Tube Power vs Solid State Power

A watt is a watt. 
See here for threads on the subject. 
https://forum.audiogon.com/search/index?utf8=%E2%9C%93&query=Tube+watts

Thank you … I should have searched 1st before posting but see others too have been fed the same line. 

It has to do with how they distort.

As mentioned watts are watts, but I do think there are a couple of other potential factors to consider that may lead to the "tube watts are louder" perception.

Wattage rating aside, and SS or tubes aside, well made amps tend to have better dynamics, better clarity, better control, and are better able to meet the demands of driving a loudspeaker than mediocre amps, so tend to sound good and have excellent impact even when pushed a bit. I suspect that many lower wattage tube amps (ie 5-40 watts) are built to a higher quality standard than most smaller wattage SS amps, due in part to specs and marketing influence. Good SS amps tend to have higher output because it’s easy and fairly inexpensive to increase that output during the design phase. While I do think there are differences in how they sound, and tube amps are known to have different distortion characteristics than SS amps (see HIlde45’s link), given comparable build quality, I think a good lower watt SS amp can sound as loud as a good lower watt tube amp.  I suspect it's often more of an apples to oranges comparison.