Well Pump Generator Sizing: Handle the Surge

A well pump draws 2 watt figures

Backup power for a home with a well pump is a sizing question that starts with 2 watt figures: the running watts the pump draws while operating, and the starting watts it draws for a moment when the motor starts. The U.S. Energy Information Administration explains that electricity is measured in units of power called watts, a unit that appears on both the pump label and the generator listing.

Like any electric motor, a well pump draws more power at startup than it does after it is running. That brief startup surge is why a generator listing states 2 watt figures: a starting figure and a running figure. The pump's own nameplate or manual lists the same 2 figures for the pump.

• Running watts: the steady draw while the pump operates.
• Starting watts: the brief surge when the motor starts.

Locked rotor amps and the startup surge

Locked rotor amps (LRA) is a term found on motor documentation and on some generator listings. It names the current a motor draws at the instant it is energized, before the rotor begins to turn. For a well pump, the locked rotor figure corresponds to the startup surge covered by a generator's starting watts.

The difference between the 2 figures is often the largest for a motor that starts against load, which is the case for a well pump. A generator that states only a running watt figure may not be giving you the information needed to cover the surge; the maker's page should state the starting figure as well.

This article does not assign a watt figure to any pump size. The pump's own nameplate, not a general rule, decides the surge. See the generator surge protection explainer for the general meaning of surge in a listing.

Read the pump nameplate before you shop

Before comparing generators, find the well pump's electrical figures. The pump has a nameplate or an installed documentation sheet that lists voltage, current, and often locked rotor amps. Copy those figures onto the same page as your generator shortlist.

If the pump was installed before you moved in, ask the pump installer or a licensed electrician to point out the nameplate; the maker's page for the pump model also lists the figures. Keep the pump's stated figures next to each generator's stated starting and running watts.

• The pump manual shows where the nameplate is located.
• The maker's page for the pump model repeats the nameplate figures.

Read the generator title the same way

Generator listings state a starting watt figure and a running watt figure. In a listing title, the first figure is often the starting figure, but not always; read the bullet list and the maker's page to confirm which figure is which.

Compare the pump's locked rotor amps and starting watts against the generator's stated starting watts, and the pump's running watts against the generator's stated running watts. If a listing does not state 1 of the figures, that comparison stays incomplete.

The inverter generator guide and the large portable generator guide both compare models on their stated watt figures.

Why this site does not publish a pump-sizing table

This site compares backup power products on their stated details: watt figures, fuel type, outlets, and features. It does not publish a table that assigns a watt figure to a pump size, because the pump's figures depend on its own nameplate rather than on a one-size rule.

For a Florida home, the practical order is: note the pump's stated figures, note each generator's stated starting and running watts, and read the maker's page for both products. The Florida backup power guide compares the full catalog of generator and power station options, and the running watts versus starting watts explainer covers the 2 figures in more detail.

• Plan your shopping before the season; the Florida Division of Emergency Management and National Weather Service pages in the sources list preparation steps.
• Keep receipts and the pump manual with your generator paperwork.

Apartments and condos: no surge question

A well pump is a motor-driven appliance, so the surge question only applies where a well pump exists. For an apartment, condo, or high-rise, there is no well pump to size, and the relevant comparison is a portable power station, which states its own continuous AC output and battery capacity in watt-hours. See the apartment power station guide and the watt-hours versus watts explainer.

What to pick for your situation

Questions

What is locked rotor amps on a well pump?

Locked rotor amps (LRA) is the current a pump motor draws at the instant it is energized, before the rotor turns. It relates to the startup surge that a generator's starting watts are meant to cover. Read the pump nameplate for the LRA figure.

Do I need the starting watts or the running watts figure?

Both. The running watts figure is what the pump draws while operating; the starting watts figure covers the surge at startup. Compare both against the generator's stated figures.

Where do I find my pump's watt figures?

On the pump nameplate or in the maker's documentation for the pump model. If the nameplate is hard to reach, ask the pump installer or a licensed electrician to point it out.

Can this site tell me exactly which generator runs my well pump?

No. This site compares products on stated details and does not publish a pump-sizing table. Read the pump nameplate and the generator listing, then read the maker's page for both.

I live in an apartment or condo without a well pump; does this article apply?

No. The sizing question only applies to a home with a well pump. For an apartment or condo, the relevant comparisons involve portable power stations; see the apartment power station guide.