I have a typical rural water system—a jet pump pulling water from a shallow well and pumping into a pressure tank. When we lose electrical power, we have no water—a situation that I would like to remedy with as much simplicity as possible.
I would like to install a DC-powered pump in the well and have it run whenever water is needed in the house, eliminating the need for a pressure tank, an elevated storage tank, or a storage tank with a pressure booster pump.
I’ve made the assumption that once the lines in the house have been filled by the pump, they would remain pressurized, and a flow sensor or pressure sensor could be used to turn on the pump. I would use batteries to power the pump with DC, and charge them from the grid. Our water usage is less than 100 gallons on most days. Even a worst-case draw of 500 gallons per day at 5 gallons per minute (which I think is more than we need) would require 100 minutes of actual pump run time. My well pump is 17 years old, and I want to be ready with an alternative when it fails.
Jim Yannaccone • Turbotville, Pennsylvania
Your project has several variables, so let’s take them one at a time. First, the pump: A jet pump (like yours) works from above, “pushing” some of the water down one pipe to help push more water up a larger pipe. It is the least energy-efficient type of well pump. In your quest to protect your water supply from power failures, you are correct to seek a more appropriate pump.
If your groundwater is no more than 20 feet below your wellhead (at pump level), a surface pump should work. A low-power, efficient DC surface pump to supply low water needs like yours would be the Flowlight Booster Pump, available from many off-grid RE suppliers.
However, if your groundwater level is (or may ever be) more than 20 feet down, a submersible pump is the best choice. Your well casing—the outer plastic or metal pipe that defines your well—will need to be no smaller than 4 inches (inside diameter). If you have only a 2-inch well casing (the minimum for jet pumps), you will need to have a new well drilled to use a submersible pump.
A variety of submersible pumps are compatible with solar electricity. Contact an experienced designer and supplier of solar pumps, specifically for a battery-based system that can supply the lift plus the pressure that you require. If you don’t have a local off-grid specialist, then check the Internet for an experienced solar pump supplier. If your water is not more than 200 feet down, you can install the pump by hand on flexible pipe, without heavy equipment.
You will need some form of storage and pressure delivery. If you use a solar pump without a battery (that is, PV-direct), you must have water storage and it will need to be elevated to deliver pressure. To deliver adequate pressure for a modern home (43 psi), though, your storage tank would need to be 100 vertical feet higher than your house. Most folks aren’t able to meet that kind of tank height to create the pressure they want. So I’d advise keeping your pressure tank—the modern alternative to the elevated tank—since it imposes no significant energy loss. Using a pressure tank also helps modulate the pump’s run time, so it can run for a few minutes, then stop. As water is drawn out and the pressure drops, the pressure switch will trip and re-activate the pump. I recommend using a pressure tank with at least 40 gallons of capacity to minimize the pump’s on/off cycling.
Along with a pressure tank, running your pump on demand (when water is needed) using a battery system would be a great solution. (You could also run some lights and other devices from the same system.) If the battery is charged from the power grid, you have what is termed an “uninterruptible power supply.” If you use a PV array for primary charging, you’ll have more security when the grid goes down. When it’s up, you can use it for backup charging.
Windy Dankoff • Founder (retired), Dankoff Solar Pumps