The graph to the right illustrates the BTUs/hour required for plastic bubble skylights and Velux skylights.  The second set of numbers on the right shows how many BTU's that can be saved daily, as well as the amount of watts potentially saved.  The numbers are dramatic, a Velux skylight uses over 50% less watts/day than a plastic bubble skylight.  The Velux skylight will save you on average 137,000 watts/month.  This translates to around $7.00/month saving on your electric bill.