Apple published a table summarizing the power consumption of the iMac 2009 at the following address:
http://support.apple.com/kb/HT3559?viewlocale=en_US
This information will be of interest to those who want to buy an inverter and to know whether it will work with their machine and those who find this point important due to their ecological conscience.
Thus, a iMac 20" intended for education consumes only 87,8W, approximately 3 times less than a 24" high-end equipped with a Radeon HD 4850 that reaches 215,7W to the maximum.