Unbilled storage refers to storage space (for sites or MySQL processes) that our system has recorded as used but which hasn't been billed to your account yet.
The usual reason for this is that the unbilled amount is less than a penny; our system won't bill your account for storage until an individual site or MySQL process accumulates at least a penny's worth. This can cause the "Unbilled" amounts on the Account info panel to be higher than one penny if you have more than one site or MySQL process. E.g. if you have two sites on one account and one has 0.6 pennies worth of unbilled storage and the other has 0.8, your account will show 1.4 cents of unbilled storage, but won't bill you for storage until one or the other gets to the whole penny individually.
The other way you can accrue unbilled storage charges is to allow your account to run out of funds. When an account is empty, storage charges won't be applied, but since we don't immediately delete your content when your account goes empty, each site or MySQL process will continue to accrue unbilled storage until one of two things happens: you make a deposit or the site/MySQL process is deleted. Once you make a deposit, unbilled storage charges in excess of a penny will post to your account. If you delete the site or MySQL process before you add funds, or if your membership expires, associated unbilled storage charges will be discarded along with the content.
If you have large sites or MySQL processes and you leave an account empty for a long time, close to the 30 day limit for example, substantial unbilled storage charges may accrue. You should definitely take this into consideration -- and our system will attempt to warn you about it if applicable -- when adding funds to an account, to prevent further disruptions in service.