Equivalent Annual Cost Compact fluorescent lamps

(CFLs) have become more popular in recent years, but do they make financial sense? Suppose a typical 60 watt incandescent light bulb costs £0.50 and lasts 1,000 hours.

A 15 watt CFL, which provides the same light, costs

£3.50 and lasts for 12,000 hours. A kilowatt-hour of electricity costs £0.101, which is about the national average. A kilowatt-hour is 1,000 watts for 1 hour. If you require a 10 per cent return and use a light fixture for 500 hours per year, what is the equivalent annual cost of each light bulb?