May be we should work out some other solution.
Because the output power should not be proportion to number of lighted LED.
For electric power generation:
All the power generated will be gone if there is no load conntected to the output of power generator.
The power generator try to output a constant voltage (with the same AC frequence).
It is the current flow through LED which will determine the brightness of the LED.

I am agree that, we can connect more LEDs with the same brightness if there is more power generated.
But the studnets will not know so much.

I am afraid this might introduce another mis-concept to students.
And they might remember if more power generated, more LEDs will be light up -- which is not true.
I do not think this is good.

If it is about "position energy" only  (unless you also want to make connection to electric power generator?)
When a ball is moving higher and let go. It will gain more energy when it moving down.
Or may be you some other better ideas?

As you can see from another simulation ([url=]Re: Magness and steel marble[/url]) I created for you:
I can use picture as background image in the simulation if you can provide me some picture (which is better for young students).