Match the generator to your pump's starting power, not its running power

A well pump needs a generator roughly two to three times larger than the pump's rated horsepower. If your pump is 1 horsepower, you need a generator between 5,000 and 7,500 watts. If it is 2 horsepower, aim for 10,000 to 15,000 watts. The gap exists because pumps draw a heavy surge of power the moment they start — sometimes double or triple their normal running load — and a generator that can only handle the running wattage will shut down or fail when the pump kicks on.

The exact size depends on three things: your pump's horsepower rating, whether it is a shallow-well or deep-well model, and what else you plan to run at the same time. A pump alone is simpler to size than a pump plus a pressure tank, water heater, or household circuits. Start by finding your pump's nameplate, which should list horsepower and voltage.

Key Takeaways

  • A 1 horsepower pump needs a 5,000 to 7,500 watt generator; a 2 horsepower pump needs 10,000 to 15,000 watts, because pumps draw 2 to 3 times their running power when starting.
  • Deep-well submersible pumps usually draw more starting current than shallow-well jet pumps of the same horsepower, so size up if you are unsure which type you have.
  • If you plan to run the pump and other loads together — lights, a pressure tank, a water heater — add their wattages and multiply the pump's portion by 2.5 to account for starting surge.
  • A portable generator rated for continuous output (not peak output) is more reliable than one rated only for surge; check the nameplate for "running watts" rather than "maximum watts".

How to find your pump's power draw

Look for a metal or plastic nameplate on the pump itself or on the pressure tank next to it. The nameplate will show horsepower (usually 0.5, 0.75, 1, 1.5, or 2 HP) and voltage (usually 120V or 240V). Write both down. If the nameplate is missing or unreadable, you can estimate: a 0.5 horsepower pump draws roughly 2,000 to 3,000 watts starting, a 1 HP draws 5,000 to 7,500 watts, and a 2 HP draws 10,000 to 15,000 watts.

The voltage matters because it affects how much current the pump draws. A 240-volt pump draws less current than a 120-volt pump of the same horsepower, which means it needs a slightly smaller generator. Most modern well pumps are 240 volts. If yours is 120 volts, add 20 percent to the generator size to be safe.

Shallow-well versus deep-well pump sizing

Shallow-well jet pumps (which sit above ground and pull water from wells under 25 feet deep) typically draw less starting current than deep-well submersible pumps (which sit underwater and push water up). A 1 horsepower shallow-well pump might need a 5,000 watt generator, while a 1 horsepower deep-well pump might need 7,500 watts. If you do not know which type you have, assume deep-well and size up — it is cheaper to buy a slightly larger generator than to buy a second one because the first one keeps shutting down.

Submersible pumps are more common in modern installations. They are sealed units lowered into the well on a pipe, and they have higher inrush current because the motor has to overcome the weight of the water column above it. If your pump is mounted on a tank above ground, it is a jet pump and can use the lower end of the sizing range.

Running other loads alongside the pump

If you only need to run the pump itself, the sizing math is straightforward. But if you want to run lights, a pressure tank, a water heater, or household circuits at the same time, you have to add their wattages together and then explore the pump's surge multiplier.

For example: a 1 horsepower pump (5,000 watts running), a pressure tank (500 watts), and some LED lights (300 watts) total 5,800 watts running. Multiply the pump's 5,000 watts by 2.5 to account for starting surge: 12,500 watts. Add the pressure tank and lights: 12,500 + 500 + 300 = 13,300 watts. You would need a generator rated for at least 13,500 continuous watts. Do not add the surge multiplier to the non-pump loads — only the pump draws that heavy starting current.

A pressure tank is worth mentioning separately because it often runs at the same time as the pump. If your system has one, its wattage is usually printed on the tank itself. If not, assume 500 to 1,000 watts.

Choosing between portable and standby generators

Portable generators are cheaper and easier to move, but they are louder and less convenient for regular use. Standby generators (also called backup generators) are permanently installed, quieter, and can switch on automatically when power fails. For a well pump, either type works as long as the wattage is correct.

The key is to read the nameplate carefully. Manufacturers often list two numbers: "maximum watts" (the peak surge the generator can handle for a few seconds) and "running watts" (the continuous power it can deliver). Use the running watts number for your sizing calculation. A generator rated 7,500 maximum watts might only deliver 6,000 running watts, which is not enough for a 1 horsepower pump.

Inverter generators are quieter and more fuel-efficient than conventional portable generators, and they are safer for electronics. If you plan to run the pump alongside sensitive equipment, an inverter model is worth the extra cost. Standard portable generators are fine if the pump is the main load.

Generator fuel type and runtime

Gasoline generators are the cheapest upfront but require regular fuel and maintenance. Propane generators are cleaner and can run longer on a tank, but propane is harder to store long-term. Diesel generators are more efficient and last longer, but they cost more and are heavier. For a well pump, gasoline is the most common choice because most people do not need to run the pump for more than a few hours at a time.

Runtime depends on fuel capacity and load. A 7,500 watt generator running a 1 horsepower pump at half load might run 8 to 12 hours on a full tank. If you expect to run the pump for extended periods — during a long power outage or while drilling a new well — choose a model with a larger fuel tank or plan to refuel regularly. Propane models can be connected to a larger stationary tank for longer runtime.

Common sizing mistakes to avoid

The most common mistake is buying a generator rated only for the pump's running wattage, not its starting wattage. A 5,000 watt generator will not start a 1 horsepower pump, even though 1 HP is often quoted as 5,000 watts. The pump needs 5,000 to 7,500 watts to start, then settles to around 2,500 watts once it is running. If the generator cannot handle the surge, it will either shut down when ready or run so poorly that the pump does not prime.

Another mistake is underestimating the wattage of other loads. A pressure tank, water heater, or household circuit can add hundreds of watts. If you are unsure, add 20 percent to your total and buy the next size up. A generator that is slightly oversized will run more efficiently and last longer than one that is constantly at its limit.

A third mistake is confusing maximum watts with running watts. Maximum watts are the peak surge a generator can handle for a few seconds. Running watts are what it can deliver continuously. Always use running watts for your calculation.

Frequently Asked Questions

Can I use a smaller generator if I run the pump alone?

No. Even if the pump is the only load, it still needs a generator sized for its starting surge, which is 2 to 3 times its running power. A 1 horsepower pump requires a 5,000 to 7,500 watt generator minimum, whether or not other loads are connected.

What if my generator shuts down when the pump starts?

The generator is too small for the pump's starting current. You need a larger model. Alternatively, some soft-start devices can reduce inrush current, but they are expensive and not always reliable. Buying the right size generator is simpler and more dependable.

Do I need a transfer switch to connect the generator to my well pump?

Yes, if the generator is connected to your home's electrical panel. A transfer switch prevents the generator from backfeeding power into the utility lines, which is dangerous. If you are running the pump directly from the generator with a heavy-duty extension cord, a transfer switch is not required, but it is safer and more convenient.

Can I run a well pump on a battery backup system instead of a gas generator?

Battery systems can run small loads like lights or a sump pump, but they cannot reliably run a well pump because the starting surge is too high and drains the battery too quickly. A gas, propane, or diesel generator is the standard solution for well pump backup power.

How often should I run my generator to keep it ready?

Run it under load for 20 to 30 minutes at least once a month. This keeps the fuel system clean and the engine in working order. If you store it for more than a month, drain the fuel tank or add a fuel stabilizer to prevent the carburetor from clogging.