Block Heater for Canada
 

I bought a 2010 Convertible that didn`t come with a block heater. Apparently they are an aftermarket installation now to the tune of about $600 -$700. I installed an oil pan heater for about $250.
Mine isn`t a diesel.
I live in Southern Alberta where the temp. can take a dip to -30 Celsius regularly and to -50 celsius on very rare occasions. I have no idea what that is in Farenheight.
Should I have bought the block heater instead...
At what temperature does my car need to be plugged in...
They were actually very unhelpful at the dealership and just plugged, ho ho good German engineering, what is the pretty red-head worried about
So, is the dealership just trying to eventually pull me back in for expensive repairs later.....or do I need to plug my car in at -15..