The eu/t versus eu/p thing is... kinda weird.
I understand the basic concept, but I think it could have been elaborated on much, much better... somewhere.
The way I understand it, and please correct me if I'm wrong, is that a packet is essentially the size of the 'bundle' of eu that is sent out in intervals that make up the eu/t. So, let's say you have four ultimate solar panels all hooked up to the same length of cable. Even though they're collectively generating well over 2000 eu/t, their individual packets do not exceed 512 eu. The cable can accept as many of these packets as you want it to, so long as none of them exceed the maximum packet size of 512 eu.
And please, if that's wrong, someone correct me. I need to understand this properly before I start screwing around with Fusion Reactors.