I'd like to record the on/off times of an electric water heater with two heating elements. A couple weeks of monitoring should be enough. My first thought is a battery-operated pro-mini, with 2 pins monitoring each element. I'd make a small circuit to connect to the 240V element with a half-wave rectifier and a 1M/10k resistor divider to output a 3.4V peak when the element has power. I'd log the on/off times to EEPROM, dump the EEPROM when the logging is finished, and write a simple script to convert to CSV.
I tried to think of a non-contact solution, but the wiring into the water heater is BX shielded.
I'm going for quick and simple, with no intention of designing a product to sell.