£1/Watt/Year

For each Watt left on for a whole 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

2022 update

... such innocent times.
This is how it looks in the current environment: 
p/kWh	pounds/W/year
13	1.14
14	1.23
15	1.31
16	1.40
17	1.49
18	1.58
19	1.67

20	1.75
25	2.19
30	2.63
35	3.07
40	3.51
45	3.94
50	4.38

55	4.82
60	5.26
65	5.70
70	6.14
75	6.57
80	7.01
85	7.45
90	7.89
95	8.33
100!	8.77