When you go through the quoting process for your solar hot water system, you get a choice between the two kinds of systems (where your situation makes that possible). To make the right decision, consider:

• The distance of the collector panels to the old tank (the further away, the more it makes sense to get a split system, e.g. two-story house)
• Your aesthetic preferences
• Your finances
• Future use and value of your house.

Some of the answers you give might rule out an on-roof system anyway, then you just need to decide on tank size.