The wattage number written on loudspeakers, only shows how much power the speaker is capable of handling, not how much sound it can generate, or how good or bad it sounds.
Maximum sound pressure level (SPL) is how loud a speaker will go - normally measured in decibels, and its efficiency is shown in decibels per watt (dB W). The higher the decibel number shown, the more sound you'll get per watt input from the amplifier.
In practice, inexpensive set ups can sound quite nice at low volumes, but as soon as the sound is cranked up, speaker cabinets start reasonating, speaker cones break up and sound harsh, and the amplifier distorts - some of this distortion can wreck speakers.
If in doubt, listen before you buy, but also be aware that new speakers tend to 'break in' over time and sound less hard than when brand new.