Generator Sizing for Homes with Well Pumps
A typical residential well pump requires 1,000 to 2,500 running watts and 3,000 to 7,500 starting watts depending on the pump’s horsepower rating. Well pumps are one of the most important loads to consider when sizing a backup generator because they have high startup surge requirements and because losing water supply during an extended outage is a serious problem for homes not connected to municipal water.
Well Pump Wattage by Horsepower
The power requirement of a well pump is determined by its motor horsepower (HP) rating. Here are the typical running and starting wattages for common residential well pump sizes.
A 1/3 HP well pump draws approximately 750 running watts with a starting surge of 1,400 to 2,300 watts. A 1/2 HP pump draws approximately 1,000 running watts with a starting surge of 2,000 to 3,000 watts. A 3/4 HP pump draws approximately 1,500 running watts with a starting surge of 3,000 to 4,500 watts. A 1 HP pump draws approximately 2,000 running watts with a starting surge of 4,000 to 6,000 watts. A 1.5 HP pump draws approximately 2,500 running watts with a starting surge of 5,000 to 7,500 watts.
The starting surge (also called locked rotor amps or LRA) is the critical number for generator sizing. When the well pump motor starts, it briefly draws 2 to 3 times its running wattage. The generator must have enough surge capacity to handle this startup demand without tripping the overload protection.
Why You Should Not Size for the Pump Alone
While it is technically possible to power just a well pump with a small 3,500 to 5,000 watt portable generator, this approach leaves the rest of your home without power. During an extended outage, you will also want your refrigerator, heating or cooling system, lights, sump pump (if applicable), and basic outlets operational. A generator sized only for the well pump forces you to choose between water and everything else.
Recommended Generator Sizes for Homes with Wells
For homes on well water, a standby generator provides the most practical solution because it powers the well pump along with your other household loads automatically. For a home with a well pump up to 1 HP and moderate other loads (no central AC), a 14kW to 16kW standby generator is typically adequate. For a home with a well pump and central air conditioning, a 20kW to 22kW generator provides comfortable whole-house coverage. For homes with large well pumps (1.5 HP or larger), multiple pumps (well pump plus booster pump), or high overall electrical demand, a 22kW to 26kW generator ensures sufficient capacity for all simultaneous loads including pump startup surge.
Soft Starters and VFDs
If your well pump has a soft starter or variable frequency drive (VFD), the starting surge is significantly reduced, sometimes to near the running wattage. This effectively lowers the generator capacity needed for the pump startup. If you are installing a new well pump alongside a generator project, specifying a soft starter can allow you to select a smaller, less expensive generator. Ask your well installer about soft start options for your pump motor.
Submersible vs. Jet Pumps
Most modern residential wells use submersible pumps installed inside the well casing at or below the water table. Submersible pumps are generally more efficient than above-ground jet pumps, meaning they draw less power for the same water output. Jet pumps (used for shallow wells) have similar wattage requirements per HP but are less common in new installations. Both types are compatible with standby generator backup when properly sized.
Protecting Your Well Pump During Outages
A standby generator with an automatic transfer switch provides the most reliable protection for homes on well water. The generator starts automatically when power fails, restoring water pressure within seconds without any manual intervention. This is especially important if you are away from home during an outage or if the outage occurs at night. Without backup power, a well pump failure also means no water for toilets, cooking, fire suppression (if on a well-fed sprinkler system), and livestock or irrigation if applicable.
IB Lighting carries standby generators from 10kW to 26kW and beyond, all compatible with homes on well water systems. Call (800) 674-9019 for help sizing a generator that accounts for your well pump and total household load, or visit our online shop to compare models.