[hvalentim]

For a permanent 1w consumption with an optimal winter tilt of 66% (facing South) and assuming an unobstructed horizon with a 5% chance of Loss of Load (the system failing) using weather data from Aberdeen (SOC under 50% will stress/wear the battery in Dec.-Jan. Also there is a 1/3 likelihood of system failure in Dec. - see table in third pic.):

   
   
   

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For a permanent 2w consumption with an optimal winter tilt of 66% (facing South) and assuming an unobstructed horizon with a 0% chance of Loss of Load (the system failing) using weather data from Aberdeen:

   
   
   

Personally, I would go for nothing less than the second option. At least a ~50W panel and a ~20Ah battery.

PS: Disregard the "cost" estimate. It includes an absurd amount for installation and maintenance costs.