💣 Recommended Generator Output
Sizing Breakdown
Full Load List
| Load | Running Watts | Starting Watts |
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How to Use the Generator Sizing Calculator
Add a row for each appliance or piece of equipment the generator needs to power, with its running watts.
Resistive loads like lights and heaters don't surge. Motors and compressors without soft start typically need 2 to 3 times their running watts at startup.
The calculator totals running watts and adds the single largest motor's starting surge on top, then applies your safety margin to recommend a generator size.
Sizing a Backup Generator for HVAC Equipment
A generator that's sized only for total running watts will often fail the moment a compressor or blower motor tries to start, because motors draw far more current for a fraction of a second at startup than they do once running. Getting generator sizing right means accounting for both numbers separately, not just adding up nameplate ratings.
Running Watts vs. Starting Watts
Running watts, sometimes called rated watts, is what a piece of equipment draws once it's up and running steadily. Starting watts, also called surge watts, is the brief spike in demand when a motor first energizes. A standard single-phase compressor or blower motor without soft-start electronics can draw 2 to 3 times its running watts for a second or two at startup, and some compressors spike even higher. Equipment with electronically commutated motors (ECM) or variable frequency drives tends to have a much smaller surge, since the electronics ramp the motor up gradually instead of slamming it to full speed instantly.
Why You Only Add the Largest Surge Once
In a typical home or small building, equipment doesn't all start at the exact same instant. The standard sizing approach assumes everything else is already running at its steady-state wattage, and then the single largest motor starts on top of that. This means the generator needs to supply the sum of all running watts plus the starting surge of just the largest motor, not the combined starting surge of every motor in the building at once. If two large pieces of equipment could realistically start at the same moment, such as two units on a shared control sequence without a delay, size for their combined surge instead.
Building in a Safety Margin
Real-world motor starting current varies with ambient temperature, voltage, and the specific motor's condition, so most generator sizing guides recommend a safety margin on top of the calculated minimum, commonly somewhere around 20%. This margin also leaves room to add equipment later without immediately outgrowing the generator.
Portable vs. Standby Generators
Portable generators typically list both a rated (continuous) wattage and a surge wattage right on the unit, and the surge rating is usually only sustainable for a few seconds. Standby generators with an automatic transfer switch are generally sized closer to their continuous rating with less distinction between running and surge capacity, since they're built for sustained duty. Either way, compare this calculator's recommended size against the generator's continuous rating, not its peak surge rating, to confirm it can actually sustain the load.
Next Steps
Once you've confirmed generator size, the transformer sizing calculator can help if the generator output voltage needs to be stepped up or down for specific equipment, and the electrical load schedule tool can document exactly which circuits the generator backs up.
Frequently Asked Questions
Motors draw a brief surge of current well above their running amperage at the moment they start, often 2 to 3 times their running watts for a standard motor without soft start. A generator only needs to cover this surge for the single largest motor starting at any one time, on top of everything else already running, but skipping that surge allowance entirely is the most common reason an undersized generator stalls or trips when the air conditioner or furnace blower kicks on.
No. In most homes and small commercial buildings, equipment doesn't all start at the exact same instant, so only the single largest motor's starting surge needs to be added on top of the running watts of everything else. If multiple large motors could realistically start at the same moment, such as two rooftop units on the same control sequence, size the generator for the combined surge of whichever combination is most likely to overlap.
Motors with electronically commutated motor (ECM) technology or variable frequency drives ramp up gradually instead of connecting directly to full voltage, so their starting surge is much smaller than a standard motor, often only 1.2 to 1.5 times running watts instead of 2 to 3 times. If your equipment uses one of these technologies, use a lower surge multiplier than you would for a standard single-phase compressor.
Compare this calculator's recommended running watts against the generator's continuous (rated) wattage, since that's what the unit can sustain indefinitely. The generator's surge rating, which is usually only sustainable for a few seconds, needs to cover this calculator's recommended starting watts figure. A generator that meets both numbers, continuous rating above running watts and surge rating above starting watts, is properly sized for the connected loads.
Related HVAC Tools
Step voltage up or down
Document backed-up circuits
Critical controls backup power
Convert between amps and watts
Variable frequency drive sizing
Total connected building load