While the output of a speaker (measured in decibels, dB) for a given amount of amplifier power is termed its efficiency, over the years marketing has emphasized wattage rating at the expense of efficiency. However, both values are required to convey maximum meaning. Without a companion efficiency rating, wattage rating alone becomes merely a "damage-limit" guide of maximum power handling capability. It's unfortunate that this has become the marketing standard.
Given only a speaker's wattage rating (without its corresponding efficiency rating), there is no telling how much audible power it will output from a given power input.
The chart shows the huge importance of efficiency.
From the chart, playing 1 watt of musical power into a speaker with an efficiency rating of 96 (1 watt, 1 meter) would result in a sound pressure level of 96 decibels, measured at one meter from the speaker. Higher efficiency speakers yield more loudness from a given power input.
Actually, neither criterion will tell you how a speaker sounds to your ear or how much you will enjoy listening to it with a particular amp. The fact that a car has 200 watt speakers and a 500 watt-per-channel amp is not a criteria for loudness or for enjoyable sound. For example, powered by two Milbert BaM-235 car tube amps, Earl's car has only 30 watts on each of four speakers above 100Hz (120 watts total).
To understand why "only" 120 watts of tube power (provided by two Milbert BaM-235 amplifiers) was plenty for Earl Zausmer to win sound-off championships and garner mention in hundreds of newspapers, it's necessary to understand how the "soft overload" characteristics of tubes are audibly preferable to the unpleasant "hard clipping distortion" of transistor amps.
See also How Loud is Five Watts?