Battery backup for a well pump: the 240V and start-current problem
Well pump makers publish the one number sump pump makers do not: locked rotor amps. Run that figure against the published surge ratings of 240V-capable power stations and most of the plans people make stop working.
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- Jackery Explorer 5000 Plus (with Smart Transfer Switch)Highest published surgeAmazon (affiliate link, opens in a new tab)
5,040 Wh with 7,200 W maximum through its 240V ports and a published 14,400 W surge, the only surge figure among the units we track that exceeds the locked rotor draw Franklin Electric publishes for a 1 HP submersible motor. The listing bundles Jackery's Smart Transfer Switch.
7,200 W continuous with a published 10.8 kW surge and 6,144 Wh per battery, expandable to 30 kWh on one inverter. The continuous rating is the widest here, which matters once the pump is running rather than starting.
3,840 Wh with 6,000 W maximum output, delivered only through its 240V-capable NEMA L14-30R port, since each 120V outlet group tops out at 2,400 W. Anker publishes no surge figure for the Plus configuration, so 6,000 W is the number to size against.
4,096 Wh with 4,000 W continuous across its 120V and 240V ports and a published 8,000 W surge. On paper that clears the 32.2 locked rotor amps Franklin publishes for a 1/2 HP submersible motor and does not clear the 48.7 it publishes for a 1 HP.
2,764.8 Wh with 3,840 W maximum output, a 7,680 W lifting rating, and a published 6,000+ cycles to 80 percent, the longest cell-life rating of the units we track. The smallest capacity here, so treat it as a short-outage answer.
Every figure is a manufacturer datasheet spec. We have not lab-tested these units.
A well pump is the load that turns a power outage into a water outage, and it is also the load that quietly disqualifies most of the equipment people buy to solve it. Two facts do the disqualifying. Most residential submersible well pumps run on 230V single phase, and a submersible motor started across the line draws several times its running current for the starting instant. The second fact is the one that matters, and unusually for this site, it is published. This guide works the arithmetic from Franklin Electric's own motor data. We have not tested any of these products, and the Amazon links above are affiliate links, disclosed at the top of this page.
The number that is published here, and usually is not
When we went through sump pump literature for our sump pump guide, no manufacturer published a starting or locked-rotor figure. Well pumps are different, because the motor and the pump end are usually made by different companies, and the motor company publishes a full electrical table.
Franklin Electric's AIM Manual for 60 Hz submersible systems, 2023 edition, lists in its "4-inch / Single-Phase / Encapsulated Motor Specs" section, for two-wire motors at 230V, the following (full load amps, maximum load watts, and locked rotor amps). Source: Franklin Electric AIM Manual, 60 Hz Submersible Systems, 2023 edition, read August 6, 2026.
- 1/2 HP: 5.0 amps full load, 960 W maximum load, 32.2 locked rotor amps.
- 3/4 HP: 6.8 amps full load, 1,310 W maximum load, 40.7 locked rotor amps.
- 1 HP: 8.2 amps full load, 1,600 W maximum load, 48.7 locked rotor amps.
- 1.5 HP: 10.6 amps full load, 2,280 W maximum load, 66.2 locked rotor amps.
Franklin also publishes a 115V version of the 1/2 HP two-wire motor at 10.0 amps full load and 64.4 locked rotor amps, which is the same power drawn at half the voltage, and therefore twice the current.
Locked rotor amps are what the motor pulls at the instant of starting, before it turns. Multiply by the supply voltage and you get the apparent power the source has to deliver for that instant: roughly 7.4 kVA for the 1/2 HP motor, 9.4 kVA for the 3/4 HP, 11.2 kVA for the 1 HP and 15.2 kVA for the 1.5 HP.
One honest caveat on that multiplication. Volt-amps are not watts, and a motor at locked rotor has a poor power factor, so the real watts drawn are lower than the kVA figure. Franklin publishes full-load power factor for these motors (73 to 83 percent) and does not publish a locked-rotor power factor, so we cannot convert precisely. We print the kVA figure because an inverter's protection trips on current, not on true power, so the current-derived number is the conservative one to plan against.
Setting that against what the power stations publish
Here are the published surge and continuous figures for the 240V-capable units we track, from each maker's own spec table:
- Jackery Explorer 5000 Plus: 7,200 W maximum through its 240V ports, 14,400 W surge.
- EcoFlow DELTA Pro Ultra: 7,200 W continuous, 10.8 kW surge.
- EcoFlow DELTA Pro 3: 4,000 W combined across 120V and 240V output, 8,000 W surge.
