Not knowing how often the lights are being turned on when it is not being used.
To solve this problem, data must be collected to understand usage pattern of the environment. From this problem, when the light switch is turned on and off, the system will take a note of the date and time of each event. Ideally, each light source would have an accompanying sensor.
Over a course of about two months, enough data would be collected to the system to start doing some preventive measurement.
However, the first measurements it would provide is how much energy is being used by the lights. This should correlate to your power bill.
Secondly, due to the pattern that it has analysed, if anything falls outside the usage pattern, a notification could be sent out. Potentially, saving money.
The sensor could be coupled to a PIR. The analysis could draw patterns on the presence of movements and the active status of the light.
In a typical family house with children, lights will be left on when they exited the room. This system will help reduce the amount of the light is switched on and no one is in the room