I have an oversized generator, it has 150 kVA and I am only using 25 kW. If the power factor is determined by the load, how can I determine the total surplus of energy that I could generate if I would for example sell that surplus?

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    \$\begingroup\$ Hi! Welcome here! What have you been able to figure out so far? You seem to know what a power factor is, which actually kind of means you know how to answer this, so I wonder how we can help your understanding, instead of doing this small math exercise for you? \$\endgroup\$ Commented Oct 12, 2021 at 16:47
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    \$\begingroup\$ @TFM, this question is very simmilar to your last question, can you delete one and edit the question that you want to keep? \$\endgroup\$
    – Voltage Spike
    Commented Oct 12, 2021 at 16:58
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    \$\begingroup\$ 1. What is the power factor of your current load ...is it truly 25kW unity power factor or it is something else? Typically the generator rating is some specified kVA at a fixed power factor eg. 150kVA @0.9.. 2. Selling power back to the utility is normally at unity power factor. \$\endgroup\$ Commented Oct 12, 2021 at 17:26
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    \$\begingroup\$ Does this answer your question? Electric generator fuel consumption while load is increased \$\endgroup\$
    – user16324
    Commented Oct 12, 2021 at 17:26

1 Answer 1


You are confusing the concept of power factor with efficiency.

  • power factor is the the ratio of real power (kW) to 'apparent' power (kVA), computed as kW/kVA.
  • efficiency is the ratio of energy out (real watthours) to energy in (liters of fuel * joules/l).

'Apparent' power, kVA, is the vector sum of real and reactive power.

More here: https://www.electronics-tutorials.ws/accircuits/power-triangle.html

Ultimately, the genset engine is supplying real power only, that is consumed (does work) in the load and as heat losses in the alternator. The reactive power component circulates in the alternator but otherwise does no work. Nonetheless, the alternator is sized in kVA to account for that, even though that reactive component doesn’t load the engine significantly.

See also: What does a power factor of a generator means?

Your genset should have a curve showing efficiency (or at least fuel consumption) vs. output power. There will likely be a ‘sweet spot’ starting from the mid-range that will be maximum efficiency.

That said, a genset of this size won’t be as economical as a large scale utility. It is unlikely that you’d come out even selling your genset’s ‘excess’ capacity back to the grid. Solar or wind are a different story.

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    \$\begingroup\$ “power factor is the the ratio of real power to (reactive + real)“ // No, the denominator is not real power + reactive power. It’s apparent power, which is the square root of the sum of the square of real power and reactive power. \$\endgroup\$
    – alejnavab
    Commented Oct 12, 2021 at 18:15

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