Shua42 hits the nail on the head.

If I can be so bold as to summarize:

You are only charged when the Warehouse is running regardless of how much or how little you use it.  We do have an auto stop feature you can configure. Essentially, you set a timeout and if nothing happens on the Warehouse in that time period (nobody runs queries) it will automatically shut down. It is a cost saving feature.  Cheers, Lou.