A 20W panel may be sufficient in good weather, in optimal conditions.
- this is a nearby solar panel array.
If you look at the daily generation graph, you see that if you want it to run worst-case, you pretty much need to be able to survive several days at 1000Wh/kW.
This will drop out several days a year.
We need 2.5 watts constantly for 24 hours per day.
This is 60Wh/day.
Charge/discharge is perhaps 80% efficient with good batteries, bumping this up to 75Wh.
Assuming 750Wh/kW, you need 100W of panel.
And perhaps 200Wh of battery, minimum. ([email protected]
) (however, if lead-acid, you need 500Wh, as it won't be safe to charge the battery with the full output of the panel)
This would have stopped working for maybe a week or two in November, and perhaps a few days in december.
If you wanted it actually to work all the time, you'd need more like a 150W panel, and a 750Wh
is a useful resource for working out how much you're likely to get in a given month for a location in europe.
However, the monthly totals it can produce are averages - as you can see from the data above, you need to cope with several days of very low output.
It may not be quite as bad as the data above presents it.
It's possible you can gain perhaps 20% with a system optimised for low-light conditions - that is efficient even when the solar panel is dimly illuminated.
In addition, of course, powersaving may be possible to use under 2.5W.
Plus, you may be lucky enough to live where it's sunnier.