I have a townhouse built in 1975 with an air conditioner on the roof. I have no idea how old it is but I can imagine it is on it's last legs. What's more, this last month's energy bill was one of our highest so I'm looking to see how much I'd save a month if it were replaced? I know ACs can be expensive but they have improved so much in their efficiency I figure at some point it'll be worth replacing the old one.
How long would I have to own the townhouse before I start to make money on a new AC?
Are there any other tips/tricks to maximize efficient usage of the old one in the meantime?