Generator Sizing for Well Pumps and Sump Pumps
If your home has a well pump, a sump pump, or both, a standby generator is not a luxury. It is essential infrastructure protection. A well pump failure during an outage means no running water: no toilets, no showers, no cooking water, and no fire suppression. A sump pump failure during a storm outage means your basement floods while the very rain that caused the outage sends groundwater pouring in. These are the two highest-stakes systems in homes that depend on them, and both require electricity to operate.
Well Pump Power Requirements
Residential well pumps vary significantly in size based on well depth and water demand. A shallow well pump (jet pump, up to 25 feet deep) draws 500 to 1,000 running watts with a starting surge of 1,000 to 2,000 watts. A standard submersible pump (1/2 HP, common for wells 100 to 200 feet deep) draws approximately 1,000 watts running with a starting surge of 2,000 to 3,000 watts. A larger submersible pump (3/4 HP to 1 HP, for deeper wells or higher demand) draws 1,500 to 2,000 running watts with a starting surge of 3,000 to 6,000 watts. A high-capacity pump (1.5 HP or larger, for very deep wells or irrigation) draws 2,000 to 3,000+ running watts with a surge of 4,000 to 8,000+ watts.
The starting surge is the critical number. When the well pump motor kicks on, it draws 2 to 3 times its running wattage for the first 1 to 3 seconds. Your generator must handle this surge without tripping the overload protection or causing a voltage drop that affects other equipment.
Sump Pump Power Requirements
A standard 1/3 HP sump pump draws approximately 800 watts running with a starting surge of 1,300 to 1,800 watts. A 1/2 HP sump pump draws approximately 1,000 to 1,050 watts running with a starting surge of 2,000 to 2,500 watts. A 3/4 HP sump pump draws approximately 1,200 to 1,500 watts running with a surge of 2,500 to 3,000 watts. During heavy rain events, the sump pump may cycle frequently (every few minutes), so the starting surge load occurs repeatedly throughout the storm.
Generator Sizing Recommendations
Well Pump and Sump Pump Plus Essentials (No AC)
If you need to power both pumps plus basic household essentials (gas furnace blower, refrigerator, lights, and outlets), a 10kW to 14kW generator with a load-managed transfer switch provides adequate coverage. The load management handles the overlap of pump starting surges with other loads. This is the minimum recommended configuration for homes with both a well and a sump pump.
Both Pumps Plus Central Air Conditioning
Adding central AC changes the calculation significantly. When your AC compressor starts (drawing 5,000 to 7,000+ watts of surge), your well pump may start simultaneously, and your sump pump may cycle on. A 22kW generator handles all three starting surges plus full household load without strain. Generac True Power Technology on Guardian models delivers less than 5% total harmonic distortion, which means clean, stable power even during simultaneous motor starts.
Both Pumps Plus AC Plus EV Charger or Other Heavy Loads
For homes with well pump, sump pump, central AC, and additional heavy loads (EV charger, electric water heater, workshop equipment), a 24kW to 26kW generator provides the capacity and surge handling needed to run everything simultaneously. The Generac 24kW (Model 7261), 26kW (Model 7327 without ATS or 7328 with ATS), and Kohler equivalent models all serve this tier effectively.
Why Starting Surge Matters More Than Running Watts
A common sizing mistake is adding up only the running wattages and selecting a generator based on that total. Motors have starting surges that dramatically exceed their running draw. If your well pump (2,000W running) and AC compressor (3,500W running) both start within the same few seconds, the combined surge can reach 12,000 to 15,000 watts momentarily, even though the combined running load is only 5,500 watts. A properly sized generator handles these surges. An undersized generator trips its overload protection and shuts down, leaving you without power at the worst possible moment.
The Consequence of No Backup
A home with a well pump and no generator loses all water service during an outage. No flushing toilets, no water for cooking or drinking (without bottled reserves), and no ability to wash anything. A home with a sump pump and no generator risks thousands to tens of thousands of dollars in basement flood damage during every storm outage. Together, these two systems represent the strongest financial and practical case for generator ownership.
IB Lighting carries Generac home standby generators from 10kW to 28kW and Kohler generators for every residential application. Call (800) 674-9019 for a free sizing consultation based on your specific pump and household loads. Free freight plus insurance included.