Here is a DIY home power usage logger, based on the AVR ATmega168 microcontroller. The output is displayed on a graphical LCD display as a bar graph. The power is calculated with the P=V*I formula. To get the current (I), he used 3 flyback coils from old TVs he had (has 3-phase power). Each coils generates a voltage according to the current of the line that is attached to. A 0.2 ohms current sense resistor allows him to measure the current. To have more accurate readings, he amplifies this current in 3 stages (X1, X10, X100) and uses the best fit gain. Data are then stored in an SD card. Read more info in his page.