Decades ago, I chased the power wars as a guitar player. I spared nothing. I had an original 1975 first year issue Marshall 2203 100 watt 4x6550 (American market, EL34's in the UK & I changed them to 6CA7) Master Volume. I ran that through a converted 4X12 angled baffle Fender Bassman cabinet with Jensen Concert speakers (so you could actually stay on stage with it), which are a brighter version of Celestion Greenbacks, similar paper cones not quite as grindy, originally used in early "hi-fi" or stereo systems, with about the same power handling. It rocked. I even had the triple-triode adaptor that plugged into the 2nd preamp tube socket to make it into an overdrive monster when I wanted to.
It was uncontrollable. First, I pulled one pair of the power tubes and discontinued using the triple triode. Then I went to a 2X12. Then after all the abuse of the decades by prior owners then me, the output transformer started to die, so I sold it. I didn't know what to get, until Marshall recently released its Origin head. This is the modern version of the Marshall 2204 head, 50 watts, one pair of power tubes. It has one great feature: a three-way power switch so you can have 1 watt, 15 watts, or the full 50 watts. I run it through a custom cabinet with one Celestion Neo speaker that has a tone between the Greenback and the Jensen. I also changed it from EL34's to 6CA7's, which give me a little more clean headroom, but can still rock when pushed.
OK - why this digression - I usually run it on the 1 watt for personal practice, and most jams and small gigs on the rare occasion I still play guitar, I use the 15 watt setting. So I can stay in front of it, I rarely, if ever, use the full 50 watt power setting.
Also, remember that Eric Clapton recorded Layla on a 6-watt Fender Champ, and Brian May recorded with the Deacy 1-watt amp.
I forget to which second tier British Invasion band this is attributable to, but when they went on tour and needed new equipment, they actually discussed whether they really needed the full 17 watts of a Vox AC-15, one pair of EL-84's.
Epilogue: the two electric guitar players in my party band that I play bass in have, respectively, a Fender Blues Jr, power just like the Vox, 17 watts from two EL-84 power tubes, and the other has a 50 watt Boss Katana modeling amp. They are both older than I am and should know better, but our bandleader, also older, after thirty years still has to constantly tell them both to turn down so they don't overwhelm the mix, which is run through a Behringer digital board and tons of wattage, subs, and mains that can support an outdoor gig or a small stadium (which is why I only run a direct box - no bass amp on stage).
So - how many watts does a guitar player really need? One and a good microphone to a good PA.