Electrical AC circuit is built with resistor R=0.25[Ω] and inductivity L=10[mH]. Resistor and inductivity are connected in series. Between circuit's power supply terminals is voltage U=230[V]. Voltage has initial phase ψu=π/2. Supply voltage has a frequency f=50[Hz]. As a result of supply voltage, current I flows through circuit. All calculations are made with application of complex numbers. Current I is designated with application of Ohm's law for AC circuits → U=Z·I. To make possible current calculation total impedance Z of circuit has to be designated. Subject of example is calculation of apparent power S, active power (real power) P and reactive power Q. Power triangle is plotted in example.