Err...interesting idea.
I don't think this really belongs in the weapon slot, nor in a normal permanent slot.
You may wish to submit something here if you really like the idea:
http://elementscommunity.org/forum/index.php/topic,32738.0/topicseen.htmlI guess it would result in a stall deck or a deck-out deck with an OTK possibility.
However, in that case, your idea would become too powerful. If you get that card early on in the game, and put a PA on it, it is no different from tying a timer bomb to him (or her...). Unless he could break through whatever stall you have prepared, he would be defeated in 14 turns.
A more moderate idea might this be:
Storm Ball (Unupgraded): Cost 5
; Weapon does 4 damage at the end of the turn; Ability: Charge
Storm Orb (Upgraded) : Cost 5
; Weapon does 5 damage at the end of the turn; Ability: Charge
*Charge: Instead of doing 4 damage at the end of the turn, you can choose to charge that 'electricity' (simply damage) into the weapon for future use. You may charge up to 4 times before you are forced to fire the charged electricity (shield ignored) to your opponent (thus dealing 20 or 25 damage to your opponent at once). If you do not charge the orb on a specific turn, it will simply fire the charges currently inside the orb. If the storm ball / orb is destroyed while charges are intact, you receive the damage.
Meh, just thought of it quickly. Understandable?