts. This is a far cry from the small diaphragm pump at 90 watts. Yet these pumps are capable of running an entire house with no issue. When sized properly, these pumps can easily supply enough water to have people taking showers, doing dishes and even running laundry at the same time if desired. When considering jet pumps, look for two things. 1) Quality of the pump housing, is it plastic or cast iron? 2) Is the motor a soft (capacitor) start or is it a hard (not capacitor) start? Always choose cast iron capacitor start motors. These will give you the longest life (average 10-15 years) on the pump with the best energy efficiency on the motor.

I can hear you saying, “Ok Mr. Off-Grid Guy, this is great info, but where do I save money in

all this?” Well that answer is three-fold. First, the high-efficiency pumps are about double the cost of the RV pump, but have a 10-15 year life span vs a 1-year life span for the RV pump. With an RV pump, you would spend 5-7 times more than the cost of the cast-iron pump over that 1-15 year period. Second, buying a high-quality pump that has the energy-efficient motor will help reduce your overall current draw on the system. Third, combining the high-quality pump with a pressure (or bladder) tank will reduce the number of times the pumps runs. The largest wear and tear on your pump and the highest power consumption is the split second it comes on. For that split second, it draws 3-5 times its rated amp draw and causes the most wear on the windings and seals. The fewer times per day that it starts up, even if it is on for a longer period of time, the better off you are. Installing a pressure tank on the system allows us to store water under pressure to be used when needed. This will allow the pump to come on fewer times per day since it only has to make up the water in the tank and not kick on and off every time you open a faucet.

For example, a 33 gallon pressure tank added to the system means the tank is capable of

supplying 11.3 gallons of water before the pressure will drop below the 30PSI mark, which is

where most pumps will start up. In this case, you could wash your hands several times, flush the toilet 5-7 times based on the toilet, and even take a quick shower without the pump ever coming on. Then once it did start, rather than remaining on for a few seconds it would run for a minute or two and replace the water in the storage tank before shutting off again.

Bottom Line – a properly sized pump and pressure tank can easily save you 20% of the

daily watt load on your system, not to mention extending the life of your pump, thereby reducing your replacement costs. As always, feel free to call us with any questions.