Generator Size Calculator
Size a standby or portable generator from the appliances you need to run, accounting for motor starting surge, power factor and altitude derating rather than adding up nameplate watts.
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Running watts is not the number that sizes a generator
Adding up appliance nameplates gives the running load, and a generator sized to that figure will stall the first time the fridge compressor starts. Any device with an induction motor draws a surge several times its running current for a fraction of a second while the rotor comes up to speed. A refrigerator running at 200 watts can demand 1200 watts to start, and a well pump or an air conditioner considerably more. The generator has to supply the largest starting surge on top of everything already running.
Order of starting matters
Because surges are brief, they rarely coincide. Sizing for the sum of every starting watt in the house produces a generator twice as large as needed. The realistic requirement is the total running load plus the single largest starting surge, which is what this calculator reports. Load management, staggering the compressor starts so two never coincide, reduces the requirement further and is how a modest generator runs a whole house.
Derating that catches people out
A generator loses roughly three percent of its output per three hundred metres of altitude and around one percent per five degrees Celsius above twenty five. A unit rated at 7000 watts at sea level delivers under 6000 at two thousand metres on a hot day. Continuous rating is also lower than the peak figure on the box, typically by ten to twenty percent, and running at the peak figure continuously shortens the machine's life considerably.
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Frequently Asked Questions
What is the difference between running and starting watts?
Running watts is the continuous draw. Starting watts is the brief surge an induction motor pulls while it accelerates, typically two to six times the running figure. The generator must cover the running total plus the biggest single surge.
Do I add every appliance’s starting watts?
No, and doing so massively oversizes the unit. Surges last a fraction of a second and rarely coincide. Size for the running total plus the single largest surge, which is what the calculator reports.
What is the difference between kW and kVA?
kW is real power and kVA is apparent power. Generators are often rated in kVA, and the kW output is the kVA times the power factor, usually 0.8 for a standby set. A 10 kVA generator delivers 8 kW.
How much headroom should I leave?
Around twenty to twenty five percent above the calculated requirement. It covers derating, load growth and the fact that continuous rating is below the peak figure on the label. Running a generator near its limit continuously shortens its life.
Does altitude really matter?
Yes. Naturally aspirated engines lose roughly three percent of output per three hundred metres of elevation, and about one percent per five degrees above twenty five Celsius. At two thousand metres in summer a generator can be twenty five percent down on its rated figure.
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How to Use
Select the appliances you need to power and read the running and starting wattage the generator must supply.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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