Excluding variable energy pricing, it’s much more energy efficient to only heat and cool your home while you’re actually at home.

Think of it like a tea kettle. It’s definitely not energy efficient to keep the water boiling for the hours when you’re not home just because you might want a cuppa when you get home. The only benefit keeping the water hot is to brew your next cup quicker. The water is cooling off at the same rate it would if the heater was off, but energy is being pumped into it to keep it hot and therefore it is constantly losing energy.

This is also assuming your HVAC’s coefficient of performance is constant, which it’s not, but it still generally is way better to avoid heating and cooling while you’re away from home, especially if you live in an older less insulated home.

If you do have variable energy pricing, that can change things, and that’s when a smart thermostat can really save you money. Instead of heating and cooling around your schedule, you do it around the pricing treating your house like a battery. See: https://youtu.be/0f9GpMWdvWI?si=LjiAjNf6t8cU8OZ2

This video really only really works if your home is relatively well insulated (as he points out). If it’s not well insulated, you’ll be uncomfortable basically all the time.

Generally if you’re on a variable rate it’s better to set the thermostat closer to the outside temperature when you’re gone for more than 5 hours. If you’re not on a variable rate, that break even point is like 30 minutes.