How does auto renewal for accounts work?

When an account is created, a renewal period is specified and the payer account is charged enough hbars so that the new account will not expire for this time. When it reaches the renewal time, the account will then be automatically charged to renew for the same time. If it does not have enough hbars to renew for that long, then the remaining hbars are used to extend its expiration as long as possible. If it is has a zero balance when it expires, then it is deleted.