Can a UPS Run a Well Pump?

No desktop or tower UPS will start a submersible well pump. The obstacle is not running power but the current the motor pulls before it turns. Franklin Electric's motor table puts that at 32.2 A for a 1/2 HP 230 V two-wire motor. A 1500 VA UPS supplies about 12.5 A, at 120 V, from outlets a 230 V pump cannot use.

What a well motor draws at start, from Franklin's tables

Franklin Electric's AIM manual for 60 Hz submersible motors lists full-load watts, maximum-load watts (at the service factor) and locked-rotor amps, the current drawn before the shaft turns. Describing full-voltage starting of its three-phase motors, Franklin says current goes "from zero to locked rotor amps, then drops to running amps at full speed," which "may dim lights" and cause "momentary voltage dips to other electrical equipment" (the voltage sag you see when a well pump kicks on). A UPS inverter has to supply that peak or it shuts down.

The only 115 V motor in the table is the 1/2 HP two-wire, and it draws twice the amps for the same 7.4 kVA. From 3/4 HP up, every motor listed is 230 V. Three-wire motors, which start through a control box, draw less at start than two-wire motors of the same size, but even the smallest needs 5.3 kVA.

Lined up against real UPS ratings

Take a common 1500 VA unit. CyberPower lists the CP1500PFCLCD at 1500 VA/1000 W, 120 VAC on battery, NEMA 5-15R outlets, two 12 V 9 Ah batteries, and 2.5 minutes of runtime at full load. At 120 V, 1500 VA is 12.5 A.

A UPS facing that load does what the UPS overload alarm guide describes: it alarms, and on battery it shuts its output off to protect the inverter. The motor never reaches speed. Franklin adds that restarting a two-wire motor "within 5 seconds after power is removed may cause the motor overload to trip," so repeated attempts can trip the motor's own protection too.

Wiring is the other wall. A 230 V pump runs on a hardwired 240 V circuit, and a desktop UPS has a 120 V plug and 120 V outlets. Backing up that circuit means an inlet and transfer switch or a hardwired system (hardwired vs plug-in UPS).

The "1 to 3 kVA for 1 HP" figure, and why it is wrong

Search for well pump backup and vendor pages say a 1 HP pump "typically requires 1-3kVA UPS." That is the line on VirtueUPS's well pump page, which sells units in that range. The same page then contradicts it: a pump's "starting current can be 5 to 7 times higher than the current during normal operation," so you "must ensure that the peak output power of your selected well pump UPS exceeds the 7000W starting power."

The second statement is the right one, and Franklin's numbers back it: 48.7 locked-rotor amps at 230 V is 11.2 kVA for a 1 HP two-wire motor. The 1 to 3 kVA range persists because it matches the running load, 1,210 W at full load and 1,600 W at maximum, which a 2 or 3 kVA UPS carries once the motor is turning. Sizing from running watts is how most UPS sizing works (VA vs watts explains the ratings), and it is exactly the step that fails for an induction motor.

What actually backs up a well

A generator sized from Franklin's table

Franklin publishes minimum generator ratings, and they are lower than the kVA-at-start column above because a generator sags rather than shutting off. Generators "must be sized to deliver at least 65% of the motor's rated nameplate voltage during starting," and Franklin notes that "using the minimum rating or sized generator acts as a soft start to the motor."

Franklin's table note: "For best starting of 2-wire motors, the minimum generator rating is 50% higher than shown." Its operating rules matter as much as the size. Transfer switches "must be used any time a generator is used as standby or back up." Start the generator before the motor and turn the motor off before the generator, because "the motor thrust bearing may be damaged if the generator is allowed to coast down with the motor running or if it runs out of fuel." UPS vs generator lays out the wider trade-off, and if the generator also feeds UPS-protected electronics, UPS not working with a generator explains why a UPS may reject generator power.

A split-phase inverter/charger sized on current

Battery systems can start a well pump, at a different scale from a UPS. One example of the class: Schneider Electric's Conext XW Pro 6848 NA, a 48 V inverter/charger with split-phase 120/240 V output, rated 6,800 W continuous and 12,000 W for 60 seconds, with maximum 60-second output current of 52 A at 240 V.

Our method: an inverter limits its output current rather than sagging like a generator, so do not reuse the generator table. Compare the inverter's surge current at 240 V with the motor's locked-rotor amps. On that test the XW Pro covers a 3/4 HP two-wire motor (40.7 A) with about 28% to spare, but a 1 HP two-wire motor (48.7 A) with only about 7%, which leaves almost nothing for anything else running at the same moment. A 1 HP three-wire motor at 41.8 A is the better match. Confirm motor starting with the inverter maker before you buy.

Constant-pressure drives: no small controller to back up

A variable-speed drive soft-starts the motor, which shrinks the source it needs: Franklin's minimum generator for a 1/2 hp MonoDrive is 2,000 W, against 3,000 W for a 1/2 HP two-wire motor started directly. But the drive is not a small control box. It feeds the motor, so a UPS on the drive is a UPS on the pump. Franklin sizes generators for SubDrive models at 1.5 times maximum input watts (3,500 W for a SubDrive15, 5,700 W for a SubDrive20) and says its drives "have not been tested with inverter-controlled generators and AC-DC inverters." Get the drive maker's answer before buying battery backup.

The pressure tank: the water you have without power

When the power drops, the tank's drawdown, the water stored between the pump's cut-in and cut-out pressures, is all that reaches the taps. Amtrol's Well-X-Trol specifications list drawdown at a 40/60 psi setting:

EPA gives 1.6 gallons per flush as the federal toilet standard and 2.0 gpm as the WaterSense showerhead maximum (standard showerheads use 2.5 gpm). A hot shower also needs the water heater running; a gas tankless heater can fire on a small UPS, but only while the tank has water to give it. A bigger tank buys minutes, not a night. Its real backup value is fewer motor starts: Franklin limits single-phase motors up to 3/4 HP to 300 starts per 24 hours and 1 to 5.5 HP to 100, each run lasting at least one minute.

Where a UPS does belong in a well system

A UPS is still useful for the low-power electronics around a well: a water-level or leak alarm, a Wi-Fi pump monitor, and the modem and router that carry their alerts. When a generator backs up the pump, a UPS keeps computers and network running through the gap until it takes over; see using a UPS with a generator. A sump pump is the same motor problem with different backups, compared in UPS for a sump pump.

Where these numbers stop applying

• Other motors. The figures are Franklin's 4-inch single-phase motors. Another maker's motor of the same horsepower can differ; read the locked-rotor amps in your motor's own data before sizing anything.
• Jet pumps. This page covers submersible motors. An above-ground jet pump has its own motor data; size it the same way, from its locked-rotor current.
• Long cable runs. In its starter section, Franklin notes that a 5% cable voltage drop at running amps causes "about 20% reduction in starting current and about 36% reduction in starting torque." A source that sags on top of that may leave the motor short of torque to start.