Is there any way to predict how large a Windows system image backup will take?

I'm looking at moving to Windows System Image for my backups as I'm realising that just backing up user files is still going to leave me with days of installing and configuring in the event of a disaster. I've got three disks - my boot SSD (128GB), an application/working data drive (1TB) and a user data drive (2TB). The total amount of data on these drives is about 2.5TB.

Currently I already have a bunch of 2TB exteal drives for backups and would obviously like to keep using them if possible. From some preliminary reading I've seen that Windows ignores empty space when imaging (unlike a disk image) and will attempt to compress any compressible files so I suspect I can fit a single image on my existing exteal drives, but is there any way to confirm this without just doing it and seeing if it throws up an error?