My 2002 Focus estate TDDI has just started showing the following symptoms:- when idling with dipped headlights on, if I switch on either of the heated window elements the headlights dim. Is this due to the regulator in the alternator on its way out or the battery starting to die?
Also on the subject of batteries what amperage should the battery be?

Switching on a high load such as a heated screen will cause the lights to dim slightly.

I find that if I switch on the front heated screen the wipers will slow down noticeably even at normal road speeds. The front screen, particularly, takes a very high current which is one reason it's on a fairly short time-out. The high current causes a significant voltage drop, partly because the alternator current will be internally limited and partly due to losses in the wiring.