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?
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.
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.
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.