For the most part, we don't turn anything on or off. It stays in it's state. The water heater or furnace have their own systems they abide by, and given they aren't alive, they aren't required to abide by the rules of the Sabbath. This goes for any autonomous system: timers for lights, crock pots, utilities, servers, answering machines, faxes, etc. We don't cook food, so either everything has to be prepared in advance, or we eat cold stuff.
We also follow more strict rules that are beyond tech. Technology is just one thing we avoid (long story short is we are prohibited from making a fire, sparks generated but turning something on is conceivably a distant, but related spark, thus we avoid turning on things). We don't write (no pen/paper) and as adults we don't usually play with chalk, but our children are less contained by this nuance. We don't chop wood. We try to even avoid making plans for after the Sabbath during the Sabbath.
We don't drive cars, do normal work, run errands, do chores (sweeping, cleaning up, etc are fine, but some people hold that even doing dishes may violate their Sabbath, but to each their own).
Given the well-known Askhenazi propensity for hair splitting, shouldn't using FETs to turn electricity on/off be ok? There really is no spark in that case.
Does it mean not using devices with screens and/or Internet connectivity or does it include avoiding things like refrigerators, washing machines, blenders, dishwashers, ovens, stoves, electric lights, doorbells, electric kettles, water heaters, fans, air conditioning, automobiles, and many more that are built with technology?