The 20GB was a MS published number for 64 bit OS (16GB I believe for 32bit but who has a 32 bit Surface) which could be on the conservative side or only apply to extreme cases.Interesting... I only had 5Gb free the first time I tried installing Windows 10, and it wouldn't let me. So I free'd up some space and doubled what I had to 10Gb. It then installed without an issue... so I wonder what the 20Gb is about!?
Interesting... my upgrade left me with 18GB less space.For the systems I have upgraded they were left with nearly the same free space after the upgrade as they had before if that helps. If you're totally satisfied with Windows 10 you can recover even more free space by deleting the Windows.old folder, which would be used to roll back if you wished.