Generator Sizing Calculator
Estimate minimum generator wattage from running load, motor surge demand, and NEC continuous load rules — with safety warnings for large units and high inrush loads on 120 V and 240 V circuits.
Load Details
W
W
Live Results
Recommended Generator Size
4,500W
Adjusted Running Watts
3,000W
Running Watts
3,000W
Surge Watts
4,500W
Voltage
120V
Continuous Load
No
Generator sizing follows NEC continuous load guidelines. Always verify final design with local electrical code and a licensed electrician.
How to Use This Generator Sizing Calculator
- Add up running watts. Enter the total running (rated) wattage of all equipment you plan to power simultaneously — use nameplate or manual values for refrigerators, sump pumps, HVAC, lighting, and critical circuits. Running watts represent steady-state power draw after motors and compressors have started.
- Enter surge (starting) watts. Input the highest combined starting surge for motor-driven loads — typically refrigerators, well pumps, air conditioners, and table saws. Surge watts are the brief inrush current at startup and often exceed running watts by 2× or more. The generator must support this peak even if it lasts only a few seconds.
- Select system voltage. Choose 120 V for standard branch-circuit backup or 240 V for whole-home panels, well pumps, and large appliances on double-pole circuits. Voltage is recorded for planning transfer switch and cord configuration — the watt-based sizing formula uses running and surge values directly.
- Mark continuous loads if applicable. Check Continuous Load when any backed-up equipment is expected to run for 3 hours or more — examples include refrigerators, freezers, medical equipment, and heat pumps. The NEC 125% rule increases adjusted running watts before comparing against surge demand.
- Review recommended size and warnings. The results panel shows recommended generator wattage as the greater of adjusted running watts and surge watts. Review large-generator and high-surge warnings, then confirm transfer switch rating, fuel capacity, and conductor sizing with the Breaker Size and Wire Gauge calculators.
Formulas & Example
Generator sizing compares adjusted running watts against peak surge demand and selects the larger value as the minimum recommended generator rating.
Adjusted Running Watts = Running Watts × (Continuous ? 1.25 : 1.00)
Recommended Generator (W) = max(Adjusted Running Watts, Surge Watts)Worked Example
3,000 W running, 4,500 W surge, 120 V, continuous load off:
Running Watts = 3,000 W
Surge Watts = 4,500 W
Voltage = 120 V
Continuous Load = No
Adjusted Running Watts = 3,000 × 1.00 = 3,000 W
Recommended Generator = max(3,000, 4,500) = 4,500 WSurge demand governs in this example because motor starting watts exceed steady running load. If continuous load were enabled, adjusted running would be 3,750 W (3,000 × 1.25) — still below surge, so 4,500 W remains the minimum. Pair this tool with the Breaker Size, Voltage Drop, Wire Gauge, and Conduit Fill calculators for complete backup power planning.
Frequently Asked Questions
What size generator do I need?â–¾
Generator size depends on the greater of your adjusted running watt total and your peak surge watt demand. Add running watts for all simultaneous loads, apply the NEC 125% multiplier if any load is continuous (runs 3+ hours), then compare that value to the highest combined starting surge. Select a generator rated at or above the larger number — for example, 3,000 W running and 4,500 W surge with no continuous loads requires at least a 4,500 W (4.5 kW) unit.
What are surge watts?â–¾
Surge watts (also called starting watts) are the brief power spike motors and compressors draw when they first start — often 2× to 3× their running wattage for a few seconds. A refrigerator might run at 200 W but need 1,200 W to start. Your generator must deliver enough surge capacity to start the largest motor loads without tripping overload protection, even if average running load is much lower.
What is the NEC continuous load rule?â–¾
The NEC defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. For continuous loads, equipment — including generators and overcurrent devices — must be sized at 125% of the load. On a generator sizing worksheet, multiply continuous running watts by 1.25 before comparing to surge demand. A 4,000 W continuous load becomes 5,000 W adjusted running watts.
Does voltage affect generator sizing?â–¾
Voltage does not change the watt-based sizing formula in this calculator — running and surge watts already account for power demand regardless of whether the circuit is 120 V or 240 V. Voltage matters for transfer switch configuration, cord and receptacle selection, and whether you back up a full panel or selected circuits. Higher-voltage circuits deliver the same watts at lower amperage, which can reduce conductor size on long runs.
Should I oversize my generator?â–¾
A modest safety margin above the calculated minimum is good practice — typically 10% to 20% — to handle measurement uncertainty, future loads, and altitude or temperature derating. Oversizing too far wastes fuel and increases cost without benefit. Avoid undersizing: a generator running near 100% capacity continuously will have shorter life, more frequent overload trips, and poor voltage regulation under motor starts.
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