Online works fine, if you've got a brain.
I use Google Drive for my online backup. I have just under 170GB stored on there. I would know within minutes if someone was encrypting my important files because the Drive icon would start showing that files were being uploaded as they've changed. I don't stare at the icon incessantly but I do notice if it's up to something. Anything encrypting my files would be quickly identified and the system would be cleaned/re-installed immediately.