> Especially in literal fields where power is hard to come by.
Is it really? A square centimeter of a photovoltaic surface should provide you with at least a milliwatt of power on average. Thas's not to be scoffed at IMO.
The device might see that milliwatt for two minutes twice a week during winter in some locations. Planning in the field devices means planning for the worst case scenarios, not the common scenarios.
A square centimeter is a very small surface, though. Scaling it up for the worst case is hardly a problem. The board in the picture has dozens of square centimeters on its own, so wherever that board can be used, clearly a much larger PV area can be used as well.