How much energy is "wasted" in an aeroplane's descent?
A meaningful numeric value is going to be hard to calculate, because so much depends on the aircraft you're working with.  In practice, if I can pull off power at the calculated Top of Descent point, descend, meet any ATC crossing restrictions, fly the approach, configure as late as possible, and push up the power right at the point where I'm required to be stabilized for the approach, then I've flown an efficient approach &amp; wasted none of the energy that I started with.

If I have to add power sooner than that, I started down too soon (or wasted energy in some way); if I have to deploy drag (which includes configuring early), then I started down too late &amp; so I have excess energy that I need to dissipate.

I suppose "amount of time speedbrakes were up" would be one way to score how much energy had to be wasted; add to that how long flaps &amp; gear were out before they were "needed" (given the requirement to be stabilized &amp; configured at a given point in the approach).

Here's the complication, though.  If I see early on that I have too much energy, I can push the nose down &amp; gain speed, which increases the parasite drag of the aircraft -- thus dissipating energy.  If you have enough time at the higher speed (not just push the nose down, gain speed, pull the nose up, lose speed -- that doesn't accomplish very much), then you can get rid of excess energy without having to deploy speedbrakes or configure early.  And the reverse, if my standard descent is at 280 knots (above 10,000', obviously) but I slow up to 250 knots early enough, I can "gain" some energy, since I'm losing less with less parasite drag.  How you could account for all of that in a "score" is beyond me.

In the ideal case, I can descend to glideslope intercept altitude, level off there, let the speed decay, then at the point when I need flaps to fly any slower, I extend the first increment of flaps, let the speed keep dropping off, start down the glideslope (speed decreases more slowly or if the aircraft is light enough, not at all), at about 500' above the "stabilized approach" gate I'll lower the landing gear &amp; extend more flaps, so that as I approach that gate I'm extending the last increment of flaps, my speed reaches VTarget, and I add power.  No extra time with power above idle, a fuel-efficient approach.

ATC may not let me do that, though; they may keep me fast until close to the runway, in which case I'm extending flaps &amp; gear closer to their maximum speeds so as to dissipate more energy more quickly and still be configured &amp; stabilized at the appropriate point.  (The 737 dissipates speed pretty quickly fully configured at idle power, even descending on the glideslope.)  So did I "waste" some energy in that scenario?  Yes.  But keeping ATC happy so they can get efficient runway utilization is a worthy goal as well.