How loud a speaker sounds isn't really related to its maximum power rating.
The maximum power rating simply states the maximum power the speaker can accept without damage.
A speaker's efficiency rating (or "sensitivity") is what describes how loud it sounds. A higher efficiency means that the speaker is more efficient at converting amplifier power (watts) into sound (decibels).
Some inefficient speakers may require many dozens of watts to sound as loud as a really efficient speaker might sound with just a few watts.
Speaker efficiency is usually specified in dB@1w-m (decibels of sound, measured with 1 watt of amplifier power, measured at a distance 1 meter away from the speaker).
A higher sensitivity number means that the speaker is more efficient, and requires less amplifier power to produce any particular amount of sound.
Not all speakers have a published sensitivity number - In particular, it is often omitted in cheap OEM or replacement speakers.
