I've got the 500ltr tank upright fitted. This is exactly what I have done in order to heat my water up. The biggest heart palpitations come when you drill the hole in the tank, but, worry not, it works well. Follow that YouTube link and you'll be fine.
It is simple and works well. Most nights, just need to plug in for 4hrs ish til before I go to bed, usually enough. (I've insulated my tank).