This, incidentally, is the answer to your question elsewhere in the thread about what banks do with deposits: They put them in the central bank, and the central bank pays them its policy rate for that (give or take the CB's own bid-ask spread). They make money on the spread between the policy rate and the rate they pay their depositors, and their costs include servicing those deposits with ATMs, branch offices and so on.

Well the McKinsey report I cite below describes profits banks had from interbank lending on deposits. And this report from the Fed shows that prior to the financial crisis, excess reserves were near zero

http://www.newyorkfed.org/research/staff_reports/sr380.pdf