Proposed EU regulations on data retention could require every provider to keep accounting logs of its customers Internet usage. Although the technical consequences of these requirements have been investigated by consultancy companies, this paper investigates what this accounting data could be, how it can be obtained and how much data storage is needed. This research shows that every gigabyte of network traffic results in approximately 400 kilobyte of accounting data when using our refinements to existing methods for storing accounting data; less by a factor twenty than previously assumed.