You can do a broad search on the 1911 Census for free, but to see specific details for a particular address, you have to pay by buying credits. Credits are £0.12p each. You can see a transcript of the data for an address for 10 credits and, for 30 credits, view the actual census return.

Once paid for, the record stays in your file, to be looked at as many times as you like. The full 1911 Census is not part of the paid Ancestry package, so you can search and view the summary books for free with Ancestry. To see more - you buy credits from the 1911 Census site.