Yes they're on the roof, and face almost due south too
I know that sounds obvious...but they were installed by the developer who built our house and looking at other houses on the estate I think they just installed them on whichever roof was most convenient for them, rather than actually orienting them to maximise the efficiency.
Our house isn't large but its only about 7.5 years old so it has pretty good insulation. Our heating is set to 19/20c when its on, but we don't have it on all day and the spare bedroom is kept closed off with the radiator turned off unless we have guests. If I get a bit cold at my desk I use fingerless gloves (
these are my new ones....they're lovely) and a
heated throw (which costs pennies to run).