How to work out how much energy my iPod charger uses?

As usual, bullrussell42 gave a good answer. But if the MIN rating gives a meaningful clue, it means that the minimum power consumption is 150mA at 5V, or .75W. A good estimating guide is that an electric rate of 11 cents per kWh, one watt costs a dollar a year, US. Best guess is that your charger costs you $.75 - $1 / year, if you leave it connected all the time. Less, if you plug it in only when you're using it.