I am trying to read a document and it says a power grid is 400kW(e) what does this mean?


The symbol kW(e) or kW${}_e$ refers to the "kilowatt electrical". It is the part of the power that is actually used by the devices connected to the power station or the grid, effectively the average of $P = U\cdot I$. The "kilowatt electrical" should be contrasted with "kilowatt thermal" i.e. kW(th) which represents the power including the thermal losses. So the figure in "kilowatt electrical" is much smaller than the figure in front of "kilowatt thermal". In your case, with 30% efficiency, the grid could be powered by 1,200 kW(th).

The unit kW(e) isn't an officially correct unit of the international system of units, SI. The correct description uses the regular universal unit, kilowatt, and the fact that we talk about the "electric power" is represented on the left hand side. So your notation would say $$ P = 400\,{\rm kW(e)} $$ but the correct recommended logic to describe the same idea is $$ P_e = 400\,{\rm kW} $$

  • $\begingroup$ 1200 $\times$ 30% = 360... Which might lead to some confusion. Perhaps use 40% and 1000 for credible round numbers. $\endgroup$ – Floris Mar 30 '15 at 11:52
  • $\begingroup$ OK, what was meant was 33.3%. $\endgroup$ – Luboš Motl Mar 30 '15 at 14:10

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