Business Green: On any given night an estimated 30 per cent of workplace PCs are left on, according a major new survey which calculated that the practice is costing UK firms more than £300m a year. The survey of more than 2,000 UK adults found that 30 per cent of respondents admit to not turning off their PC overnight and at weekends, increasing corporate energy bills by an estimated £17 per PC. The findings from polling company Harris Interactive mark an improvement on the last survey, …