An easy-to-implement technique for measuring the dissipated plasma power of RF-excited plasmas at a frequency of 13.56 MHz is presented in this work. Specifically, the AD8302 phase detection board is used to measure the phase difference between voltage and current signals of the COST reference plasma jet. The system was characterised and power measurements were successfully carried out. Power curves of the COST-jet operated in pure helium and helium with 0.1 % of oxygen were well reproduced using the phase detection board. The current measurements were not disturbed by the addition of the circuit board and the ignition process as well as mode transition of the plasma are remarkably apparent in the phase measurements of the phase detection board. This work provides the basis for how the AD8302 phase detection board can be used for power measurements of RF-excited plasmas and what requires careful attention. This work is of particular interest for research groups, that would like to work with RF-driven plasmas, like the COST-jet, but are not able to perform power measurements involving fast and expensive oscilloscopes. However, the dissipated plasma power is the most important parameter when comparing different plasma sources or even comparing results produced by the same plasma but operated under different conditions. The measurement technique using the AD8302 phase detection board is easy to implement, requires no expensive oscilloscopes, and can be carried out in almost any laboratory that uses RF-driven plasma jets. Nevertheless, this method is only suitable for symmetrical waveforms and may lead to errors in the case of asymmetrical voltage and current waveforms. Care is required here, and proper calibration and a preliminary check of the waveforms must be carried out.