Electric power is the rate at which a circuit transfers electrical energy. It is measured in watts (W), with one watt equal to one joule per second. Radio stations use power figures for several different quantities, including DC consumption, RF output and peak envelope power.
For a steady DC voltage and current:
P = V I P = I² R = V² / R (resistive load) Energy = P t (constant power)
V is in volts, I in amperes, R in ohms and t in seconds; energy is then in joules. A device drawing 5 A from 13.8 V consumes 69 W. That does not mean it radiates 69 W: some energy becomes heat and some supports other circuitry.
If a hypothetical transmitter converts 40 W of a 69 W DC input into RF output, its efficiency at that operating point is:
Efficiency = P_out / P_in = 40 / 69 ≈ 58%
For sinusoidal voltage and current at the same frequency:
P_average = V_rms I_rms cos(φ) S = V_rms I_rms
φ is the phase difference. P is real power in watts; S is apparent power in volt-amperes (VA). For a purely resistive load, φ = 0 and the power factor is one. Reactive loads require the phase relationship, not just two meter readings. Distorted waveforms require a more general calculation.
A 10 V RMS sine wave across 50 Ω produces 2 W. The same sine wave has a peak voltage of about 14.14 V; using that peak value in the RMS formula would overstate average power.
Average power and peak envelope power (PEP) describe different aspects of a modulated signal. Voice SSB has a changing envelope, so a slow average-reading meter will not normally show its PEP. dBm expresses a power level relative to 1 mW.