Power Measurement
This page is the practical answer to "what power am I really measuring?" because amplifier power checks get sloppy fast when voltage, current, and phase are mixed together carelessly.
DC supply power tells you what the amp is taking in. Speaker-side RMS power tells you what it is sending out. The difference between them is where efficiency and heat start to matter.
Core Formulas
Real power:
P = V_rms x I_rms x cos(phi)
Apparent power:
S = V_rms x I_rms
Reactive power:
Q = S x sin(phi)
Useful Car-Audio Shortcut
For practical amplifier checks, most people first measure supply-side DC power and speaker-side RMS power:
P_supply = V_supply x I_supply
P_output = V_speaker_rms^2 / R_speaker
Efficiency = P_output / P_supply
What To Measure Where
- Current clamp - on the amplifier's positive supply lead.
- DC voltmeter - across the amplifier power terminals.
- RMS voltmeter - across the speaker load during the test signal.
Common Mistake
Do not assume that supply-side watts and speaker-side watts should match. Amplifiers are not lossless. Heat and efficiency explain the gap, and reactive speaker behavior makes the speaker side more complicated than a simple resistor would be.