ASK THE EXPERTS: Water Supply Backup

It’s easy to double your pressurized water storage for use during power outages by adding a second pressure tank.

I recently bought my dream home in the woods of southern Ohio. The home is served delicious spring water that is 400 yards away and some 80 feet downhill. A submersible 240 VAC pump sits at the bottom of a 1,000-gallon cement tank, powered by a separate electrical service from the local utility. A second 120 VAC pump and small pressure tank in the house provide good pressure there.

The problem is that we get many power outages due to downed trees, and each power cut equals losing our water service to the house. A backup gasoline generator keeps the house loads powered during outages.

I’m researching backup power options for the well pump. A PV array is not an option due to the spring’s location, which is at the bottom of a steeply wooded hill. Access to the spring is not easy; mechanical transport is not an option.

I was considering a battery backup solution with a trickle charge, which would keep the batteries topped off until the power goes out. But I’m having difficulty finding specifications that would accommodate a 240 VAC pump. An hour or two of backup would be great, and I would also like to add a generator to recharge the batteries during prolonged outages.

Tom Martin • Sugar Grove, New York

Battery backup for an hour or two may cost more than you think. I’m assuming your pump at the spring is rated at 1/2 hp, which equates to about 500 watts when running. Pumping for one hour would require 500 watt-hours (Wh). The battery bank and inverter reduce efficiency by about 30%, which means about 650 Wh would be required for an hour of pumping.

A lead-acid battery bank, both for longevity considerations and/or to deal with decreasing capacity in cold temperatures, would need a capacity of about 1,000 Wh (1 kWh) to serve this load. Batteries are traditionally rated in amp-hours (Ah). For example, a 24-volt battery bank would be rated at 1,000 Wh divided by 24 V, which equals about 42 Ah. The least expensive option in this case would be to purchase two 100 Ah, 12 V RV/marine flooded lead-acid batteries, at about $300. The downside is that these batteries require regular maintenance. Sealed lead-acid batteries are maintenance-free, but would be about double the cost.

Because your pump runs on AC, you’d also need an inverter to supply AC from the DC batteries, and, if your inverter doesn’t have 240 V output, a transformer to step 120 V up to 240 V to match the pump. You can expect to spend between $1,000 and $2,000 for this equipment.

Alternatively, you could replace the pump with a 120 VAC pump. You would also need wiring, safety devices, and weatherproof housing for the equipment, which may add another $1,000. Depending on how the batteries are maintained and the number and length of outages, you may need to budget battery replacement in five to seven years.

You could also consider replacing the 240 V AC well pump with a lower-voltage DC pump made especially for PV or battery power. It will use about half the energy and eliminate the need for an inverter. With this pump, you could then use a single 12 V battery and, as you suggest, an inexpensive trickle charger. A variety of DC pumps are available, ranging from about $100 for an RV-marine pump, to more than $700 for a long-lasting, specialized pump made for PV power. You will still have a significant installation cost, likely more than $2,000. DC pumps generally pump over longer periods of time. They may not last as long as a conventional AC pump, which runs only for short periods.

A simpler, low-tech approach would be to store water in a holding tank or cistern at the house. A tank is far less expensive than a battery backup system, and it is relatively maintenance-free, long-lasting, and reliable.

If you don’t run your backup generator continuously during power failure, you may wish to extend your supply of pressurized water by replacing your pressure tank with a larger one. Or, keep it and add a second pressure tank to your system—your new capacity will be the sum of the two, and they need not be matched in size. The second tank can be installed anywhere on a main pressure line and won’t require a second pressure switch. Even if you do nothing else, this will extend your water supply to weather most short-term outages.

Windy Dankoff • Founder (retired), Dankoff Solar Pumps

[Ed. note: Veteran solar installer Drake Chamberlin contacted the reader directly by phone. Their agreed-upon solution was to install a cistern for storing water at his house, which will get him through periods of utility outage. Tom is also considering a future PV system for his home.]

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