Smallholders Insurance from Greenlands

Author Topic: What size solar panel do I need?  (Read 5113 times)


  • Guest
What size solar panel do I need?
« on: September 01, 2011, 12:16:56 am »

Just a quickie here-I hope!,what size solar panel do I need to keep a 12volt battery topped up,I have a solar panel some 5 watts,seems to keep it reasonable for a few weeks but it does need regular re-charging from the mains.

any advice appreciated!


  • Joined Mar 2009
  • carmarthenshire
Re: What size solar panel do I need?
« Reply #1 on: September 01, 2011, 01:09:39 am »
How big is the battery?

What type of battery is it (car, sealed gel or AGM)?

Does the battery have anything draining it (car alarm, immobiliser, radio, etc), or is it just a maintenance charge (i.e. just to offset the battery self discharge)?

where is your 5w panel (in the window or outside in the sun, facing the south, etc)?

if it's just a car/leisure battery up to 100Ah, then if you are really getting 5W (0.4A ish at 12v) then that ought to be OK for maintenance charging. Having said that, is you panel a good quality one or a cheap amorphous one? (the good ones will charge the battery even when it's overcast, the cheap ones need full sunshine to produce enough volts)?

Oh, and one more question: does your panel have a blocking diode (stops the battery draining though the panel when it's dark)?


  • Guest
Re: What size solar panel do I need?
« Reply #2 on: September 01, 2011, 08:02:45 am »
Ok,it is a car battery,I can take the lids off if need be whilst recharging.

The battery runs my electric fence around the pigs,half an acre by just over half an acre.

The solar panel is up high over the top of the feeding shed facing east.

The battery is just checked!

I paid 45.00 for the I have no idea of the quality.though a light does come on whilst charging and this does work even in overcast days.

What is a blocking diode!?


  • Joined Mar 2009
  • carmarthenshire
Re: What size solar panel do I need?
« Reply #3 on: September 01, 2011, 06:40:59 pm »

What is a blocking diode!?

it's like a one-way valve for electricity - it allows the charge to flow from the panel to the battery during the day and stops it flowing the other way at night; I guess that if the light on your panel goes out at night then it has the diode built in; if it doesn't you may have to check with the manufacturer.

Unfortunately there isn't a simple answer to your question; there's a big difference in the size of panel you need for winter and summer - particularly as the battery's capacity goes down when it is cold.

I don't suppose you know what the power consumption of the fence energiser is?

If I assume 120mA (that's the nominal consumption of my ESB200 1.6Joule), then that's about 3Ah/day. with a solar panel facing east (I assume there's a good reason it's not facing south) I guess you would need double the size of panel c.f. a south facing panel. So for summer; I would guess a 12w panel would suffice; for winter; 45-50w (but you would then need a charge controller for summer with this sized panel).

Note that this is all very vague, wooly figures, so there's no guarantee.  :)

You might try googling solar calculators - I did find this one:

which, if you accept my double-it figure for east facing and power consumption figures, very broadly agrees with my guess.

hope that helps



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