Basically, what I saw people do so far is this:
P1: I gives u powa 4 1 min 4 five cole
One minuit passes but P2 still didn't use the EU making him to waste coal. (PS: Noobs, go to school -_- )
What I do:
SplitterCable SplitterCable :CustomerHouse:
This setup is quite easy and qickly understandable.
Generator: Self Explanitory
First splitter cable: Prevents the flow of EU while the batbox is charged and your client is using the EU.
BatBox: Stores the 40k EU you're selling to him.
Second Splitter: Prevents the flow of EU if your client didn't pay or the batbox got drained.
This will allow you to charge your client for every 40k EU he uses. Neither you nor your client gets ripped off. Feel free to add ideas, bitch around or bump the topic.
Note: best used in City Building servers. If used in PvP, you can get ripped off by a nuke :o
You could do it in ssp but that will make you a HUGE forever alone.