Oh boy, you just triggered my nerd infodump mode. Brace yourself.

Disclaimer: I only researched the practical applications in the Czech Republic. I'd guess Slovakia's situation is 99% similar, the neighboring Germany is 95% similar (just with more solar and no nuclear power) and most of Europe is 80% similar (with varying energy mixes). Similarly to Teletext, I am pretty sure Alec from Technology Connections (YouTuber from Chicago) would have made a video on this if it was widespread where he lives.

Basically yes. Residential boilers in Germany are part of load management.

Technically, the electric water heaters themselves are not "smart" in any way, they are just resistors and thermostats in a water tank. The "smartest" units you'll find use a decades-old trick with two thermostats and heating filaments to achieve a larger virtual tank size. Turn them on, and they'll do their best to maintain a steady 70 °C (or whatever) at the output. Similarly, gas furnaces are all rather dumb too. There are two contacts on the units; bridge them to allow at most 100 mA to flow, which will energize a contactor (big relay) to switch the heating element or signal to the furnace electronics to go through the ignition routine. Air conditioners typically have another contact for cooling.
This is great: the protocol basically every heater uses could not be simpler and that allows anyone to build a thermostat, so available products range from the most basic bimetallic units (temperature knob and manual day (T)/night (T–5°C)/off switch) to AI Smart Home Automation IoT Buzzword Salad™ devices. The most common kind nowadays has two AA cells, a thermistor, a simple LCD display, rubber buttons to temporarily adjust the temperature manually, and a flap to reveal more buttons that set the time and weekday, create the weekly temperature schedule, adjust hysteresis, start Vacation Mode etc. There is either a set of wired contacts and a latching relay for the aforementioned wired "protocol", or a wireless transmitter that controls a wall-plug-based receiver close to the furnace.

However, a ripple control system receiver can be connected in series to the thermostat, only allowing heating at certain times set in your energy contract. Or more often, people hook it up to the boiler. All energy you consume at these times is metered separately and up to 2x cheaper so the system saves you money at rare discomfort, you can use a switch to override it at any time if you accept the extra cost.

Various companies use ripple control system receivers for energy intensive, time-independent operations, such as

• water tower pumps
• baking, annealing furnaces
• computationally heavy tasks (rendering)
• heating water for swimming pools
• charging electric vehicles

All such uses have different tariffs, hours (pltheirntractual limits to unscheduled switches) and thus need different RCS channels. Also, RCS receivers can control municipal lighting etc., and at higher frequencies that don't propagate too far, power plants would call each other but that's obsolete, now we use that band for smart city LAN (traffic lights, various sensors, parking ticket vending machines). This is called Power Line Communication (PLC) and the railway uses it too. I've seen a claim that such system caused an aircraft (Invicta International Flight 405 on 1973-04-10) to crash into the Alps after misinterpreting it as a radio beacon at Basel Airport but that person did not cite sources (interference was likely involved but nobody else claimed it was from PLC).

First dual-tariff systems used switching clocks inside the tamper-proof electric meters. This did not allow for regulation so a simple device with mechanical logic (rotary decoder of series signal) was introduced, with a starting tone (3 seconds of an approx. 250 Hz "tone" on top of the mains voltage) followed by a series of 44 gaps or tones (1 second long, 0.5 s apart) that signalled on/off states for each of the 44 channels. This was later revised into a "1 of 4" + "1 of 8" coding for 32 device groups, each then having 16 channels controlled by 2 bits each, with "10" meaning ON, "01" meaning OFF, any other means ERROR; ABORT RECEPTION. This division of the 44 pulses into groups of 4+8+2x16 allowed for 512 channels and more robustness. This protocol from the 1980s is used to this day, and so is a similar one in Germany called DECABIT. The later VERSACOM, used in both countries, then uses proprietary licensed data schemes with up to 128 bits and allows addressing individual customers. It always starts with 4 "1" bits so that dumb receivers expecting their "1 of 4" code (0001, 0010, 0100 or 1000) abort reception. VERSACOM receivers have clocks synced by special transmissions and keep a regularly broadcast weekly schedule in memory so that the bandwidth is free for the old protocol commands at switching times. Every household is assigned one of the 512+ channels and these groups switch minutes apart to prevent jumps in mains load. For some reason, frequencies used to send this signal sometimes differ between or even within control regions and sometimes not, and include 🟪183⅓ Hz, 🟨191 Hz, 🟩216⅔ Hz, 🟦283⅓ Hz, 🟥760 & 1060 Hz (no, the image doesn't exist in better quality other than cropped versions, I've looked everywhere).

If you want to look up info for Germany, the keyword is Rundsteuersystem; for Czech Republic and Slovakia it's Hromadné dálkové ovládání.

But mains beeping is not where the future lies. Our ripple control device is built into the meter and uses 2G, I imagine newer IoT protocols are being used too. People in the US have started opting into dynamic-pricing contracts and using smart thermostats that download info on quarter-hour prices 1 day ahead and switch accordingly. There are various settings that trade off comfort and cost but I think significant savings can be achieved with negligible discomfort. Some Americans would say that the utility controlling their meters feels like communism but participating in the free market and chasing discounts doesn't, even though the result is the same. EV owners charging overnight will probably soon be able to choose between options like

⚪ charge ASAP (higher battery wear, estimated cost $3.11)
⚪ charge by ➖6 am➕, constant power (lowest battery wear, estimated cost $2.81)
🔘 charge by ➖6 am➕, lowest energy cost (higher battery wear, estimated cost $2.07)

Target state of charge: ➖90 %➕ (250 km, battery wear ▓▓▓░░)

☑️ Remember selection

Many Texans are installing battery backup solutions and maybe will realize they can make money off them by "renting" them to the utility company in seasons when blackouts are unlikely. Or will just save money by smart-charging them at the price dips, which has the same effect but with less "communism".

You will be able to see the price graph in an app and decide what delay to use on a washing machine, dryer or dishwasher (provided they are far enough to not disrupt your sleep if the dip is at night). Maybe the machine will do it by itself, knowing when the most power-intensive part of the cycle is.

People with unused basements sometimes choose to install "sand batteries" that store heat (or "cold" in summer) from heat pumps, which can be retrieved at another time of day. These installations always come with dynamic power pricing.