Running Watts vs Starting Watts
Every electrical device has a running wattage, the power it draws in steady operation, and many also have a higher starting wattage, the brief surge needed to get moving. Lights and heaters draw essentially the same at startup as when running. Motors, compressors, and pumps can demand several times their running wattage for the first moments of startup. To size a generator, you must account for both: the generator has to supply total running watts continuously and absorb the largest starting surge on top of that without stalling or dropping voltage.
Totaling Your Load
List every device the generator must power and note each one's running watts. Add them up to find your continuous running load. Then identify the device with the highest starting surge, because motors generally do not all start at the same instant. Add that single largest starting surge to the running total of everything else to find your peak demand. Nameplate data, tool specifications, and motor ratings give you the figures. When numbers are listed in amps, multiply amps by volts to estimate watts. This total is the floor your generator must clear.
Allowing for Motor-Start Surge
Motor-driven equipment is the usual reason a generator must be larger than the running load alone suggests. An electric motor can draw a large inrush current at startup, and that surge, though brief, will trip a generator sized only for running watts. Pumps, air compressors, HVAC compressors, and large power tools are common culprits. When several motors are present, sequence their startups so they do not surge together, and size the generator to handle the biggest single start on top of the base load. Soft starters and variable frequency drives can reduce surge where they are fitted.
Single-Phase vs Three-Phase Power
Single-phase power suits homes, small shops, and light job sites with modest loads. Three-phase power is standard for larger motors and industrial equipment because it delivers power more smoothly and efficiently, letting motors run cooler and start more easily. Choose the phase your equipment requires: three-phase motors need a three-phase supply, and you cannot simply run them on single-phase. Many industrial generators offer selectable voltage and phase. Confirm the voltage, phase, and frequency your loads need and match the generator's output connections accordingly before you buy.
Sizing for Jobsite vs Standby Duty
How you will use the generator shapes the size you buy. For jobsite power, size for the real simultaneous load of the tools and equipment running at once, with headroom for the largest motor start and for adding equipment later. For standby duty, decide whether you need to back up an entire facility or only critical circuits, and size to the loads that must stay live during an outage. In both cases, build in a margin of roughly 20 to 25 percent so the set is not running at its limit, which improves reliability and service life.
Frequently asked questions
How do I calculate what size generator I need?
Add the running watts of everything the generator will power, then add the single largest starting surge on top of that running total. Build in roughly 20 to 25 percent headroom. The result is the minimum generator capacity your load requires.
What is the difference between running watts and starting watts?
Running watts is the steady power a device draws in normal operation. Starting watts is the brief surge motors and compressors need to start, often several times their running draw. A generator must cover the running load continuously and absorb the largest startup surge at the same time.
Do I need a three-phase generator?
You need three-phase power if your equipment has three-phase motors or industrial loads that require it, which is common in larger machinery. Homes and light shops typically use single-phase. Match the generator's phase, voltage, and frequency to what your equipment specifies.
Is it bad to oversize a generator?
A modest margin is good practice, but a diesel generator that runs far below its rated load for long periods can suffer wet stacking, where unburned fuel accumulates. Size generously enough for surges and growth, but not so large that it idles lightly most of the time.