Have about 40 gu10 10w lights. Considering changing these to the 3.5w variety but trying to get my maths right.

New ones = 40 * 3.5w = 140w

Old ones = 40 * 10w = 400w

If charged at the tariff of R1.1858 per kwh then would the cost difference be R46 odd?

(5 Hours a day of 3w = (140w*5) = 0.7kwh * 1.1858) = R0.83006 = R24.9018

(5 Hours a day of 10w = (400w*5) = 2kwh * 1.1858) = R2.3716 * 30 days = R71.148

40 lights is a cost of R800 odd so ROI in a year an a half. Worth the effort?