Powering an off grid instrument.


Solar Enthusiast
I am currently experimenting with powering an unattended off grid networked instrument that continuously draws 500mA (6W) in 50° latitude maritime climate.
The requirements are quite different from providing grid power or even providing off grid home power.
The challenge is to have a panel that is not too large (which would have been difficult to mount on a street lamp pole and would attract vandals) and a battery that does not mind freezing.

The problem is that you get far too much power form the panel in summer and absolutely not enough in winter.

A 100W panel is not sufficient: i will only provide 5W for 6 hours on cloudy days, you need at least 25W for 6 hours to balance the 500mA permanent consumption. More, if the battery needs to catch up a series of cloudy days.
The battery must last for a week or more if fully charged, which happens only rarely in winter.

Of course, two or three hours sunshine are a game changer.
I am just now wondering if having two 100W panels oriented SE and SW will not be better?
On a cloudy day the orientation is almost irrelevant, but if you get two hours sunshine (whenever it comes) having at least one panel oriented well makes a difference, isn't it?
Tracking is not a solution either: too hard to maintain, does not work on cloudy days, the whole thing should be unattended.

What is your opinion?