HA is certainly not about being green. Anyone who tells you that is wrong. It is possible to reduce certain energy usage by using controlled modules, but there are other things never considered. If you have an exterior light that uses a 100W bulb and runs 24x7, you use 2400Wh per day. If you use a dusk/dawn sensor, you can reduce that by approx half (1200Wh). If a motion sensor is used, you can even go lower depending on how many times motion is sensed so maybe 1 hour (100Wh). If you use a HA module of some kind, you can also control it remotely which doesn't save energy but does make it more flexible.
A standard light switch uses 0W as it's simply a on/off toggle switch. If you use anything else, it will use some power even when off. A photo sensor is basically 0W. A motion sensor uses some power, but I can't locate anything that indicates how much. I'll assume it's fairly small (1-2W). A HA module gives no real benefit to efficiency over a motion sensor/dusk/dawn type light but uses more power. I've read that X10 modules use at least 3W per module. In my case, I have at least a couple dozen modules plus the various transmitters and other equipment.
I'd guess I'm using close to 100W just for the modules and that's on 24x7 (2400Wh per day). So, if my goal was to reduce my energy footprint, I would eliminate all HA modules and install dusk/dawn/motion sensors on outdoor lights. Also, a RPi is much lower power than a full PC, but it still uses some energy. If you want to save the world, HA is not the way to do it. If you want to come home and have lights turn on for you, the fireplace kick on, music start playing, and a voice say "welcome home", you are in the right place.
