I have an older thermostat I'd like to replace with the more modern Nest thermostat. The online Nest compatibility checker tells me that I have a compatible device. However, I'm not sure how it could work since my existing thermostat runs on batteries. How would the Nest draw it's power? The older thermostat (pictured) has R, W, Y wires and a B which is not connected to anything. The other end of the wire (pictured) mixes colors with two smaller wires (the heater and the AC I'm assuming?). Is this a DIY job or something that should be handled by professionals?
Thanks in advance!