A Simple To Build Power Meter
Here is the schematic for a very easy to build power meter. None of the parts are especially critical, but you do have to adjust a bit based on amplifier power.
Build two for stereo (obviously).
For R2 you need to select a value that gives you at least full scale deflection of the meter at your amplifier's maximum output. 33k should work for most amplifiers.
For C1 you need to pick a voltage rating above the maximum voltage your amplifier feeds to your speakers, 1uf at 50 Volts is good enough for up to about 150 watts.
VR1 is used to calibrate the meter. Run your amp up to the edge of clipping and set the meter to read +3 VU.
M1 can be any 100ua panel meter that strikes your fancy with a VU marked scale.
Also note this is one of the very few VU meter designs that will work with the BTL outputs on most chip amps.