It is better to charge them for whole project not according to per hour. Lets say a client have PSD of website and he/she wants you to convert it to wordpress. If that site is limited to 5pages then you should charge $500. If your client requires design mockup from scratch then you should add $100 , which makes $600 in total.
Another possibility here is that it requires more than 5 pages ,then you may charge $50 according to per page.
It is a good practise to charge with fixed amount as it satisfies clients by letting them know what total amount they are going to spent.

Happy coding !