I think the members here way overstate the actual power needed for a gig.
Power is not what we should all be focusing on. Find a rig that sounds good and gets loud enough for most of the work you do.
IMHO it's generally best to have more headroom than need. You can always turn down if you are too loud. But if the rig is not loud enough, you are stuck.
It's somewhat of a different matter if you like pushing a tube output section into the magical realm of tubey goodness. Then the headroom needs to fall within a range that reasonably meets both your volume requirements and the desired tone quality. If your rig has too much headroom, you won't be able to push it hard enough to reach your tone goals without being too loud.
I can usually get the job done with a rig that is rated for 118dB calculated. But there is a difference between getting the job done, and getting the job done well. With only 118dB of potential headroom, I often find it difficult to hear well, and I have to change my technique to avoid unintentionally clipping the amp. 118dB is plenty loud for jazz trio. But in my experience, it often feels extremely inadequate in a cover band.
A rig capable of 124dB is about the minimum I want. I find that 124dB is loud enough for most gigs and, to be honest, I don't want to play very much louder.
My last touring rig was capable of almost 127dB calculated. It had a very efficient cab, but only 250W. I have done gigs where this rig was not loud enough to hear well, but at some point I chose to smile and act like it sounded perfect rather than making it even louder. I don't ever remember pushing that rig to clipping.
How much power is required for 124dB. That depends on usable power and the efficiency of you cab. Usable power is the minimum between how much power the amp can make and how much power your speaker(s) can handle. If the usable power is not high enough, you can't reach the goal.
Start with the speaker's 1W/1m sensitivity rating.
Let's figure out the necessary power with these sensitivity ratings:
Cab A = 94dB 1W/1m
Cab B = 97dB 1W/1m
Cab C = 103dB 1W/1m
If the target is 124db, we need to figure out the decibel change from the 1W sensitivity rating to our goal.
A = 124 - 94 = 30dB change
B = 124 - 97 = 27dB change
C = 124 - 103 = 21dB change
Next we figure how much the power must be increased over 1W to produce the required decibel change.
Before we do the calculations, let's consider this. Since we know that you get a 3dB change each time you double the power, we can conclude that cab B requires half as much power as cab A to reach 124dB. Also, cab C requires 1/4 as much power as cab B, and 1/8th as much power as cab C.
The formula we will use to figure out the required power is
[Log(X) x 10] = (decibel change)
We need to solve for X, so rearranging the formula from common log form to exponential form, we get
X = 10^(decibel change/10)
Required power A
X = 10^(30/10)
X = 10^3 = 1,000W
Required power B
X = 10^(27/10)
X = 10^2.7 = 501W
Required power C
X = 10^(21/10)
X = 10^2.1 = 126W
Hopefully you see that the amount of power required to meet our 124dB target varies drastically based on the sensitivity rating of the cab. If you have cab C (rated 103dB 1W/1m) you need a 126W to produce 124dB calculated. You get about 123dB with 100W...maybe it's close enough.
However if you have cab A (rated 94dB 1W/1m), the usable power must be 1,000W to produce 124dB calculated. With 100W you only get about 114dB. IMHO, totally inadequate, but YMMV.
My personal feeling is the sweet spot in the solid state amp market is between 500W and 800W. Sure you can get the job done with less power if you buy the right cab. But these days power is cheap and an 800W class D amp will still be relatively small and light. Buying a more powerful amp ensures a wider range of cabs will meet your minimum volume requirements. Just make sure your cab can handle the power.