ESP's only a power hog when it's using the WiFi... In deep sleep it can go as low as 10 uA! .15mA gets you the low-power processor for doing sensor interrupts.

In reality Watchy currently draws around 1mA while sleeping and 55 hours of battery for me, not using WiFi (not really a priority for me).. I think that can be improved with firmware updates, seems like there is potentially some low-hanging fruit. It's all open source firmware, so you can eg. program it to deep sleep while you deep sleep to save battery :)