Simple Plug in Energy Monitors to find out how much electricity an applicance is using:
Monitors to measure electricity usage whole house (without solar) These energy monitors are hooked up via the meter box. The sensor is placed on a cable in the meter box (this should be done by an electrician for any of these type of monitors). If you were to turn the airconditioner on you would immediately see how much your energy usage has jumped.
If you have solar panels or any type of photovoltaic system installed on your house you would need a monitor such as the Wattson or the Watts Clever Solar Meter. With an extra sensor it can detect the flow of power in both directions.