Note that "least consumption" is not the right indicator, different people have different levels of consumption. What you want is a break in the consumption pattern, a home with lower than usual consumption of electricity.
They don't need to spend any time on it past the initial setup, you can have an automated system integrating the consumption pattern of an entire neighbourhood, and pointing out which owners are likely on holidays.
Note that "least consumption" is not the right indicator, different people have different levels of consumption. What you want is a break in the consumption pattern, a home with lower than usual consumption of electricity.