Reading through various articles about the upcoming launch of the New
Horizon probe I did not manage to locate any solid information about
the amount of energy & light available from the Sun on Pluto.

The sun emits about 1400 watts per square meter at Earth's orbit.

Pluto is (roughly) 40 times as far from the Sun as Earth.

Per the inverse square law, Pluto intercepts 1/1600th as much sunlight
as Earth, per square meter.

Therefore, Pluto receives approximately 0.875 watts of sunlight per
square meter.

If you want more precise numbers, you can google up precise values for
Pluto's current distance from the sun and the exact quantity of
sunlight received in Earth orbit.

If it's true I find it really remarkable that an earth based
telescope would manage to "see" Pluto...

Pluto is an object 2274000 meters in diameter. As a quick
approximation, that means it presents a disk of some 4.06E12 square
meters, intercepting a total of 3.55E12 watts (3.55 terawatts) of
sunlight. Using Pluto's lowest albedo value of 0.49, Pluto reflects
about 1.74 terawatts of sunlight for terrestrial telescopes to spot.
That's quite a bit of light.

Of course, Pluto doesn't focus its light like a manmade spotlight but
rather reflects that light across a hemisphere, but even minor
quantities of light add up when you're illuminating objects 2000km in
diameter.

Mike Miller