Hi everyone.
Firstly I hope to have made this post into the right category (if it wasn't, let me know it so I can fix my mistake).
I would like to create a thermostat for my house with an Arduino Uno that controls a relay to activate the system, an LCD display 16x2, a TSIC 506F TO92 as thermal sensor and 3 buttons for some configurations. The whole system has to be supplied by batteries.
Then, I discovered that its energy consumptions are too high (reading on some websites I found out that it can last 20 hours at best) and someone advised to use an Arduino nano because it's "cheaper" as far as energy is concerned.
Firstly, is it true? And How many hours it could last?
Thank you for every advice you will be able to give
For battery powered devices an Arduino Pro Mini may be better. They do not have the power robbing USB to serial converter nor the LEDs of the Uno and Nano. Still the same Atmega328 processor of the Uno. Sleep modes on the processor and peripherals can also reduce current consumption. The Pro Mini can be had in 3.3V / 8MHz. That will also reduce the current.
I once built that stuff to control the temperature of the house. No way I used batteries. They will run down and You will notice it when the climate goes wild. Go for mains!
Have you got an idea about how much time the system could last with your set up? because my main problem is exactly that
I would like to do so but I don't have any way to plug it into my house energy line so I must use batteries
What does the relay control? A relay will draw coil current all the time that it is on. If a MOSFET can be use instead of the relay the current will be nearly 0.
The thermal sensor draws very little (30uA).
If you turn off the LCD backlight the LCD draws little current (1 to 1.5mA).
With sleep modes you can reduce the Pro Mini current to very low levels (microamps).
There is no way that I can estimate the time that a battery will last without knowing the specs of the battery and the average current draw.
What kind of house heating/cooling system do you have that does not use mains power in some way?
Paul
While I entirely agree it is not good idea to have a battery powered thermostat, I must point out that latching relays are available.
They have two coils, snick and snack. Pulse the snick coil and the contacts switch and hold. Pulse the snack coil and they switch back and hold. Milliseconds of relatively high current each.
I have a sunset sunrise mains switch running off a 9 volt smoke alarm battery. It is 100 percent isolated from the mains power lines it the relay switches.
Low power and sleeping techniques, all well know and documented (see Nick Gammon) let the battery last well over one year.
When the battery fails, the device stops working: either the light is stuck on or it is stuck off… no large deal - I notice and change the battery.
A larger concern in an HVZC controller…
Edit: sitting here in the heat wave, I realize my thermostat has no power of its own, except for batteries backing up the program… it just went snick and the AC kicked in, it goes snack when it cuts out, I’m thinking it has a… latching relay in it.
Not messing with the AC just now to see. 
a7
As a battery you can take a 9V E type (Duracell Battery Products | Coppertop Batteries this one can be an example).
I would use the realy in order to activate the heating system. basically it activates the pumps that push the water into the radiators but it's only a trigger, the energy supply of the pumps is autonomous. Honeslty I am not an expert about relay and stufs like that, as you can surely see, so I don't even know what is a MOFSET, I am sorry
Edit: I have just discovered that the page uses asynchronous call to the server in order to update its content so the battery you see when you load the page is the wrong one (basically the URL remains the same but the content changes). You should click on 9V one to see the battery I was talking about
Hi Paul
In my house there is obviously a power line, the problem is that there isn't any wall socket or a stuff like that near the thermostat that I use at the moment (it runs on batteries indeed), the one whch I would like to substitute the arduino. I guess there was a misunderstanding
I don’t see an E type, just the good old “transistor radio battery” but with modern chemistry.
Thermostats must harvest power from the “calling for heat” line somehow.
The AA batts in there last 4-ever, no way are they the only sauce of power.
a7
I am really grateful for your answer but I didn't catch what you meant while you were talking about snick and snack (even if it is very lickely that is due to my ignorance given that I didn't have to work with relays before)
Do you have google in your part of the internet?
Try
5 volt latching relay
for a description better than I have patience for.
a7
I would like to use them but I must stick with quite small spaces so I can't use 6 of them to supply the arduino (if I am not wrong, arduino needs at least 5/6 Volts and those batteries can only provide 1.5 Volts each)
uhhh, cool man, thank you so much!
I said my thermostat uses AAs for program keeping (memory backup), and somehow steals power from the HVZC control lines for operation.
My sunrise switch uses a plain 9 volt alkaline battery, Duracell always, yeah! like you get at the drugstore and put in you smoke alarms twice a year.
Read, not even between the lines.
a7
Are you sure it runs on batteries? My house thermostat runs on the 24 VAC from the transformer in the heat pump system. The thermostat does have two batteries for backup power, but only to hold the parameters when the 24VAC is off.
Paul
Understood 
Thank you for everything alto 
Honestly, I haven't read it anywhere, but I think that it runs on batteries because it runs properly even when it is removed from its socket on the wall and it dies when the batteries are gone (even if it is plugged in the wall)
But, does the heating/cooling system still work when the batteries are removed?
Paul
Is the brand Honeywell, by any chance?