Apart from a SUS (System Usability Scale) assessment – which is an established method of measuring usability – there are several other assessments that can be used for a website.
The SUS assessment is a good method for measuring the users experience of a website and how they grade the usability of a system, but it does not give any indications to what can be improved or where the users get stuck. A quality assessment can give you more values. Now, we’re still talking about experiences of a website, and not specific tasks or paths a user will take though a website, we are measuring the feeling associated with the system. Before you start you should also have a good reason for measuring and a plan for how to treat or act on the result.