How loud is a 15 Watt Tube Guitar amp in Solid State Wattage?

Watts are the same regardless of whether the amp is tubes (valves) or solid state. There is no such thing as, "solid state wattage". I think I know what you mean and a lot of what you say is broadly correct. But...

Tube amps are usually (but not always) louder than solid state amps of the same wattage but that's because watts is not a measurement of loudness (that's SPL which is measured in db).

Certainly one 15 watt tube amp could be as loud as one 80 - 90 watt solid state amp, but equally it might not be.

An important factor is the sensitivity of the speakers. The best sounding speakers aren't necessarily those with the highest sensitivity. Replacing less sensitive speakers with the most sensitive available could make a big difference in terms of loudness, especially in a set up using more than one speaker - a player who replaces all the speakers in a stack using 8 speakers should notice a big difference. Also what the amplifier is being asked to amplify, and how, makes a difference: a person with a guitar with high output pickups who plugs into an amp and turns the volume control flat out is going to be a lot louder than someone who uses maybe an old Tele with low output pickups and needs a really clean sound so keeps the volume control down.

Add to this the fact that, all things being equal, to double the volume, ten times the wattage is required (so a 100w amp would only be twice as loud as a similar 10 watt amp) and things get very complicated.

The 15 watt amp you describe could easily be suitable for a live gig but if you were playing in a fairly noisy band, wanted a clean sound, were on a large stage and you weren't mic'ed up I think you'd have problems.

A problem is that watts is a measurement of electrical consumption and not of volume. Obviously, there some correlation between wattage and volume but comparisons are difficult: a 100 watt amp certainly wont be twice as loud as a 50 watt amp and one 20 watt amp could be a lot louder than another 50 watt amp. You are right to be concerned about volume. A 15 to 20 watt tube amp could be far too loud for home use. As you say, turning the master volume down isn't the same and sort of spoils having a great tube amp. The only way really is to try out these amps and make sure they will do as you need them to at the volume you want.

15 Watt Tube Amp

Great answer from Tony. Basically there are too many factors to just answer that question. But let me give you a real world example. My friend has a 100 watt Fender Mustang solid state amp. If he cranks it up to a point it is as loud as my Mesa tube amp will get, when set at 15 watt, his amp sounds horrible.

A good tube amp is going to sound better on 10 than with the volume on 3. Most solid state amps will start to sound worse as you get close to maximum volume. So even if that 100 watt SS amp is louder on 10 that my 15 watt tube amp is on 10 it really doesn't matter because the SS amp sounds like crap on 10.

So if we are talking real "useable" volume, most good 15 watt tubes amps will get you as much useable volume as a 75 - 100 solid state amp.

15 watts for a tube amp is very loud and will do gigs fine, it is for sure much louder than a 80-90 watt solid state amp.

Speaker size has a lot to do with it too hence how a cabinet sounds much louder then a single miniature amp..