One of the things I'd like to do down the road is to add a relay to control a 240V device such as a water heater or an EV. I have ToU billing on my electric and the Tesla can't deal with the concept properly. They claim to have an awareness of ToU, but it is very, very limited and doesn't work as you would expect. Very odd for such a high tech company to be so lame when it comes to this issue. In the Tesla forums they say it may suck, but it sucks less than other EVs. lol!
About ten years ago I used some of these remote control 240V outlets to control my hot water heater, 2400W oil column heater, and a great big fan directed at the heater.
https://www.jaycar.co.nz/remote-controlled-3-outlet-mains-controller/p/MS6147I used an Arduino Uno to schedule and control it all, and run PID control for the heater (using a thermistor on a shield daughterboard I made myself). With 100x a second analogRead() and exponential averaging with a few second time constant I got very smooth and reliable temperature sensing to 0.01 C resolution. I was able to make any reasonable temperature change (up to a couple of degrees C) in the room in about 20 minutes (twice the thermal constant of the heater) accurately and without significant overshoot. I was just turning the heater on and off on a 30 second interval.
These outlets are controlled on 433 MHz. It's one-way and sometimes (about once a day) there would be radio noise at the same moment so the command will be missed. I just sent the command for the current desired state every 30 seconds so if one was missed it was no big deal. Not receiving two commands in a row was extremely rare.
At first I hacked the supplied remote to fake pressing the buttons, but after a couple of months I reversed engineered the protocol and switched to using a $3 433 MHz transmitter board from dealextreme, which I added to my protoboard.
I used this system every winter for four or five years until I moved from NZ to Moscow. My winter heating bill went down NZ$500 compared to my previous controlling the heater with a Honeywell mechanical thermostat AND the comfort level improved a lot because I was getting 0.05 C temperature swings (up to 0.2 C with a big transient such as starting cooking or stopping cooking in the same open-plan area), instead of 2 C with the Honeywell.
The Honeywell itself had been a huge improvement, 15 years earlier, over manual control of the heater, or using a heater's built in thermostat.
Sigh. And in the rented house I'm in since June, temperature control is adding another log to the wood fire, or not, or stirring what's already in it. Electricity now (at least in this remote region) is 40 c/kWh, vs 18-20 c/kWh in Wellington in 2010-2015 before I went overseas for five years. And there is an abundance of cheap firewood.