Planning storage requirements

I'm planning a roll out of about 35 vm's for dev/qa. they would like to be able to refresh the vm's periodically. I've got acronis set up and loaded in our environment to back up the vm's.
How should I calculate the amount of space required to store the images? Does it back up the 80 gig vm or the actual amount that is in use on disk (like 20 say)? big delta there.
I plan to do full backups over each weekend, with a differential one each night and a clean up of anything over 15 days. So should I take each systems full allocated disk space then multiply by 3 or 4 to give me enough room/fudge factor? Searching for other user's experience.
Thanks in advance