I was just reading Robert Harley's chapter on amplifiers in his book, "The Complete Guide to High-End Audio," last night. He explains all this fairly clearly. He makes it pretty easy to understand and calculate an amps potential versus speaker requirements, how to determine the dBW power rating, what it means, and why it is useful. Give it a read. You'll get all you need.
Speaker efficiency vs. power requirements
Recently someone gave me the "math" behind speaker efficiency ratings and power requirements. Although I am not sure if the information below is 100% accurate, it is what I have been told. Can we lay this on the table for discussion and try to resolve this confusing issue once and for all?
0 db is a power ratio of 1. Records and tape have dynamic ranges of 30-40db. To achieve a 30db dynamic range requires a power ratio of 1,000:1 and 40 db requires 10,000:1. So if you assign 1 watt of power to a speaker yielding 90db SPL, you need 1000 watts to deliver a true 30db dynamic range. With digital material we find dynamic ranges of 60-70db requiring power ratios of 1,000,000:1 & 10,000,000:1 respectively. Power amps of 1-10 million watts are not feasible today but the point is, more power offers more dynamic realism. Forget power vs. loudness because that really is not a factor in the overall scheme.