PEAK power can only be determined for waveforms that are relatively constant, such as power lines at 60 Hz. With purely sinusoidal, constant level waveforms, the math is a snap - and you can easily measure it with a calibrated oscilloscope.
PEAK ENVELOPE POWER (PEP) is a little more complex, since it involves the waveform as modulated by a human voice or by a multitone audio signal. RF wattmeters typically measure average (not RMS) power. There are some meters that have a "PEAK" position on their switches, but these are NOT reliable in that switch position.
Meters that will accurately read PEP must have active circuitry to analyze the RF waveform, do the math, and display it on the instrument's face. In order for the instrument to do that, it needs to have either an internal battery, or an external power source, to provide the power for that active circuitry.
Some MFJ meters (and others, I suspect) have wallwarts available that will power the lights for the meter face. Check the specifications carefully and ignore switch position markings. You're the buyer, so beware.