Digital metering with a peak hold function is more accurate than physical meters.
P=I squared R or V squared/R It is hard to get away from the fact that current and power are extremely closely related. You know an amp has an adequate power supply if it doubles down to 2 ohms.
There is no such thing as too much power. How much power is adequate depends on the speaker and listening habits of the individual. Since every 3 dB doubles the power requirement power requirements increase dramatically with volume. To get bass at realistic levels and depth always requires a lot of power especially if room correction is utilized, in the thousands of watts.