VOLTCRAFT ES-1 Bedienungsanleitung Seite 26

If the load is purely inductive, the angle is 90° and with a purely capacitive
load the angle is 90°.
In both border cases the power factor is zero and it is not real power but
rather reactive power only that is absorbed by the consumers (see figure
on the next page).
In the given formulas, we are dealing with the so-called effective value of
current and voltage.
The effective value "I" of an AC corresponds to the permanent value
resulting from the instantaneous values of the current "i". This permanent
value generates the same heat in an ohmic resistor as a DC current of the
same strength.
With sinusoidal functions of current and voltage, the effective value can be
very easily computed by multiplying the peak value (the maximal voltage
value or the maximal current value) by the factor;
i.e. to calculate the real power, there are basically two possibilities.
The real power can be determined either with the help of the formula by
using the real voltage "U", of the effective current "I" and the phase
difference "ϕ", or by multiplying the instantaneous value of voltage "u" and
current "i".
For the calculation with the "ES-1", the second possibility is used, as one
must not assume that the connected consumer effectively consumes a
sinusoidal current.
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