This graph shows the logging of power on the upright display cases at Parklane Superspar in PMB in winter 2009.
The test was done over 5 weeks with no covers being used for the first 2 weeks and then used at night for the next 3 weeks. A multiplex system was in use.
The average drop in power with the covers on is clearly visible from the yellow lines. An average 26% saving was recorded.