We understand this question to mean load factors, rather than efficiency. DECC produces load factors for onshore wind, on a quarterly basis. These can be found for the last four quarters in the following table.
|Year/ Quarter||Onshore wind load factor (percentage)|
|Note: Quarterly wind load factors are produced on a standard basis, rather than the unchanged configuration basis used for annual load factors. Source: Energy Trends table ET 7.1—available at: http://www.decc.gov.uk/en/content/cms/statistics/energy_stats/source/renewables/renewables.aspx|