wattage

Loudspeaker specs can be a little misleading. According to your logic you might need 80 watts if the loudspeaker is a 4 ohm model. But that only pertains to sound output at 1kHz. It could take even more watts to produce equivalent levels from the lower midrange down into the deep bass.

As a practical matter 50 to 80 watts should be sufficient for 80% of the loudspeakers on the market in typical rooms. It's just that even greater wattage may be required if you want to optimized that loudspeakers performance.

I agree with Onhwy61's comments. I'll add the following:

First, the 86 and 98 db numbers you cited are presumably at 1 meter. Neglecting room effects, the SPL of dynamic (box-type) speakers will fall off at about 6 db per doubling of distance, meaning a reduction of perhaps 10 db or more at typical listening distances. (The rate of falloff will be significantly less than that for planar speakers, though). Add 3 db back, however, to take into account that two speakers are being driven.

Second, keep in mind that speaker sensitivities are often specified on the basis of an input of 2.83 volts, rather than 1 watt. For an 8 ohm speaker impedance, that makes no difference, since 2.83 volts into 8 ohms corresponds to 1 watt. But 2.83 volts into 4 ohms corresponds to 2 watts, so subtract 3 db to get efficiency on a per watt basis if the speaker has a nominal impedance of 4 ohms and is spec'd on the basis of 2.83 volts.

Third, as can be seen from measurements provided in conjunction with reviews in Stereophile and elsewhere, it is not uncommon for speaker sensitivity and efficiency specs to be optimistic by 2 or 3 db or more.

Fourth, and perhaps most importantly, while the doubled power capability you referred to will provide for transient peaks on many recordings having narrow dynamic range, keep in mind that SOME recordings will have enormously greater differences in volume between the peaks of brief transients and their average volume. One common example being well recorded minimally compressed classical symphonic music. I have more than a few such recordings in my collection which when listened to at average levels of about 75 db at my listening position reach brief peaks of 100 to 105 db at my listening position. A 30 db difference (105-75) between the volume of brief peaks and average volume means that 1,000 times as much power is required for those brief peaks as for that average level.

Finally, see my post here for a description of the detailed calculations that are involved.

Regards,
-- Al

P.S: I hope you won't mind if I point out that it's "strictly," not "strickly." A common mistake.

1 watt at 86 db,2 watts for 89db,4 watts for 92, 8 watts for 95, 16 for 98db, 32 watts for 101, 64 watts for 104 and so on. So yes,40 is usually sufficient but keep in mind that thx is 110db and ultra thx is 112db.

Extra wattage is always a good thing with transient peaks (symbol crashed or bass drums).
Finally, you should have a "perceived" doubling of volume for every 10db increase.