Low power temperature monitoring
One option is to use nRF24L01+ based chips which have great support on Arduino. Use the ones with Power Amplifier (PA) + Low Noise Amplifier (LNA), as mentioned here Which nRF24L001+PA+LNA should I get?
With these you can have one receiver that is powered from the mains and the rest can do unidirectional communication with the temperature value every 5 minutes. You can even kill power to the rest of the circuitry with a MOSFET switch in this off-time to have few uA/nA current consumption in this duration. Hope this helps.