If, however, you are open to different hardware you can find boards which are designed specifically for low power consumption.
I chose Atom Lite with Tail Bat because of:
- Out of box solution e.g. board with battery including case.
- It is small and it looks nice.
- Cheap / good price.
- Bunch of sensors which can be easily connected.
I don't know about any other HW which fits these categories. I'm using also other products for home automation which can be on battery for 2 years at least but it is x3 more expensive so I'm kind of surprised with the result :(