Money-saving light bulbs

How many environmentalists does it take to change a light bulb? Two, but apparently it takes them two months. Shortly after moving into the flat we replaced most of the bulbs with their energy-saving equivalents, but we left the 100W bulb in the lounge as the energy-saving bulbs we had weren't bright enough. It turns out that it's hard to find higher power energy-saving bulbs, but this afternoon we finally located a 18W (100W equivalent) bulb. Even better, it was buy-one-get-a-second-for-1p.

Coincidentally, we received our first letter from the electricity company today, so we can now work out the monetary savings from energy-saving bulbs (it's always nice to be able to save money and the planet):

Suppose we have the lounge light on for an average of 2.5 hours a day. The savings are (100W - 18W) * 2.5 = 205 Wh. For an average 30-day month, that's 6.15 kWh - around 4% of our monthly electricity usage. On our electricity plan, after a 10% discount for paying by diret debit and opting for paperless billing, that's £1.40. The energy-saving bulb pays for itself in only two months, and over its estimated 8-year lifespan will save us £134 - and yet people still complain about the price of energy-saving bulbs.

(image from http://commons.wikimedia.org/wiki/File:Compact_Flourescent-bw.jpg)