The FCC defines Peak Envelope Power for radio amateurs as, "The average power supplied to the antenna transmission line by a transmitter during one RF cycle at the crest of the modulation envelope taken under normal operating conditions." [47CFR97.3(b)(9)]
Figure 1. Peak Envelope Voltage
Once that cycle is identified, we calculate the average power over its complete duration. That's the red area in Figure 2.
Note that we calculate the power in both the positive voltage half-cycle and the negative voltage half-cycle. They don't cancel out because, as shown below, the PEV is squared, making both of them positive.
Figure 2. Peak Envelope Power Sine Wave
Start by measuring the amplitude of the peak, usually in volts. That is PEV in Figure 1.