COOL project, and this is something that i will make in the near future (not this year though). I've done some research. You can use current transformers for this project.
A current transformer is a device like a small donut with 2 wires. The phase wire goes through the center of the transformer, and no galvanic connection is required. The output of the transformer can be used to determine the amount of current that flows through the phase wire. The power is indirectly measured (P=VxI). A current transformer usually costs around 50 euro, but is easy to use because the project is completely isolated by the mains power.