Electric fence solar panel to charge battery

Starting with the math for a "conservative" system:

• 8 Watts * 24 hours per day load = 192 WH per day
• 192 WH per day * 2 days "no sun" * 1/0.50 max planned discharge * 1/12 volt battery = 64 AH @ 12 volt battery (FLA)

A 100 AH battery is certainly large enough and covers a few more "bad sun" days.

Charging at "non-winter" 5% to full time off grid 10-13% rate of charge:

• 100 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 94 Watt array minimum
• 100 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 188 Watt array nominal
• 100 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 245 Watt array "typical" cost effective maximum

So--That is just sizing based on Battery Bank AH capacity... 10% is a good "nominal" rate of charge. And "oversizing" the battery does call for a larger charging current array.

And based on hours of sun per day. Assuming fixed array around Auckland New Zealand, facing north:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Auckland
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 38° angle from vertical:
(Optimal winter settings)

Jan	Feb	Mar	Apr	May	Jun
5.07	5.03	4.87	4.30	3.64	3.36
Jul	Aug	Sep	Oct	Nov	Dec
3.60	3.91	4.52	4.56	4.79	4.83

Using June as "break even" winter harvest:

• 192 WH per day load * 1/0.61 DC off grid system eff * 1/3.36 hours of sun per day (June) = 94 Watt array "June break even"
  

Somewhere between 94 Watt and 188 Watt array seems to be an OK fit (assuming 8 Watt continuous load)... And there are the "fudge factors"--Does the system run 12 months out of the year? Do you plan on charging the battery during "bad weather" at times during the winter--Or would you prefer the 50% (to 65%) "base loads" (loads that must run 24x7) fudge factor for the larger array:

• 94 Watt June break even array * 1/0.50 base load fudge factor = 188 Watt array

I agree that an 80 Watt array sounds a bit on the "small side" for your system (assuming Auckland NZ) and you get some "poor" marine layer weather. An 188 Watt array would certainly give you some more head room for the system.

If you run the system only spring and summer (growing season, keep the sheep out of the garden)--You could flatten the panels out a bit to better catch spring/summer/fall sunlight and get away with a bit less solar panel:

Auckland
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 53° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
5.80	5.54	5.10	4.27	3.47	3.14
Jul	Aug	Sep	Oct	Nov	Dec
3.39	3.82	4.63	4.92	5.41	5.57

But--Your marine layer/amount of sun does not increase that much during the summer--A larger panel/array will still give you some better peace of mind.

Your thoughts/corrections?

-Bill