Power Consumption Monitoring

Hi

i intend to work on a project where i find the energy consumed by each device of a house/office and give it to a Computer so that user may use the information to reduce his power consumption wherever required

Though i am going through online literature for this i have a doubt:

Is it necessary for me to monitor the energy consumed at each instant or can i find the power consumed by a device (say for eg air conditioner) once and based on its ON time i can multiply them to find the energy consumed.

i understand there are good amount of power fluctuations when a device is turned ON or OFF but otherwise while working normally does the power consumed change??

under normal working you just take the average power and multipy it by the time ..
that means you are buddy..