Well pumps and construction power tools share a trait that trips up a lot of generator sizing: their starting surge dwarfs their running wattage. A generator that looks plenty big based on the running-watts number on the box can still fail to start one of these loads, which makes this pair of use cases worth its own dedicated sizing pass beyond the general framework.
Well pump sizing
| Pump size | Running watts | Starting watts |
|---|---|---|
| 1/3 HP shallow well pump | 600–1,000 | 1,400–2,000 |
| 1/2 HP well pump | 1,000–2,000 | 2,000–3,000 |
| 3/4 HP well pump | 1,500–2,500 | 3,000–4,000 |
| 1 HP deep well pump | 2,000–3,000 | 4,500–5,000+ |
Well pump motors typically start under load (against water pressure already in the system), which pushes their surge toward the higher end of these ranges. Your pump's nameplate lists its horsepower and, on better documentation, its locked rotor amps (LRA) — multiply LRA by voltage (240V for most well pumps) to get a more precise starting-watts figure than the general table above.
Construction tool sizing
| Tool | Running watts | Starting watts |
|---|---|---|
| Circular saw | 1,200–1,800 | 2,000–3,000 |
| Table saw (portable) | 1,800–2,400 | 3,000–4,500 |
| Air compressor (1–2 HP) | 1,500–2,500 | 3,500–5,000 |
| Angle grinder | 1,000–1,500 | 1,800–2,500 |
| Portable welder (110V, light-duty) | 3,500–5,000 | — |
| Shop vac / wet-dry vac | 800–1,400 | 1,600–2,500 |
Jobsite generators face a different challenge than home backup: multiple motor-driven tools potentially starting at unpredictable times as different trades work simultaneously. The practical fix is discipline — stagger tool startup one at a time rather than switching on several motor-driven tools at once, which avoids stacking surges that would otherwise require a much larger (and more expensive) generator to cover a worst-case simultaneous-start scenario that rarely actually happens.
Clean power matters for tools too
Modern power tools increasingly include variable-speed motors and electronic controls sensitive to power quality, similar to household electronics. An inverter generator with low total harmonic distortion protects this equipment the same way it protects a laptop or CPAP machine — worth considering even in a jobsite context, not just a medical or RV one. See our clean power generator picks for THD-rated options that apply equally well here.
For the general sizing framework this article builds on, see how to size a generator for whole-house backup.
Reading a pump or tool's nameplate correctly
Most sizing mistakes trace back to skipping the nameplate and estimating instead. Every well pump and most power tools have a nameplate or label listing horsepower, running amps, and sometimes locked rotor amps (LRA) directly — the most reliable sizing data available, more accurate than any general reference table. To convert amps to watts, multiply amps by voltage (120V for most tools, 240V for most well pumps): a pump rated at 15 LRA on a 240V circuit has a starting draw of roughly 3,600 watts, a number worth confirming against your actual nameplate rather than assuming from the horsepower rating alone, since motor efficiency varies by manufacturer and model.
Jobsite generator selection beyond wattage
Contractors and serious DIYers sizing a generator for regular tool use should weigh a few factors beyond raw wattage capacity:
- Multiple 120V outlets rather than relying on a single outlet and a power strip, which reduces cord clutter and trip hazards on an active jobsite.
- Durability rating for frequent transport and rougher handling than a occasional-use home backup generator sees — look for models marketed toward contractor or commercial use specifically.
- Fuel efficiency at partial load, since jobsite generators often run for full workdays rather than the shorter bursts typical of home backup use, making fuel cost a bigger factor over time.
- GFCI-protected outlets, required by OSHA on most active construction sites and a genuine safety feature regardless of regulatory requirement.
Well pump generator sizing for irrigation and agricultural use
Larger agricultural or irrigation well pumps can exceed the residential sizing figures in the table above substantially, sometimes requiring 3 HP or larger motors with correspondingly higher starting surge. For these applications, consulting with the well or irrigation system's original installer about specific generator sizing recommendations is worth the call — agricultural pump systems vary enough in configuration that a generic sizing chart becomes less reliable at this scale, and an undersized generator repeatedly failing to start a large agricultural pump can cause real damage to both the generator and the pump motor over time.
Frequently Asked Questions
What size generator do I need for a well pump?
A typical 1/2 HP well pump needs roughly 1,000-2,000 running watts and 2,000-3,000 starting watts; a 1 HP pump can need 2,000-3,000 running and up to 4,500-5,000 starting watts. Deep or high-capacity wells with larger pumps need proportionally more — check your pump's nameplate for its exact HP and, ideally, its starting current (locked rotor amps) rating.
Why do power tools need so much starting power?
Motor-driven tools (table saws, air compressors, welders) draw a large inrush current the instant the motor starts turning, often 2-3 times the running wattage, before settling down to their continuous draw. This starting surge, not the running wattage on the tool's label, is usually what determines whether a generator can start it at all.
Can I run a well pump and power tools on the same generator at once?
Yes, if the generator's running watts cover both continuous loads combined and its starting watts cover whichever single item has the largest surge on top of everything else already running. Stagger starting equipment one at a time rather than switching on multiple motor-driven tools simultaneously to avoid unnecessary surge stacking.