(Warning: Requires Star Trek Next Generation Tech Manual. Also, for prelude, see http://ocean.otr.usm.edu/~randers2/STSWtlvpha.html )
*(Brief rundown of above site for the lazy)
1. Based on width of turbolaser bolt, the famous TESB asteroid (hereafter FTA) was smaller than 20m.
2. Based on chase scene asteroid collision, the FTA may also have been shattered, not vaporized.
(Though argued differently, this corresponds with the recent argument on this group about heat transfer rates (that the FTA must have been shattered, not vaporized), as well as a web link referring to the "conglomerated" (or "porous") nature of most asteroids (i.e. much of the volume is empty space).)
3. Given 1 and 2, a 10.75m diameter asteroid (volume roughly 650m3) of 6000kg/m3 density could be destroyed similarly to the FTA by 4.2GJ.
4. As per the TNGTM, a type-II hand phaser could perform a similar feat at maximum setting in .28 seconds.
5. The TNGTM lists the type-II as being limited to .01MW.
And now, the continuation:
6. If a .01 MW device can produce 4.2 GJ in .28 seconds, then if the Type-X phaser elements onboard the Enterprise-D acted just the same, but at 5.1 MW, then they could produce about 2100 GJ in .28 seconds, or 7500 GJ in 1 second.
7. There are 200 individual emitter segments, each capable of directing 5.1 MW.
8. 200 X 7500 GJ/s = 1,500,000 GJ/s (In English, the dorsal phaser array of the saucer is capable of 1,500,000 GJ/s, or 1,500,000 GW, or 1500 TW.)
The comparison of this figure to an ISD's firepower compares either quite favorably or quite unfavorably, depending on who you listen to, since there is no universally acceptable figure.
Back to ST-SW intro.