What other appliances are you using in the house. I'm sure you aren't recording the hours the other appliances are on for;
one month you could have used the tv more than the last month.
What you do is multiply your electricity tariff by the kilowatts of the appliances to work out the amount it costs to run per hour.
- 100w / 150w = 0.25kW
- 0.25 x 0.89 = 0.22 (0.2225)
So it costs $0.22 to run them each hour. If it doesn't add up, complain to your energy company.
Also, there is quite a bit a power lost in magnetic ballasts.