lol! That's a good one!

if it's a modern thermostat you would want to save the settings in a power outage and the current draw is so low that a 9V will last a long time.
not only that you don't need to include a (AC-to-DC) transformer in the thermostat.
it also means the thermostat can be screwed into the wall anywhere that you want to poke a hole for the control wire and won't require a junction box or an electrician to install it.

I know, but I still like my reason better- easier for the simple folk to understand.

someone asked why the thermostats have batteries instead of being connected to the regular power. I replied that it was so you could still adjust the air conditioning even during a power outage.

They make thermostats with batteries? Why don't I have one to work with my GAS heater which becomes unusable during a power outage? (The stove similarly has electric ignition running off house current. But I can always light the burners with a match, and the oven isn't a daily essential.)

I once lived in a place where the smoke alarm ran off house current. The town had underground power cables-- this was in an area that had Winters-- so outages were not a regular problem, but still...

The batteries are only to maintain memory in a "smart" thermostat on a central air system.

Forced air fans draw a lot of current, and A/C compressors prodigal amounts. I LOL'd at the power outage quip, thinking of hoisting 72-lb RV batteries, several of which could run a small A/C for a matter of minutes. Modern furnaces have complex flame and draft proving circuitry that needs a main connection. These traditionally have a 24VAC thermostat circuit for operation, allowing simple wiring.

Old fashioned convection furnaces use a "pilot generator" to convert pilot flame heat to a few hundred millivolts, just enough to control the gas valve. This type of furnace keeps working during power failures; the downside is, heat distribution is always lousy. Batteries could power a smart thermostat for this type, as this alone needs about as much power as a digital clock.

Yes, that's what I meant. It's not a house with a furnace-- those would obviously have a forced-air component requiring electricity-- it's just a single heater. Not even the fancy kind with a fan that's powered by rising heat. Electric ignition, but unlike the oven it doesn't use house current. Just a spark thingy.

My utility bills plummeted when I gave up the 75-gallon aquarium. Heating it used more electricity than everything else in the house put together.

The electricity is required for the "smart" thermostat, not sure about current mechanical ones but I know the older mechanical ones have a mercury switch in them which I'm pretty sure they can't manufacture anymore.

There is two small wires going to the thermostat at least for basic heating system. The thermostat on the wall is basically a switch and it can't draw power from that line because it would need to complete the circuit.

My utility bills plummeted when I gave up the 75-gallon aquarium. Heating it used more electricity than everything else in the house put together.

That is basically a radiator. If you have electric heat you shouldn't have seen any drop. If you have gas heat then of course the electric bill would drop and the gas bill would go up. In the summer your electric bill would drop dramatically because the "fish tank radiator" is no longer using electric to heat the cold air you just used electric to produce.

Hee. In this climate, air conditioning is very much optional. I've never been in a house that had it. Maybe some large windowless public buildings, but even then, air circulation alone should do it.

Aquariums are typically heated to at least 10 degrees (F) above "room temperature". Different story if you've got a saltwater tank with so much lighting that even if it's fluorescent (i.e. little heat involved) you need a chiller to bring it down again.

When I spent a month in New Delhi a few years back the electricity went off about 30 times a day, which is pretty lame for a capital city. The office had a big room full of batteries that kept everything running temporarily while the generator kicked on outside, so technically the A/C thermostat did have battery backup :)

The part that sucked was the generator could only sustain about half of the A/C in the building and it toggled a relay every time the lights went out so your location was cooled every other time.

Now the fun part was they still had elevator operators at my hotel, don't remember the elevators having A/C at all, or often if it did, as all I remember was a fan in the ceiling, and that poor guy forced to wear an elevator operator outfit with long sleeves was sweating like a wild pig in a hot desert.

Imagine the fun of being trapped with the little man in the smelly suit when the lights managed to go out when I was in the elevator... ugh. Not to mention the great conversation we had as his English appeared to consist of "what floor?" and numbers to ten.

technically the A/C thermostat did have battery backup

A/C? In Delhi? You hung out in posher places than I did. I remember bedtime: soak the bath towel and wring it out, turn on the ceiling fan, use the wet towel as a top sheet and hope you get to sleep before it evaporates.

That was a top-floor room, of course. Elevators? You bloody well took the stairs. All five flights of 'em.

Most places the power didn't go out anywhere near 30 times a day, though. There wouldn't have been time. Once it went out, it was down for the count.