Generator Size Calculator

A generator size calculator establishes the minimum running rating and minimum surge rating needed to support an anticipated electrical load. The result is used during early generator load planning to determine whether the expected connected equipment can operate continuously and whether motor or equipment starting demand can be carried without excessive voltage dip, overload, or generator shutdown.

For the values shown, the calculator produces a minimum running rating of 6,000 W (6 kW) and a minimum surge rating of 10,800 W. These are planning values, not a selection of a specific listed generator, engine-generator package, transfer switch, feeder, or overcurrent protective device.

Generator Load Demand

A generator must satisfy two different load conditions:

• Continuous operating demand, represented by the running load.
• Short-duration starting demand, represented by the largest starting load and any other simultaneous starting load.

The running rating supports the equipment after motors, compressors, pumps, fans, and other loads have reached normal operating speed. The surge rating addresses the temporary additional wattage required while starting loads accelerate or energize.

The calculator separates these conditions because a generator that can support 5,000 W of steady load may still be unable to start a motor-driven load if the momentary demand rises materially above its available surge capability.

Calculator Inputs

Input	Purpose in generator planning
Running load	Total running watts expected on the generator after connected equipment is operating normally
Largest starting load	Largest additional starting watts expected from one load starting
Other simultaneous starting load	Additional starting watts expected to occur at the same time as the largest starting load
Power factor	Power factor used to convert the watt-based demand into an estimated kVA requirement
Planning margin	Extra capacity added above the entered load condition

Running load is the baseline operating demand. It should reflect the loads intended to run together, rather than the total nameplate load of every circuit that happens to be supplied by the generator.

Largest starting load is additional starting wattage, not the full running wattage of that same load. The calculator adds it to the running load to represent the temporary peak.

Other simultaneous starting load covers additional equipment expected to start during the same event. If no other starting loads are expected to overlap, this value is zero.

Power factor does not change the displayed watt ratings. It is used for the kVA estimate, since generator nameplates and equipment specifications commonly state both kW and kVA capacity.

Planning margin applies a multiplier above the entered demand. It allows capacity for expected variation, small future loads, and uncertainty in preliminary load assumptions.

Running and Surge Formula

The calculator applies the planning margin to both the running and starting-load condition.

\(\displaystyle \text{Margin multiplier} = 1 + \frac{\text{Planning margin}}{100}\)

\(\displaystyle \text{Minimum running rating} = \text{Running load} \times \text{Margin multiplier}\)

\(\displaystyle \text{Minimum surge rating} = (\text{Running load} + \text{Largest starting load} + \text{Other simultaneous starting load}) \times \text{Margin multiplier}\)

The kVA estimate associated with either watt demand is:

\(\displaystyle \text{kVA} = \frac{\text{W}}{1000 \times \text{Power factor}}\)

A lower power factor increases the kVA required for the same real-power load in watts. Generator selection must account for both the kW and kVA limits stated by the generator manufacturer.

Calculation Example

Use the following entered values:

Input	Value
Running load	5,000 W
Largest starting load	3,000 W
Other simultaneous starting load	1,000 W
Power factor	0.8 PF
Planning margin	20%

The planning margin produces a multiplier of:

\(\displaystyle 1 + \frac{20}{100} = 1.2\)

The minimum running rating is:

\(\displaystyle 5{,}000\text{ W} \times 1.2 = 6{,}000\text{ W}\)

\(\displaystyle \boxed{\text{Minimum running rating} = 6{,}000\text{ W} = 6\text{ kW}}\)

The starting-load condition is:

\(\displaystyle 5{,}000\text{ W} + 3{,}000\text{ W} + 1{,}000\text{ W} = 9{,}000\text{ W}\)

Applying the same planning margin:

\(\displaystyle 9{,}000\text{ W} \times 1.2 = 10{,}800\text{ W}\)

\(\displaystyle \boxed{\text{Minimum surge rating} = 10{,}800\text{ W}}\)

At 0.8 PF, the estimated generator demand is 7.5 kVA under the 6 kW running condition and 13.5 kVA during the 10.8 kW surge condition:

\(\displaystyle \frac{6{,}000}{1000 \times 0.8} = 7.5\text{ kVA}\)

\(\displaystyle \frac{10{,}800}{1000 \times 0.8} = 13.5\text{ kVA}\)

Generator Selection and Field Verification

The calculated ratings establish a load-planning target. A field selection still requires verification of the actual generator’s continuous kW rating, kVA rating, transient motor-starting capability, voltage regulation, frequency response, and permitted operating conditions.

Motor-driven equipment requires particular attention. A motor nameplate, controller type, starter method, variable-frequency drive, compressor controls, and the sequence in which loads start can produce a field demand different from a broad starting-watt estimate. Large HVAC equipment, well pumps, refrigeration compressors, and fire-pump-related systems require equipment-specific review.

Generator output also affects the distribution equipment supplied downstream. Feeder and branch-circuit conductor ampacity, AWG or kcmil conductor selection, terminal rating, insulation temperature rating, overcurrent protection, voltage drop, raceway fill, grounding and bonding, and transfer equipment must be evaluated separately. The calculated watts do not establish conductor size or transfer-switch rating.

Verify the installation with the applicable electrical code requirements, the generator and transfer-equipment listings, manufacturer instructions, utility isolation requirements, and the AHJ.

Related calculations: Inverter Size Calculator, Electrical Load Calculator.