By "smart" I just mean programmable. I had settings for morning, daytime (nobody home), evening, and night. It makes no sense but my total costs went up. This was years ago, so I don't remember exactly what temps I had set for the different times of day. Now I just set the A/C at 76 and leave it there.
Instead, now you leave it keeping 76°F, and the thing can do it working at 10%. You consume more energy at the end of the day, but the consumption is spreaded through the cheapest hours in the middle of the day, and only a small fraction of it is consumed in the expensive hours from 6 to 10 PM. This is how freezers work, cooling intermitently as the thermostate goes off limits.
I would test if it is worthy money wise, not energy wise, to put the A/C at full blast in the cheapest hours. Say from 4 to 6 PM (the cheapest here in Spain), to get the house as cold as possible, and then try to play with A/C at maybe 5% letting it rise until reaching 76°F at 9 PM.
Depending on how big the house is and how well insulated it is, it very well may make for a big thermal battery and this would work well.
If small and/or poorly insulated or drafty then this will not work well.
Not to mention, you can be late, or early, or not arrive for the whole night.
I know when I turn on the A/C in my office, it’s not going to cool the concrete wall/floors/ceiling much in the 8h I’m there before I shut it off for the day. And that’s fine.
Even the eco-friendly bit isn't always clear, depending on your equipment. It might use less energy overall to maintain your desired temperature 24/7, even when you aren't there. Your particular equipment may run less efficiently if it has to go on full blast for an extended period of time to do a large temperature change when you first get home.
For example, where I live the energy produced from 10:00 to 18:00 is almost 100% solar, eolic and nuclear (https://www.esios.ree.es/es/balance?date=04-07-2024&program=... , orange and red bars are solar, light green are wind, dark blue are nuclear). Energy after 19:00 starts to draw gas (yellow bars) to cover the fall in solar production, peaking at 21:00. Incidentally, this also matches prices going from ~5 cents/kWh to ~20 cents/kWh (https://t.me/tarifaluz/3665). So for me it's both ecofriendy and moneyfriendly to shift as much consumption as possible to the 13:00-18:00 hours.
I haven't been able to find even specific case examples showing which consumes less energy: turning down heating/ac when no one's home or just keeping it on constantly.
I'm sure it's pretty complicated because buildings vary, but someone who understands heat transfer should be able to do it at least for a few ideal spherical cows...
> daytime (nobody home)
But that's so last century :) We WFH now.
It does make sense if you were scewing around with the temperature values in excess of 2°C, because the HVAC would work harder to reestablish a new steady state. Most houses have thermal mass and heat inertia and sometimes it takes hours to reach a new temperature set point, the HVAC working at full load to do so. It depends on many factors and it's not easy to optimize without logging the temperatures, the operating cycles of the HVAC and understanding the whole process.
Due to inflation, the expected state is your costs will go up, even in the case of a small improvement in energy use. A number of western countries also seem to be in a state of energy crisis that could explain the price going up beyond inflation.
So that is less informative than it could be. Did you see your real energy use rise?