This is a great question, you’re at the same point in your career as me.
You need to report on the metrics that measure your departments performance, these are monetary values and I know they may be difficult to measure and it’s not accept to suggest the business wouldn’t run with the IT group. Although this statement is true, it doesn’t address the department’s performance.
Try breaking up what you do for the company into Service Desk, Service Support and Change Management. The number of helpdesk enquiries has value to the business, they’d pay maybe $10-$50 per call if an outside desk was used and even more if administrator would be involved and you’re saving the company money.
Maintenance work in a Service Support role and managing system changes could be related to the cost for a contractor and the reduced cost of running the system versus additional revenue generated by the users to determine how much you’re saving the company money.
That’s the small stuff, now work out (with the other departmental/divisional managers) how much of *their* department relies on IT and relate that proportional of their revenue to the value you support for the company. Especially important to look at sales staff if they use a CRM tool that you support.
If all else fails remember it’s how much your department supports the production side to do or enhance their job that counts then second is the cost you incur on the company.