Surge Explained: Why a ½ hp Pump Needs a 3 kW Inverter

A half-horsepower submersible well pump is close to the smallest motor in a house. Franklin Electric’s own specification sheet for one says it draws 5.0 amps and 670 watts. A different table in the same manual says do not run that motor from a generator smaller than 3 kW. Both numbers are correct, and the distance between them is where a lot of off-grid budgets get spent twice.

The failure looks the same every time. The battery is fine. The panels are fine. The daily watt-hours were worked out carefully. Then the pressure switch clicks, the inverter drops the load, and the lights go out.

The plate carries two currents, and only one is the running one

Every induction motor nameplate carries a full-load amp figure. Most also carry a second, much larger number, printed as LRA or “locked rotor amps.” That is what the motor pulls at the instant power arrives, while the shaft is still standing still.

The reason is mechanical. A spinning rotor generates a voltage of its own that opposes the supply and throttles the current back. A stationary rotor generates none. For that first moment the motor looks to the supply like a short circuit with a bit of resistance in it, and it draws whatever the wiring will deliver.

The US Department of Energy’s Premium Efficiency Motor Selection and Application Guide sets the band in its Table 6-3 at 600 to 700 percent of rated load current for medium polyphase NEMA Design B, C and D motors. Design A motors have the same torque limits, and the table notes their starting currents are not limited at all. Six to seven times is not an unusual motor. It is the normal one.

Household motors are single-phase, and Franklin publishes the same two numbers for those. Its 2-wire ½ hp submersible draws 32.2 A against 5.0 A running, about 6.4 times. The 3-wire version of the identical motor, which starts through a capacitor, draws 23.0 A, about 4.6 times.

What the first half second actually looks like

Franklin describes the event plainly: the motor goes from zero to full speed in half a second or less, and the current goes from zero to locked-rotor amps, then drops to running amps at full speed. The manual notes this may dim lights and cause momentary voltage dips to other equipment.

That shape matters more than the peak height. This is not a load your system carries. It is an event your system has to survive.

Two things follow from the shape. First, the energy involved is trivial — a third of a second at 7 kVA is well under a watt-hour, so stored capacity is never what runs out. Second, the current is not trivial at all, and current is what inverters, breakers and wiring actually respond to.

Watts, amps, and the trap in between

The running numbers hide a second problem. That ½ hp motor draws 5.0 A at 230 V, which is 1,150 volt-amps, while its input power is only 670 watts. The gap is power factor, listed on Franklin’s sheet as 58 percent at full load. Motors are inductive, so their current and voltage do not peak together, and the supply has to carry current for the whole 1,150 VA even though the meter records 670 W.

An inverter is rated in watts on the box and limited by amps in the hardware. Size a unit against 670 W and the hardware still has to pass the current of an 1,150 VA load, which is 72 percent more than the watt figure implies, before the motor has even started. This is why pump loads eat inverters that handle a 1,500 W kettle without complaint.

Reading your own plate

Three markings do all the work.

LRA. Locked-rotor amps, printed directly. Multiply by the supply voltage to get the momentary apparent power the motor asks for. That is the number to size surge against.

The kVA code letter. A single letter, from A upward, that classifies the motor by its locked-rotor kilovolt-amps per horsepower. Higher letters mean a harder start. Franklin prints code R on the 2-wire ½ hp motor and code M on the 3-wire version of the same pump — the same horsepower, a gentler start, a different letter.

2-wire or 3-wire. A 2-wire submersible has its starting components sealed inside the motor. A 3-wire runs from a control box on the wall with a start capacitor in it. The capacitor gives the rotor a stronger initial shove, so the motor reaches speed with less current.

If you would rather put your own well’s numbers straight into a sizing answer, the well pump off-grid calculator takes horsepower, depth and daily draw and returns the inverter and battery it needs.

What an inverter means by “surge”

Every inverter datasheet carries a continuous rating and a peak rating. The peak is not a second, softer continuous rating. It is a short window during which the electronics will pass more current than they are designed to pass, and its length is measured in seconds.

Victron MultiPlus-II datasheets (120 V and 230 V editions) and the EcoFlow DELTA 3 product specification on the US 120 V store, all read September 2026. Where a hold time is published it is very short: the 120 V 12/4k model quotes 6 kW for 2 seconds, with separate cold-start ratings of 4.5 kW for 30 minutes and 4.0 kW for an hour listed on the same line.

Roughly twice continuous is the ordinary ratio, and the units above span 1.8 to 2.3 times. That is the ceiling most people bump into. A pump wanting 7.4 kVA against a unit whose peak sits near 5,500 W looks impossible on paper, yet the 3 kW class is exactly what the motor maker specifies.

The resolution is that locked-rotor amps are a worst case, measured at full rated voltage with the shaft physically held. In a real start the supply sags a little, the rotor moves immediately, and the current is already falling by the time the inverter’s protection has finished deciding.

