Neither the Department nor its agencies has the information to answer the question as requested.
As recorded in the answer I gave the hon. Member on 3 June 2008, Official Report, column 852W, the Department and its agencies all have policies of switching off computer devices (PCs, monitors, and printers) when not in use, and at night, and staff are regularly reminded to do so.
Apart from occasional compliance surveys at headquarters buildings there is no mechanism to estimate the number of individual computers left on overnight and when not in use, and there are no plans to introduce this.
DfT (Central) operates an automatic shutdown of non-essential computers from 7 pm every Friday. A monitoring analysis of the effect of this indicates annual savings of 3.7 tonnes of carbon emissions (13.6 tonnes of carbon dioxide) and a cost saving of £2,600.