Unfortunately, after my calculation, for my circumstance, an annual Travelcard is only worthwhile if I travel for at least 5 days per week, with a majority of the days involving morning peak travel. And my circumstance is that I only usually need Zones 1-2 where I usually travel for 6 to 7 days per week.
The recent fare freeze means that a season Travelcard is becoming less and less worthwhile. For example, a monthly was a good buy if I travelled for around 5 days per week, and now, I have to travel for 6 days per week, mostly during peak, to break even.
For outer zone residents, it is even less likely that a season Travelcard can bring value. More often a National Rail only season ticket (e.g. a Croydon Stations to London Terminals annual season offers validity on most of the Southern Metro routes), or paying by PAYG, will save money unless you are travelling across all your zones for 6 days per week, multiple trips per day or more.