Well, amplifiers for home audio are voltage devices in that they regulate (or try to) their output voltage based on their input voltage. So the first step is to convert to volts. Assuming a reasonable load this simplifies things a lot.

Once everything is in volts all you need is to convert a linear scale to or from db, depending on what exactly you want out. The db scale is set up such that each 'bel' (Alex ? ) is ten times as large as the preceeding 'bel', with each bel divided into ten 'decibels'.

If all you're worried about is your speakers surviving, I wouldn't. Any reasonable speaker can absorb an amazing amount of clean music signal, as long as the bass information isn't beyond the capabilities of the driver(s) and the spectral and temporal distribution is anything close to typical for music.

Thermal limitations (which is what watts are) seldom are a factor in a HT system. Physical limits can be in the case of too much deep bass into a speaker not able to reproduce it, but this is generally a very audible problem long before anything detonates.

Every speaker I've ever blown was from too little power with only one exception.