Jimmy T is correct, the Strata Manager probably refused to pay at all in the first instance because your Property Manager simply arranged for the repairs and then sent him/her the Contractor's Account, which based on my past experiences was probably made out to the Property Management Company and not to the Strata Plan.
By the way that really does annoy me, and to the extent that I flat-out refuse to pay any Third-Party Invoices that generate in this way!
Additionally, whilst it seems odd that the hotwater supply from your Unit's heater was in the backyard (Common Property?), if it was indeed the supply pipe from the heater (i.e. hot water), and that heater services only your Unit, then the entire cost of the repair was your responsibility anyway.
So far as the electricity use is concerned, if the heater is on the off-peak supply there will be a separate meter and corresponding charge on the Account. All you need to do is obtain the “same time last year” average from that Account, and reimburse you tenant for anything additional during the quarter when your Unit's hotwater supply pipe was leaking.