@ScruffR's suggestion is the best you can do at the moment:
And for really low power, you might want to check out the Solar Shield for the Electron. We designed it with a MOSFET that completely disconnects power to the Electron for periods of time controlled by the user.
https://docs.particle.io/datasheets/photon-shields/#electron-solar-kit
These are not the droids you are looking for.