- BLUETTI Apex 300: 3,840 W maximum output, 7,680 W under BLUETTI's "lifting power" label.
- Anker SOLIX F3800 Plus: 6,000 W maximum, available only through the NEMA L14-30R port. Anker publishes no surge figure for the Plus configuration, so we do not print one.
Line those two lists up and the picture is unambiguous:
- A 1/2 HP motor (7.4 kVA at start) is cleared on paper by the DELTA Pro 3's 8,000 W and, narrowly, by the Apex 300's 7,680 W.
- A 3/4 HP motor (9.4 kVA) is cleared only by the DELTA Pro Ultra and the Explorer 5000 Plus.
- A 1 HP motor (11.2 kVA) is cleared only by the Explorer 5000 Plus at 14,400 W. The DELTA Pro Ultra's 10.8 kW does not reach it.
- A 1.5 HP motor (15.2 kVA) is not cleared by any published surge figure in this group.
That is a datasheet comparison, not a test result, and a surge rating is a duration-limited claim whose duration these makers mostly do not publish. But it explains a pattern people run into: the same power station that carries a refrigerator, a freezer and a furnace without complaining trips the moment the well kicks on. The running load was never the problem. Our continuous vs peak power guide covers why those two columns behave so differently.
Franklin's own sizing table says the same thing in different units
Franklin publishes minimum generator sizes for its motors, and the figures are much larger than the running watts suggest. For three-wire and three-phase motors its table lists 2.5 kVA minimum for a 1/2 HP motor, 5.0 kVA for 1 HP and 6.25 kVA for 1.5 HP on an externally regulated generator, with a note that for best starting of two-wire motors the minimum generator rating is 50 percent higher than shown. Applied to a two-wire 1 HP motor, that is a 7.5 kVA minimum, against a motor whose maximum load is 1,600 W.
Franklin also publishes the reason, and it is about voltage rather than watts: "Generators must be sized to deliver at least 65% of the motor's rated nameplate voltage during starting to ensure adequate starting torque." A source that sags below that during the start does not merely run the pump inefficiently. The motor may not develop enough torque to come up to speed at all.
None of these makers publishes a voltage-sag figure for its inverter under a motor start, which is the specification you would actually want here. Ask before you buy.
The exception worth knowing about
A constant pressure system changes the problem rather than solving it with more battery. Franklin's SubDrive and MonoDrive controllers, and the equivalent products from other makers, run the motor through a variable frequency drive that ramps it up instead of connecting it across the line. There is no locked rotor event to survive, which is why Franklin publishes separate and materially lower generator sizing guidance for those systems than for the same motor started conventionally.
If your well already has a constant pressure controller, the start-current section above mostly does not apply to you, and the remaining question is capacity: a 1 HP motor at Franklin's published 1,600 W maximum load, running perhaps 10 to 20 percent of the hours in a normal household day, is a load a 4 to 6 kWh unit can carry through a storm-length outage. If your well is a conventional pressure-switch installation, the start current is the gate.
Where the fixed-battery route lands
Once the pump is more than about 3/4 HP, the honest answer stops being a plug-in box. Installed systems publish larger numbers, and one maker publishes the right number in the right units: Tesla's Powerwall 3 datasheet lists a Load Start Capability of 185 LRA alongside 11.5 kW continuous. That is a motor-start rating in the same amps the submersible motor's nameplate uses, and 185 against Franklin's 66.2 for the largest single-phase motor in the table is a wide margin. Most other makers publish a peak-watt figure instead, which is a less direct comparison.
A whole-home installed system also solves the connection problem you would otherwise still have. A well pump is a hardwired 240V load on its own breaker. There is no cord to plug into a power station, so a plug-in unit still needs an electrician-installed transfer switch or backup panel before it can reach the pump at all, and that is covered in our transfer switch guide and our 120V vs 240V guide. If you are heading toward an installed system anyway, the whole-home backup guide is the right next read, and the plug-in guide compares the portable units above on the specs that matter for a house.
What to do before you buy anything
Read three things off your own equipment, in this order. The nameplate voltage, which decides whether a 240V-capable unit is even a candidate. The horsepower and the motor manufacturer, which gets you to a published locked rotor figure like the ones above. And whether there is a variable speed controller in the path, which decides whether the start current matters at all.
Every figure above is a manufacturer datasheet claim recorded with its published basis: motor data from the 2023 edition of Franklin Electric's AIM Manual for 60 Hz submersible systems, power station figures from each maker's own spec table, and the Powerwall 3 figure from Tesla's datasheet. None of it is a measurement we made, and none of it is a promise about your well, your motor or your pressure setting.