This is a UK matter, but I am sure it is universal and common to other countries, so here goes.
I have recently gone onto HD with Virgin Media Cable TV and almost had a heart attack.
My video storage of what I thought was 80 hours, plummeted through the floor. Although I knew I only had about 45 hours of stored programs, the indicator showed only 6 hours free and then went critical.
Well, after a look at the Internet, I discovered to my amazement that recording HD programs takes around 4 times the storage capacity than recording Standard programs.
So I suggest you WATCH HD programs but RECORD Standard programs in order to save storage capacity.
Here it is :-
Virgin Media - Standard storage = 80 hours. HD storage = 20 hours.
Sky TV - Standard storage = 185 hours. HD storage = 60 hours.
Why this 4 to 1 difference between Standard and HD storage is a technical mystery which I dare not investigate for fear of having an intelligence melt-down..