Look at your last or next power bill or call them up and find out your price per kilowatt hour or kw/h.
A kilowatt is 1000watts for one hour, so a light that draws 135 watts will cost 0.135 x the price of a kilowatt hour to run for one hour.
Here say a kilowatt hour is 20pence/cents then 0.135 x 20 = 2.7pence/cents an hour to run.
For 24 hours thats 2.7 x 24 = 64.8pence/cents a day to run
and then 64.8pence/cents x 7 = 453.6pence/cents or 4.54pounds sterling/american bucks a week
and anally 4.54 x 52 = 236.08sterling/bucks a year..... or so so