# Electric fence solar panel to charge battery

Solar Expert Posts: 471 ✭✭✭✭
Hi . Im looking to provide solar to keep a 12 volt fence energiser running .the specs for the energiser say average 8 watts . At 12 volts that is .66 amps  . Times that by 24 hours and we have 15.84 AH .  2 days autonamy  and we have 32AH.
The manufacurer recomends their 80 watt kit . And a 100 AH battety .
A 100 ah batery sounds right . But 80 watt pannel sounds too small .
Any sugestions ?
2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home

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:

### AucklandAverage 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:

### AucklandAverage 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.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 16 ✭✭
Which fence charger is it?

I'm running a 3.1-joule Parmak U012 and a Group-27DC 109aH Wallyworld.  The 27DC will keep my 1/4 mile of polywire HOT for 30 days without charging...another 15-20 days before I would charge it.  The support guy at Parmak told me that the U012 uses about one amp a day.  I tend to agree with that in respect to the length of time it will run without charging the battery.  I *just* set it up on solar maybe two weeks ago.  I connected a little 12 or 14 inch square surplus panel that I bought many years ago and that had been sitting in a shed ever since then.  On a good day it puts out about one amp.  The battery status light shines strong.. Bill,@BB. , here on NAWS, gave me some feedback and info on some questions I had about the battery staying charged "too good" and a good bit of other things.

I'm using a waterproof solar charge controller that I got off of Amazon.  It seems to be working great, thoughI haven't tried anything on the load circuit, yet.   The only thing I'm not thrilled about is the really, really, *really* bright green state-of-charge indicator LED...I'm thinking of lightly painting over it to subdue it a bit...as it is now I can tell from my computer chair at the house whether the battery is charged up or not...200-feet away, roughly...but, I think I could see it if I flew over at about 10,000-feet, too!<grin>.  Here's the link for the controller:  https://www.amazon.com/dp/B07CK2PCC7?psc=1&ref=ppx_yo2_dt_b_product_details

• Solar Expert Posts: 471 ✭✭✭✭
Wow bill . Thanks for your in depth analysis. You are right . Near auckland .   It will run 24 7 . All year round  . So will err on the larger size.
Thanks again for the the time you spend with your clear  workings .   Much appreciated .
2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home
• Solar Expert Posts: 471 ✭✭✭✭
Which fence charger is it?

I'm running a 3.1-joule Parmak U012 and a Group-27DC 109aH Wallyworld.  The 27DC will keep my 1/4 mile of polywire HOT for 30 days without charging...another 15-20 days before I would charge it.  The support guy at Parmak told me that the U012 uses about one amp a day.  I tend to agree with that in respect to the length of time it will run without charging the battery.  I *just* set it up on solar maybe two weeks ago.  I connected a little 12 or 14 inch square surplus panel that I bought many years ago and that had been sitting in a shed ever since then.  On a good day it puts out about one amp.  The battery status light shines strong.. Bill,@BB. , here on NAWS, gave me some feedback and info on some questions I had about the battery staying charged "too good" and a good bit of other things.