Generator Load Calculator
Enter your generator's prime rating and the connected load. See load as a percentage and whether you're in the recommended 50-80% efficiency band.
Common conversions
| Input | Result |
|---|---|
| Refrigerator or freezer | 700 W running / 2,200 W starting |
| Sump pump (1/2 hp) | 1,050 W running / 2,150 W starting |
| Well pump (1 hp) | 1,500 W running / 4,500 W starting |
| Central air conditioner (3 ton) | 3,500 W running / 8,900 W starting |
| Furnace blower fan | 800 W running / 2,350 W starting |
| Window AC unit (10,000 BTU) | 1,200 W running / 3,600 W starting |
| Microwave oven | 1,000 W running, no surge |
| Electric space heater | 1,500 W running, no surge |
| Table saw (1.5 hp) | 1,800 W running / 4,500 W starting |
| Ideal generator loading band | 50-80% of rated kW |
The math behind it
- Connected load = 0.7 + 0.8 + 1.5 + 8 + 2 = 13 kW
- % = 13 / 20 × 100
Everything you need to know
Sizing a generator starts on paper with a nameplate kW rating, but the number that actually matters day to day is your connected load, the sum of every appliance running at once, and how that load compares to what the generator is rated to deliver. This page focuses on building that load figure appliance by appliance and reading what the resulting percentage means for fuel use and engine health.
Add up your connected load first
Walk through every circuit the generator will carry and record its running watts, not its starting watts, for anything that will be on for more than a few seconds at a time. A refrigerator contributes 700 running watts even though it spikes to 2,200 watts for an instant at startup; only the running figure counts toward continuous connected load. Add a single starting-surge allowance on top for whichever motor is largest, since motors rarely all start at the exact same moment. Convert the total to kW and use it as the "connected load" input above.
Why the 50-80% band matters
Diesel and gas generator engines are designed to run cleanest and most efficiently when working between half and four-fifths of their rated capacity. Below 30% load for extended periods, diesel engines suffer from wet stacking, where unburned fuel and carbon build up in the exhaust and cylinders because the engine never gets hot enough to fully combust. Above 80 to 100%, most manufacturers only rate the unit for short-term standby duty under ISO 8528, not continuous operation, and running there regularly shortens engine life.
Reading your load percentage result
Under 30% signals an oversized generator for the job and a wet-stacking risk on diesel units. 30 to 50% is acceptable for light-duty or intermittent use but not ideal for continuous prime power. 50 to 80% is the target zone for fuel efficiency and mechanical health. 80 to 100% should only happen briefly during motor starts. Anything calculated above 100% means the generator is overloaded and will trip its main breaker or stall.
Common applications
Total the running watts of essential circuits, add the well pump or AC starting surge, and check the percentage against the generator's rated kW before an outage hits.
Contractors log welder, compressor, and tool loads through the day to confirm the rented genset stays in the 50-80% band rather than idling underloaded.
Before wiring a workshop or EV charger into a standby generator's circuit, recompute connected load to confirm the existing unit still has headroom.
Facilities teams apply a known resistive load bank and use the same percentage math to verify a standby generator meets its rated output before certifying it for service.
Common mistakes
Summing all the starting surges instead of just the largest one wildly overstates the load and leads to buying a generator two or three sizes bigger than needed.
Backup generators left running a fridge and a few lights at 15% of rated capacity for days during an extended outage risk wet stacking and glazed cylinders on diesel units.
Treating connected load as if every circuit runs at once produces an unrealistically high number; residential demand factors of 0.65 to 0.75 better reflect actual simultaneous use.
A generator that was fine at 60% load can silently climb into overload territory after a home adds central air or a workshop sub-panel without anyone redoing the math.
Frequently asked questions
How do I calculate my home's connected load in kW?+
List every appliance likely to run at the same time, add their running watts (not starting watts), and convert the total to kW. A typical home lands between 8 and 15 kW of simultaneous connected load before adding a motor's starting surge.
Should I use running watts or starting watts to total my connected load?+
Use running watts for everything, since motors only draw their starting surge for a fraction of a second. Add the starting surge of just the single largest motor on top of the running total as a separate allowance.
Does a refrigerator really need 3 times its running watts to start?+
Yes, most compressor-driven refrigerators need roughly 2.5 to 3 times their running wattage for less than a second at startup, which is why a 700-watt fridge is often listed with a 2,200-watt starting figure.
What is wet stacking?+
Wet stacking is unburned fuel and carbon accumulating in a diesel engine's exhaust and cylinders because the engine has run too lightly loaded, below roughly 30%, for too long to reach full combustion temperature.
Is it bad to run a generator under 30% load for a long time?+
Yes, sustained light loading on diesel units causes wet stacking and glazes cylinder walls over time. Gasoline and propane units tolerate light loads better but still run less fuel-efficiently below 30%.
How much load percentage is too high?+
Above 80% for anything but a brief motor-starting spike is too high for continuous duty. Sustained operation near or above 100% will trip the main breaker or stall the engine under demand.
Do I add up the starting watts of every appliance on the circuit?+
No, only the single largest motor's starting surge needs to be added on top of the running total, since it's extremely unlikely every motor starts at the identical instant.
What's the difference between connected load and demand load?+
Connected load is the sum of every device's running watts if everything ran simultaneously. Demand load applies a diversity factor, often 0.65 to 0.75 for homes, because not everything actually runs at once in practice.
Can two large motors starting at once overload a generator sized for one?+
Yes, if a well pump and an air conditioner happen to start in the same second, their combined surge can exceed a generator sized for only the larger of the two, which is why staggering motor starts with delay timers helps.
How often should I recalculate my load as I add appliances?+
Recalculate any time you add a major appliance, a sub-panel, or a new large motor. A generator that was comfortably at 60% load can climb past 85% after adding a hot tub or a second AC unit.