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1 Often, the symbol e is used for the elementary charge. However, in order not to confuse the elementary charge with
the Euler number, we use q, just as many others in the solar cell and semiconductor device communities.
2 The energy content of food usually is given in the old-fashioned unit of kilocalories (kcal). The conversion factor
is 1 kcal = 4.184 kJ. An average adult male human requires about 2500 kcal a day.
3 Or biomass authors note.