I have been running a simple weather station composed of a Sparkfun Pro Micro (8kHz/3.3V), a DHT22 and a NRF24L01+ transceiver during the last couple of months, and I can tell you up here in Canada weather stations save lives...
The board checks the temperature and the humidity once every second and transmits the new measurements back to my home server over RF if the changes are above some threshold. As such, the board sleeps most of the time, and the transmissions happen rarely (about once to thrice per minute).
The system has been running off an iPhone charger up to now, but I am now thinking of weaning it off the mains. The maximum power consumption of the whole circuit when DHT22 is on and RF chip is transmitting remains below 200mA and many articles suggest that with some optimization one can reduce the power draw by levels of magnitude. Actually, from what I read I gather that the average consumption of the circuit can be pushed below 1mA.
So, I was thinking of adding a solar panel, a charging circuit and a Li-ion battery to my setup, when I discovered these low-cost solar-powered USB power banks on several vendor websites. They make outrageous claims about their charge capacity, that in my experience one has to divide at least by two (20,000 ~= 10,000), but they are pretty rugged and seem appropriate for outdoor installation. The units seem to use a 6V/0.6W (100mA) solar cell that should theoretically be sufficient for charging the battery even if some days are less sunny or shorter.
I wonder if anyone has used one of these as a continuous power supply for an arduino kit, or can inform me about the possible limitations. The idea is not to deal with the power apparatus myself each time I want to assemble a kit.