Generator Size Calculator
Starting watts decide the generator, not running watts.
Generator Size Calculator
Recommended running = running watts x (1 + headroom); kVA = running watts / power factor / 1000.
Calculated instantly
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What This Calculator Does
The Generator Size Calculator takes the running and starting watts of the loads you want to back up, adds headroom, and reports the generator size to shop for in watts and kVA. It is a load-planning tool, not a substitute for a transfer switch and a licensed electrical design.
How It Works
Running watts are summed across everything that will operate at once. Headroom is added because generators are rated at sea level under standard conditions and should not run at their limit. The larger of the surge watts and the recommended running watts becomes the starting requirement. kVA is running watts divided by power factor and 1,000, because generators are often sold in volt-amperes rather than watts.
Example
A backup panel with 3,500 running watts and a 6,000 W well pump surge, plus 20 percent headroom, needs 4,200 running watts. The surge to cover is max(6,000, 4,200) = 6,000 W. At a 0.8 power factor the generator is 4,200 / 0.8 / 1000 = 5.25 kVA.
Formula
running watts x (1 + headroom)max(surge watts, recommended running)running watts / power factor / 1000surge to cover - running wattsReading the Results
Recommended running watts
The continuous load plus headroom. A generator run near its rating for hours is loud, inefficient, and short-lived.
Surge to cover
The largest short-term demand, usually a motor starting. If the generator cannot supply it, the motor stalls or the breaker trips.
Generator kVA
The volt-ampere rating to compare with generator labels. Watts and volt-amperes are only equal at a power factor of 1.
Sources and References
NFPA 70 National Electrical Code - Article 702, Optional Standby Systems
Installation requirements for standby power
Covers transfer equipment and the separation of utility and generator supply. Local amendments apply.
Portable Generator Manufacturers Association - sizing guidance
Running versus starting watts
Industry guidance on motor starting and generator load planning.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
What is the difference between running watts and starting watts?
Running watts are the continuous draw once a device is operating. Starting watts, also called surge watts, are the brief higher draw when a motor starts. Refrigerators, well pumps, furnaces, and air conditioners can draw two to three times their running watts for a few seconds.
Should I add up the starting watts of every appliance?
No. Start the largest motor first, then let it run before starting the next. The generator only has to cover the surge of the last motor plus the running load of everything already on. Adding every surge together over-sizes the generator and wastes fuel.
Do I need a transfer switch?
Yes for a permanently connected generator. A transfer switch prevents backfeeding the utility, which can injure line workers and damage equipment. Portable generators should use a properly rated inlet and interlock, never a suicide cord.
Why is the generator rating in kVA instead of watts?
Alternators are limited by current and voltage, which gives volt-amperes. Real power in watts depends on the power factor of the load. A 5 kVA generator at 0.8 power factor delivers about 4 kW; resistive loads use the full ratio, motors do not.
Does altitude or temperature affect generator capacity?
Yes. Engine-driven generators lose output at high altitude and in high ambient temperatures, and the manufacturer derating chart should be applied. A unit rated at sea level may not carry the same load at 8,000 ft or in a heatwave.