Disclaimer: I know nothing about electronics or guitar amps.

(Actually, I do know one thing: that "wattage" is synonymous with "volume" when speaking about guitar amps and since watts are measured on a logarithmic scale, a 100 watt amp is only twice as loud as a 10 watt amp...I think.)

I am very confused.

My question applies to any guitar amp, but as a random example, I will use the Fender Blues Junior III amp.

According to the product description on the official Fender website (and all product descriptions of this amp online), it has 15 watts:

Screenshot:Fender "Blues Junior III" amp production description

But, according to the plate on the back of the amp itself, it has 180 watts:

File:Fender Blues Junior III manufacturer plate

How come there are two different watts numbers with guitar amps? What does each number represent, and how can they both simultaneously represent the amp's "wattage"?

  • \$\begingroup\$ Although Barry answered your question, I want to correct a misconception: wattage is not synonymous with volume. Speaker sensitivity (how efficiently the speaker converts electrical energy to acoustic energy) and how the amp behaves when distorting/whether it is distorting are two significant factors that affect how loud an amplifier may be at a given wattage. \$\endgroup\$
    – uint128_t
    Jan 19, 2016 at 4:06
  • \$\begingroup\$ FYI: that amp in particular has five vacuum tubes. Each vacuum tube contains a tiny, red-hot "filament" (heater) which is necessary to make it work. The tube filaments consume a significant fraction of the total power. \$\endgroup\$ Dec 6, 2021 at 0:05

1 Answer 1


Although both values are in watts, they are referring to two different power quantities. The "15 watts" refers to the audio output power, i.e. the power delivered to the speaker. This is the power that generates the sound that you hear. The "180 watts" listed on the back of the amplifier refers to the power drawn from the AC input line. This is the power supplied to the amplifier from the AC wall socket (120 volts at a frequency of 60 Hz). It needs this power to run its internal electronics which produces the 15 watts of audio output power. The AC input power will always exceed the audio output power since the amplifier cannot be 100% efficient. In this case, its efficiency is pretty low (15 watts out for 180 watts in or an efficiency of 8.3%).

  • 1
    \$\begingroup\$ The efficiency is higher in solid-state amps than in tube amps like this one. The output power is a bit nebulous for guitar amps: it is usually measured as maximum power before distorting, but that's not how little guitar amps are usually played. I've played through that amp and it gets very, very loud. \$\endgroup\$
    – markrages
    Jan 19, 2016 at 3:57
  • 1
    \$\begingroup\$ @Barry Ah, I understand now that when people refer to an amp's "wattage" they are actually referring to the "audio output power" wattage. Thank you. I realize it can never be 100%, but where on Earth do the other 165 watts go?? (I did the math on a different amp and it has an efficiency of 8.9%, so it's not just this amp that eats power.) \$\endgroup\$
    – Kyle
    Jan 19, 2016 at 5:31
  • 1
    \$\begingroup\$ It gets dissipated as heat. \$\endgroup\$
    – Ladislav
    Jan 19, 2016 at 7:09
  • 3
    \$\begingroup\$ @Kyle, they get wasted as heat. Your amp is also a 165W heater. \$\endgroup\$
    – Austin
    Jan 19, 2016 at 7:10
  • \$\begingroup\$ It says right on the back there, (see OP's picture,) "CAUTION chassis surface hot" \$\endgroup\$ Dec 6, 2021 at 0:07

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