Franklin makes the same point from the other side. A generator at its minimum rating acts as a soft start, it says, because a source that small cannot deliver the full locked-rotor current in the first place.

That tolerance is also why generators and inverters behave differently on the same pump. A generator has a heavy spinning rotor and a voltage regulator that pushes back; it sags and recovers.

An inverter has an electronic current limit with no inertia behind it, so it either rides the event through or shuts down cleanly in milliseconds. Franklin’s requirement that the source still deliver at least 65 percent of nameplate voltage during starting is the line between the two outcomes.

Cutting the surge at the source

Buying a bigger inverter is one answer. Making the motor ask for less is usually the cheaper one, and there are four established ways to do it.

Method	What it does	Effect on starting current	Cost of it
Capacitor start (3-wire pump, control box)	Extra winding energized through a capacitor for the first moment	Franklin: 32.2 A falls to 23.0 A on the same ½ hp motor at 230 V, and its minimum generator drops from 3 kW to 2 kW	A control box on the wall, and one more component that can fail
Long cable run	Voltage drop in the wire limits the current the motor can pull	Franklin: a 5 percent drop at running amps cuts starting current about 20 percent	Starting torque falls about 36 percent, so it buys headroom by spending margin
Reduced-voltage or soft starter	Ramps voltage up over a second or two instead of applying it all at once	Micro-Air claims up to 75 percent reduction on air conditioner compressors	Torque falls with the square of voltage; the motor must still be able to break away
Variable frequency drive	Starts the motor at a low frequency and ramps it, so the rotor is never far behind the field	Franklin sizes drives for its three-phase submersibles on nameplate maximum amps, with no surge allowance at all	Highest cost; needs a drive matched to the motor, and cable length limits apply

The drive row is the one worth reading twice. Because a variable frequency drive never presents the motor with a field it cannot follow, the locked-rotor condition simply never happens.

Franklin’s instruction for its three-phase submersible motors is to match the drive’s continuous amps to the motor’s nameplate maximum amps and nothing more, on pain of voiding the warranty.

This is the same reason an inverter-driven mini-split has no meaningful surge at all, as the mini-split heat pump numbers show.

The trade behind every method is torque. Reduce the voltage the motor sees and the torque falls with the square of it, which is why Franklin insists an autotransformer starter still supply at least 55 percent of rated voltage, and why a soft start that is set too gentle leaves a pump straining against a full column of water instead of accelerating through it.

Four loads, worked through

The well pump. Take the ½ hp 2-wire at 230 V. Running: 670 W but 1,150 VA. Starting: 32.2 A, so 7.4 kVA for under half a second. Franklin’s minimum generator is 3 kW. A 3 kW-class inverter with roughly 5,500 W of peak is the equivalent, and a 1,500 W unit rated at 3,000 W peak is not, however comfortably it carries the 670 W afterwards.

The window air conditioner. The Friedrich Chill Premier CCW08B10C, an 8,000 Btu window unit, is listed at 115 V, 8.0 amps and 730 watts running. The compressor inside is a single-speed motor, so the same multiplier logic applies to whatever LRA its plate carries.

A soft starter is the standard fix here rather than a bigger inverter, because the compressor restarts many times a day and every one of those starts is an event your system has to absorb. Our window AC calculator runs the daily side of that.

The refrigerator. A household compressor is small, so its surge is small in absolute terms and the plate figure is easy to look up. The catch is timing. A fridge starts on its own schedule, so sooner or later it starts while the pump or the air conditioner is starting, and the two peaks add. Size for the worst overlap the house can actually produce rather than for each load on its own.

The table saw. Here the plate is the whole answer: find the LRA, multiply by 120 or 240, and you have the momentary kVA. What makes a saw different from a pump is the blade and arbor, which are heavy and take time to come up to speed.

The Department of Energy’s guide notes that motors of 200 hp and below tolerate only about 12 seconds of acceleration per start before heat becomes the limit, so a start that drags on is worth investigating rather than living with.

Once you have a list of motors and their plate figures, the sizing question turns into arithmetic, and the inverter and charger sizing guide covers how to turn it into a specific unit. If your motors are 240 V in a US house, the split-phase guide covers the extra wrinkle of getting both halves of the supply from one machine.

Common questions

Sources: Franklin Electric AIM Manual, 60 Hz edition (M1311) and 50 Hz edition (M1339), 4-inch motor specification tables, generator minimum rating tables, starts-per-day limits and reduced-voltage starting notes; US Department of Energy, Advanced Manufacturing Office, Premium Efficiency Motor Selection and Application Guide, Table 6-3 and Table 5-4; Victron Energy MultiPlus-II datasheets, 120 V and 230 V editions; Friedrich Chill Premier submittal sheet, revision 09-15-25, model CCW08B10C; Micro-Air EasyStart product specifications.

All read September 2026.