Thanks for your reply. I want to use wifi mainly for sniffing every 2 hours. So some writing to flash would be needed for a short amount of time.
Maybe a lightsensor could help to get an indication about the ambient light and thus the solar cell / supercapacitor.
But when it sees a trusted network, like office or home it should calculate if there is/was enough light and if a trusted network is within reach to upload the log of wifi sniffing. The device will be outside, so logging light every 2 hours will also give an indication if it is morning or evening. In the evening the supercapacitor should have enough power to run for 5 seconds or something like that.
If that won't work, the device will be at the office or at home, at a trusted location, so we can pick it up, power it and upload the log as well.
What type of capacitor should work in this case? How easy can the circuit be?