I looked into this in some detail a few years back though its al a bit hazy now. I found information that indicated one acre of woodland should be capable of keeping one house going. Of course there are so many variables (size of house, heating usage, insulation, type of wood burned) involved its not easy to say x is enough, however id say with 5 acres you will be well in. As it was only planted in 2016, youl have a few years before you can start to realise it however ! We planted one acre of woodland (about 700 trees if I recall correctly) with our retirement in mind...I expect ill be to knackered to chop down the tress and process them by then though !
Unfortunately for us all the best time to plant a tree is twenty years ago.
In recent times we have had the odd mature tree blow down and have found one mature tree lasts us a winter quite comfortably. Though we only burn in the evenings and at weekends.
When we first bought our place I looked at ground source ( very expensive to install ) etc etc. but when watching Grand designs one day a couple proudly announced it cost something riduclous like £160 a year for all their energy costs. It was then that it hit me like a ton of bricks, its not what you burn (oil gas wood whatever) but how much of it you burn that matters. Their house was super effiecient not because the fuel was cheap, or the heating system super effiecient, but because it was insulated to very high standards and hence retained its heat. It doesn't really matter what your heat source is if you use very little of it. Say my fuel cost doubled...its still only £320 a year ! So I conclude , spend your money and time insulating, you only do it once and it pays back over and over and over.