Here's how I estimate my power usage. It's going to be an estimate, at best, until you get to measuring exactly how much power your amp is pushing out during a listening session. Also, there's often a substantial difference between your average power requirements and the power requirements during peaks - just how much will depend on the music/recording.
I have a Radioshack SPL meter I've used at my sitting position during listening. Average levels hitting the mid-80's in dBs is fairly crankin' with rock music. It's quite loud. I've seen peaks go to the mid-90's during such sessions, but not to 100dB (that I can remember). So, let's say I wanted to estimate out how much power handing a peak of 100db requires in my setup:
Start with speaker sensitivity - yours are 87db/Watt/1 meter. I doubt you're going to sit only 1 meter away, so you have to attenuate for your actual distance. SPL is cut by 6dB every time distance doubles. So say you wanted to sit 2.6 meters away (in your room size this is about as much as you can get), the equation is:
db attenuation = 6*log_base_2(actual_distance / 1 meter)
For the case of 2.6 meters, we get -8.27db
(* if you don't have a calculator on hand that does log_base_2, use the equality: log_base_2(x)=log_base_y(x)/log_base_y(2))
Next, consider that you have stereo speakers; this doubles the output power because it consists of 2 channels.
For this, you get +3dB
Next, consider room gain. You will gain some dBs of loudness by virtue of all the sonic energy getting reflected back to your ears at subsequent passes that may have missed your ears on first pass (your speakers, being monopoles, radiate sonic energy in all directions - some of this is absorbed and some reflected back into the room in a different direction). This is going to be a guess at best, maybe you can google it to see if you can get an estimator for rectangular rooms of various sizes. Ignoring bass modes, smaller rooms will have higher gains. I would guess 3db-6db. 4db would likely be a safe bet for your small room (it's probably more). This is DEFINITELY the fuzziest part of these calculations.
So, +4db here.
Now, we're at 87dB - 8.27dB + 3dB + 4dB, which totals about 85.7dB at your sitting position FOR 1 WATT POWER. So, how much will it take to increase this by 14.3db to 100db:
db = 10*log_base_10(amp_power / 1 Watt)
=> 10^(14.3/10) = amp power, in Watts
=> you need 26.9 Watts
Now, a good robust amp will likely have at least 3db of dynamic headroom for handling transient peaks. So in reality, you could count on such a stereo amp rated for 24 continuous Watts per channel to give you 100db peaks. If your average level is 85db, this will only require LESS THAN 1 WATT, which means your amp will be running quite comfortably most of the time! You can also see how high efficiency speakers really take the burden off of your power requirements...a 93dB/Watt/Meter speaker would require only a 6 Watts/ch amp (9 Watts peak handling) for the same parameters as above!
You have many options at 25Watts+. I'm partial to some of the respected vintage tube amps once restored; you get a lot of sound quality for the money if you only need 25 Watts or so. Also, the pure class-A SS amps are available at this power level.
