Let's take the simple case where mass is assumed to stay constant during the descent.
The formula for kinetic energy is KE=(1/2) * m * (v^2), where m is mass and v is velocity.
Compare the kinetic energy at start and end of descent, using True airspeed. If True airspeed is constant, Kinetic energy is constant.
Now compare the (gravitational) potential energy at start and end of descent. Potential energy = mgh, where h is elevation and g is the gravitational constant and m is mass.
The energy lost is the decrease in potential energy, minus any gain in kinetic energy. Or the decrease in potential energy, plus any loss of kinetic energy.
If airspeed is constant throughout the descent, we are losing ALL of the potential energy we had at the start of the descent-- NONE of it is being converted to kinetic energy, or to any form of stored energy.
Too bad we aren't coming down by using a prop as a windmill to power up a battery as well as create drag, rather than by opening spoilers and such.
Alright, expanding this answer to consider comparing the energy lost in the actual descent to touchdown, to the energy lost in in some representative "ideal" maneuver.
What is the "ideal" maneuver?
Ideal Option 1) -- at the same point in time that the descent would be normally be initiated, we instead smoothly pull back on the yoke to transition to the angle-of-attack that gives the max ratio of lift/ drag (unless this would arc the flight path up beyond vertical unto the second quarter of a loop, then we have to use a slower rate of increase of angle-of-attack to pull less "G".) Anyway we end up doing a (possibly rather steep) "zoom" climb to gain altitude and lose airspeed and then we let the flight path arc back down into a steady-state glide at the angle-of-attack (and airspeed) that gives the max ratio of lift/ drag. As we overfly the point where we'd touch down with the real-world method, we note our altitude and airspeed; this gives us a basis to compare the actual energy lost in the real-world method with the energy lost by the "ideal" maneuver.
2) Ideal Option 2-- we roll back time to the point where we can glide all the way to touchdown with the plane in a clean configuration, at the speed that gives the flattest glide, after initiating a highly efficient transition to the best-glide speed such as that described in Ideal Option 1. At the moment of touchdown, compare the fuel in the tanks at the moment of touchdown with this Ideal Option, to the fuel in the tanks with the standard method of a longer cruise period followed by a steeper shorter descent. Also consider the difference in airspeed at the instant of touchdown. Now we have a basis to say how much energy is "wasted" with the standard method (but how do we evaluate the energy stored in the fuel? Do we consider ALL the energy stored in the chemical bonds, or just the energy that the engine would actually be able to extract?)
3) Ideal Option 3-- instead of descending we just maintain cruise speed and altitude so we lose neither kinetic energy nor potential energy. So no energy at all is expended with this ideal option. Well, that's a little silly and shows that we really do need to take into account the potential chemical energy we're taking out of the fuel tanks, both with the actual method and with the ideal method.
Since we're now considering the energy stored in the fuel, we now ought to revisit the ideal option 1. How much power should we carry? Just enough that the engines create zero thrust-- keep the fans turning under power so we're not using them as giant windmills? Maybe just a little less than that? Surely we don't want to chop the fuel flow to zero do we? Then the fans would make way too much drag.
On the other hand if we are considering, say, the energy that could be released by the fuel in a nuclear fusion reaction, then we better leave it all in the tanks for the "ideal" descent option, as it is too precious to waste any.
While we're at it we should really note that with the real method-- and arguably with the ideal methods too -- all the kinetic energy that we're carrying at touchdown is wasted-- for the real-world method at least I guess we really need to measure all the way to the point where the plane comes to a stop with zero energy. Also consider all the fuel wasted as we apply reverse thrust. None of this comes into play for the "ideal" methods; we get to use the most efficient method of flight possible to keep the plane moving through the air till we are overflying -- the touchdown point? The point on the runway where the plane would come to a stop in the real world? The gate at the terminal? With the no-landing ideal methods such as #1 and #3, as we're about to overfly the gate at the terminal, high above it, do we get to ease the yoke further back and bleed off airspeed all the way to stall speed, helping us to maintain altitude for a while without burning (much) fuel, or even to gain some altitude? Sure, why not.
Lots of different ways to set up this problem.