Sorry to bring an old thread back from the dead, but the Capacitor is being buffed up a bit in 2.1 and I recently got one, so I wanted to toss out some bad math and see what I may be missing
New Proc: Equip: You gain an Electrical Charge each time you cause a damaging spell critical strike. When you reach 3 Electrical Charges, they will release, firing a Lightning Bolt at your current target for 694 to 806 damage.
It's confirmed that the proc does not use any spell damage. Not confirmed, but believed to use your base crit and +to hit.
With the above in mind, I believe it's value is roughly:
The average non crit will be exactly 750 damage. The average crit will be 1125. Depending upon your base crit, (mine is 23%), the expected average proc damage would be 836 damage.
Each regular spell crit would be worth roughly 278.5 damage. Depending upon your crit rate, mine is around 33% with fireball / 37% with scorch self buffed, would mean the trinket would be a flat +89 damage per spell.
The flat +89 damage per spell is the tricky part. Unlike other +damage trinkets, that +89 can not crit. And it's always +89 damage on a spell that can crit. One extreme is spamming AoE where it can build up the proc all on one cast, or just simplying standing in the middle of a bunch of low level mobs and watching Ignite proc it over an over. The far extreme is casting a long spell like Pyroblast.
On first glace, I assume the trinket does not scale at all. But it will actually scale slightly as you gain more +crit. My current gear gives me roughly 2.7 damage per 1% of crit. Pretty crappy compared to other gear, but still something.