[dcb]

The tricky bit is getting a handle on the usage "on the cheap",
some electric meters come with a light that blinks every "x" watt hours, that would be relatively easy to monitor. The spinny disk kind might need some other setup, i.e. laser scanning or optical mouse type approach, to monitor the usage in real time.

Polling the system voltage (and current with a hall sensor) might be a more practical approach and easy to install.

but getting a handle on automatic monitoring of mechanical gauges has other uses, i.e. the gas meter and the water meter and ???