£1/Watt/Year

If electricity costs 11.4p/KWH

11.4 / 100 (pounds) / 1000 (watts) * 24 (days) * 365.25 (years)

= £ 1.00 / Watt / year

10p/KWH is £0.88/Watt/year
11p/KWH is £0.96/Watt/year
12p/KWH is £1.05/Watt/year