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What happens to excess solar power when the batteries are full?

Posted by Chris Rudge on 20 June 2013 at 9:25 am

Q: I want to find out how my solar panels deal with excess solar power when the batteries are fully charged. It is a 24 volt system: 4 x 165 watt panels  into 2 kW inverter   stand alone system.

A: As an off grid, stand alone system, I assume your power usage is quite low if the batteries are getting fully charged and you have an excess of solar generated power. A couple of things spring to mind. Firstly is your battery bank large enough? It would be interesting to know what size it is. Certainly if it is coping with your demands, then it will probably be adequate. 

Another aspect to bear in mind is that, in the UK, effective solar battery charging only occurs in the summer months. It is still adequate in the spring and autumn periods, but will drop to less than effective in winter. 

Your solar system is 660Wp, and really should be left dedicated to charge the battery set through a charge controller. An ideal solution would be to install a voltage-sensitive relay. This handy unit will only switch on, when your main battery is at fully charged voltage. A suitable 24v unit can be found here.

Your system is too small to use the popular heating element devices for heating water, but ideal for lighting, occasional pumping, radio/tv etc, so with the seasonal variations in mind, maybe using the relay above to float charge another smaller battery set when the main set is fully charged could be the answer. 

For more in depth advice, it would be good to know what your exact set up is, and possible future requirements as well.

Photo Credit: via Compfight cc

About the author: Chris Rudge is a qualified electrician who specialises in renewable energy.

If you have a question about anything in the above blog, please ask it in the comments section below.

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1 comments - read them below or add one


CydneyComment left on: 13 August 2013 at 12:44 pm

I have a 2.3KW PV system. Ideally  i would like to be able to store excess electricity to use after dark. Without having to have a big bank of batteries.I've seen this device advertised called MIDNIGHT SUN. Has anyone had one fitted and can give me some info on it ? Performance & cost etc.


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