What size generator do I need for a well pump?

The first two numeric columns are Franklin Electric's published minimum generator ratings for starting a submersible motor on an ordinary portable generator with nothing else connected. The last column runs the calculator above for the most common real setup: a three-wire pump sharing the generator with a refrigerator and some lights.

Minimums are for externally regulated generators (the ordinary portable kind) at a 35% maximum voltage dip during starting, from Franklin Electric's engine-driven generator table. Two-wire motors are 50% higher, per the same source. Running watts are planning figures for a 230 V pump; the nameplate governs. Generator ratings mean rated (running) watts, not peak or “starting” watts.

The usual advice multiplies the pump's running watts by three and calls that the surge. It is a reasonable guess, and it is how people end up with a 5,000 watt generator that trips, stalls, or just hums at a 1 HP pump that only draws about 1,800 watts once it is going.

What actually decides whether a motor starts is voltage, not watts. A submersible motor draws several times its running current for the first second or so. The generator's voltage sags under that load, and if it sags far enough the motor never develops the torque to spin up. It sits there stalled, drawing starting current, until the generator's breaker or the motor's overload gives up. Franklin Electric states that a generator must hold at least 65% of rated voltage during the start.

How well a generator holds voltage depends on how it regulates it. Most portable generators are externally regulated: a separate regulator senses the dip and reacts, a beat late. Internally regulated (self-excited) generators respond faster and can start the same motor from a smaller frame — which is why the manufacturer's table has two columns, and why this calculator defaults to the larger one. If a spec sheet does not say, assume external.

The second thing the multiplier misses is wiring. A three-wire pump keeps its starting capacitor and relay in a control box in the house, which gives the motor a stronger, shorter start. A two-wire pump has those parts built into the motor down the well and needs a generator half as large again for the same horsepower.

Generator guides say to start the well pump first and add the other loads after. That is good advice for exactly one start. After that, the pump is under the control of the pressure switch, which restarts it every time the pressure tank drains down to the cut-in pressure — in the middle of a shower, while the fridge compressor is running, while the furnace blower is on.

So the start that decides the generator size is not the first one. It is the pump restarting while everything else is already drawing power, and the calculator sizes to that. The one staging habit that still helps is not starting a second big motor at the same moment: if you are about to switch on a sump pump or window unit, let the well pump finish a cycle first.

A larger pressure tank→ does not reduce the size of each start, but it does mean fewer starts per hour, which is easier on both the pump and the generator during a long outage.

A submersible pump is a few hundred feet down a well, so the nameplate is not something you can walk over and read. There are four places the answer usually is, in order of how reliable they are.

• The control box. A grey box on the wall near the pressure tank, wired between the pressure switch and the well. Its label gives the horsepower and voltage. If there is one, the pump is three-wire.
• No control box — wires go from the pressure switch straight toward the well — means a two-wire pump. The horsepower is then on the well completion paperwork or the invoice from whoever last replaced the pump.
• The well log. The driller's or pump installer's record often lists the pump model. The model number gives the horsepower.
• The breaker. Least precise, but always available. The pump is on a double-pole breaker in the panel, and its size narrows the horsepower down.

If none of that turns anything up, you can estimate the horsepower the well needs from its depth and your pressure switch setting — work out the horsepower from your well→ — and then size the generator from that.

Nearly every deep-well submersible runs on 240 volts. A generator can have more than enough watts and still be useless for it if its only outlets are 120 volt household sockets, which is common on inverter generators under about 4,000 watts.

Look for a true 240 volt receptacle: usually a four-prong L14-30 twist-lock on portable generators, or a 14-50 on larger ones. That is also the outlet a transfer switch or interlock inlet connects to. A 120 volt well pump is normally a shallow-well jet pump in the basement or pump house, and it will say 115 V on its own label.

A well pump is hard-wired, so there is no plug to move to an extension cord. The generator has to feed the pump's circuit, and the only safe ways to do that isolate the house from the utility first.

• Interlock kit. A mechanical plate on the main panel that makes it impossible to have the main breaker and the generator breaker on at the same time, plus a generator inlet outside. Commonly about $400 to $900 installed.
• Manual transfer switch. A small sub-panel for the circuits you choose, including the well. Commonly about $1,000 to $2,200 installed.
• Never a double-ended cord into a dryer or other outlet. It back-feeds the utility line, which can electrocute a line worker, and it energises the plug's exposed pins.

Two operating rules come straight from the motor manufacturer. Start the generator and let it settle before the pump is allowed to start, and turn the pump's breaker off before shutting the generator down — a motor coasting down on a slowing generator runs its thrust bearing at too low a speed. Keep the generator at its rated frequency; a pump on a generator running slow will not deliver its normal pressure.

• Variable-speed constant-pressure systems. These drives ramp the motor up gently and draw far less on start. Use the drive manufacturer's generator guidance instead of this table.
• Three-phase pumps, and anything over 5 HP. Farm and irrigation duty needs sizing from the motor's locked-rotor current by an electrician.
• How long the fuel lasts. That depends on the generator and how much the pump actually cycles, which a calculator cannot know.
• Whether the pump is healthy. A pump that struggles to start on grid power will not start on a generator. If the pump will not start on grid power either→