How many watts for Home Theater is enough???
Easy and simple way to calculate your needs is to use the sensitivity rating of your speakers and create a Watt vs. SPL chart.  But first, let's understand the basic rules.
SPL = To HEAR an Audible Increase in loudness, the SPL source must increase by 3 dB min.
Watts = To Increase SPL by 3 dB would require Doubling the power output ( Watts x 2 )
DD-5.1 HT Level = 105 dB (actually 106 dB SPL 'fast' peaks - see below)
OK, let's use my JBL N24 Bookshelf Speakers
Sensitivity = 86 dB @ 1 watt (from) 1 meter
MAX. Power = 100w per channel
1........86 (N24s rating)
64.......104 (Dolby Digital Home Theater SPL Level)
From Dolby Labs Website - Poster (Canyon) JBL SC305 Center Channel Speaker -Click for Anatomy of HT
IS 100watts x 5 Power Rating - Really 100watts x 5?
What's a consumer to do??
I've compiled
Stereo Review's Sound & Vision Magazine Test Results, just concentrating on
"Mfg. Pub. PWR Rating x 5 CH & THD%"   Vs.   "Actual Test Results"
(Issue Date is listed for your Reference - I will update the spreadsheet, as required)
Click to see Comparison Spreadsheet
Click the button above for the spreadsheet
My Yamaha RX-V995 is rated @ 100 watts x 5 w/.04% THD - so, . . . .my setup can reach the Dolby Digital Home Theater SPL Level easily @ the Calibration REF* Level. (*see Calibration Techniques - below link)

For most consumers, this is too loud and generally would watch their movies about 6 dB - 10 dB lower.  But if you were home alone with your hamster, you could watch your favorite Dolby Digital 5.1 DVD @
REF Level and it will be clear, dynamic and glorious!!!  My setup is compatible with my needs but keep in mind, that if your Center Channel sensitivity rating is 3 dB higher (90 dB), then you can reach that 104-106 dB SPL level with a REC/AMP that produces 32 watts of clear, undistorted power!! 
AudioSource SW Fifteen 200w 15 inch Sub - Click for Anatomy of HT
Adding an powered sub-woofer will also minimize your REC/AMP power requirements, because the LFE channel demand would be handled by your powered sub-woofer's Amp, and not your REC/AMP.  Actually, the 104-106 dB Dolby Digital spec comes from the LFE sub-sonic impact mixed in some DVD movies.
Dolby Digital Professional Encoding Manual - 4.6.6 Mixing Level (pg. 4-42)
"This parameter indicates the absolute acoustic Sound Pressure Level (SPL) of an individual channel (usually the Center channel dialog level) during the final audio mixing session. The valid range for this parameter is a value from 0 to 31. The peak mixing level is 80 plus the Mixing Level dB SPL, or 80 to 111 dB SPL."
Using Dolby Lab's Spec. by taking the MAX value of 31 & adding it to 75 dB = 106 dB
What does all this mean?? An honestly rated  70 watt to 80 watt x 5 AV Receiver will serve the purpose for HT glorious sound!!  Having more watts won't hurt, but beyond 80 watts will not add another 3 dB of SPL! 
Got that?? Now go calculate!
*Note: To go beyond 104-106 dB SPL or increase the SPL another 3 dB, would require an REC/AMP rated @ 128 watts x 5.
Use the following SPL CALCULATOR to see how "LOUD" your system will go @ your 'sweet spot'!
JBL page Select to run C.M.Collins excellent SPL Calculator
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PI - October 1999