There's been a few posts about the no. of hours needed to do a PhD lately, so you might want to check other threads too. 18.75 hours a week is doable, although the 30 hours a week will take up just about all of your free time. Even though you don't have dependents, there's always other commitments - housework, exercise, and a bit of a social life etc. But I worked 20 hours a week and studied for about 30 hours, and it was fine. Hard work, but definitely achievable.