I don't put a lot of faith in 1 and 5 watt amps that are trying to emulate something much larger, but getting into the 15-30 watt class, really many amps can be working at their peak performance at the volumes that modern players want, or are limited to by venues.
Good point, though going by wattage alone can be a bit confusing, because some low-to-medium power designs are very punchy and loud, and others rated at the same wattage are relatively quiet.
There's also the question of how to measure power, such as peak power vs RMS power - it's really the Wild West when it comes to standards of measurement.
I think there's a better solution to the problem of home playing, and being useful in multiple venues, than buying a tiny amp. I'll get to that below. But first a few observations about volume:
I mentioned that Wattage alone can be a good or bad measure of volume potental.
The AC30, a truly loud amp, is 30 Watts. So is the DG30, a Matchless C30, a Magnatone Twilighter, my '66 Ampeg Reverberocket II, and many other amps that can handle real, live stage, big-boy/big-girl playing. My original Hot Cat 30 might have been the loudest amp I've ever had.
Even an AC15 is fairly loud, so is a Deluxe.
My 30 Watt HXDA can be opened up, and it's loud when you do that, but it's easily tamed by the Master Volume. It's a great MV amp.
On the other hand, there are amps in the 35-50 Watt range that aren't all that loud.
My 35 Watt Mesa Blue Angel wasn't loud enough to be useful onstage in a band with another guitar player and keys. I tried it several times. The amp didn't have a Master Volume, it was old school. But: Couldn't hear it over the bass and drums. Even miked up, cranked, I found it difficult to hear what I was doing in the stage wedges; I depended on the amp as well as the stage monitors to hear what I was doing, and it didn't get me there.
My 50 Watt Fillmore is loud enough to be useful onstage, but it isn't louder than my 30 Watt amps. Nor was my '67 50 Watt Bassman.
If you want an amp that can handle home, studio, small rooms, as well as larger venues - or playing outdoor gigs, which require more power because there aren't walls to reinforce the sound due to reflections - there are some real-deal solutions:
1. Amps that can be switched to multiple Wattage: Here's an example. My 100 Watt Lone Star can go to 50 Watts or 10 Watts. 10 Watts is
very quiet, though the amp somehow manages to sound pretty much like itself. The 50 Watt setting is my studio usual. The 100 Watt setting can handle any stage. My son used one in several recent stadium concert gigs (though it was miked up as well), at some medium venues for more intimate shows, at rehearsals (geez this band rehearsed 12 hours a day before the tour) and in the studio. Amps like this are very versatile. My Two-Rocks could also be switched to a lower power output.
Similarly, the Fillmore can go from 50 to 25 Watts. Very useful, though I tend to run it at 50. My Mark V could be switched down to 10 Watts, too, if memory serves. I have to give Mesa credit for making amps that are extremely useful at several settings. That Mark V had so many great-sounding options that it made my head spin. Granted, I moved on to more specialized amps, but even set to only 10 Watts that thing still sounded pretty darn good!
The great thing about multiple wattage amps is that they sound good at all settings, loud to quiet, because you're still getting the output tubes involved; it isn't all on the preamp tubes that tend to sound fizzy. In other words, you're getting everything going in the amp. I like this solution best because it can take you anywhere.
2. There are excellent Master Volume amps. My beef with MV is that the power tubes are often left out of the party. But some sound excellent. Lots of amps, like my Mesas, have both switchable power and master volumes. Generally, I can work out something that's reasonably controlled in volume, but still gets the output tubes to the party to an extent. As previously mentioned, the HXDA has an excellent MV. So did my Two-Rocks.
3. There are power scaling amps. They can sound great, though I find that most are not all THAT different in tone from a good MV amp.
Finally, if all you're doing is playing alone in your basement or bedroom, and you don't need the ability to handle live venues, or you don't record at higher volumes, a very small amp can be great. There's a very low power amp made by Audio Kitchen in the UK that some pretty famous producers are using for recording, and it sounds wonderful. I think it's called the Little Chopper.