The problem of charging the capacitor from a voltage source via a resistor is a common part of introductory courses of the circuit theory. None the less, the paper brings some new information concerning the energetic aspects of this process, particularly as regards charging via a time-varying resistor or charge-dependent resistor, the so-called memristor. The efficiency of the charging process is also specified for special cases, which can appear due to certain pathological characteristics of the resistor or memristor. The results of computer simulations fully correspond to the theoretical conclusions.