I am using the full daytime/daylight charging cycle for my Humanoid. Of course I have not managed to get energy out of zero, yet! What I am planning is to partner up with high density, green battery technologies such as the urine fuel cell developed in the UK.
The purpose of my humanoid is to be a drop-in replacement for human beings in all their daily functions, most important being:
a) moving, including obstacle avoidance, door handling, step climbing
b) detecting human identities in their immediate environment by audio and visual processing
c) engaging in interaction scenarios with the humans, whether enabling scenarios like helping seniors to walk or disabling scenarios like blocking a criminal's escape route
d) Call-for-help scenarios where the correct human agent(s) are introduced to the scene and given sufficient information to quickly and effectively engage
e) the companion scenario, whereby artificial voice and artificial face provide a suitable "partner" for anyone from toddlers to the elderly and even to their pets for baby-sitting and pet-sitting
My Project started a year ago, nominated as the country's best Sustainable innovation in 2015, and still working on it. Hopefully the new version coming in a year with over 90% charging from the nature: (hot or cold weather, sun or wind or rain).