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Solar PV to Battery

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Dave 56

Joined: 16 Dec 2013
Posts: 1

PostPosted: Mon Dec 16, 2013 11:58 am    Post subject: Solar PV to Battery Reply with quote

Hi all
maybe the wrong forum site to ask? I have solar Pv installed nearly 2 years now, only 2.88KW capacity. Would it be possible for me to use my generated capacity that goes into the grid to power a battery charger during the day, feeding a bank of 12v batts, then during the night, through one of those transformer rectifier things you can get to go up to 230v AC to power a TV and fridge/freezer. TV is LED, not sure as how pure a sine wave is needed to power, but I think the F/F should cope. and yes I know the probs with certain types of batteries. any help would be good Smile Certain electricians I have asked do not know.

Many thanks to those that may reply
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Joined: 02 Jul 2007
Posts: 8061
Location: North Somerset

PostPosted: Mon Dec 16, 2013 12:39 pm    Post subject: Reply with quote

Presumably your present installation is grid tied ?
If so it cant be used for battery charging without the grid being present.

You can of course use a standard grid powered battery charger, just like any other appliance, when the grid is present and the sun is shining.

The battery thus charged may be used via a suitable inverter to power any mains voltage appliance up to the limit of the inverter and battery capacity.
For most appliances you will need a pure sine wave inverter, for refrigeration equipment you will need a generously sized inverter due to the substantial starting currents of such appliances.

This is however most unlikely to make economic sense due to the losses involved and the cost of the battery and eventual replacement thereoff.

Consider a typical deep cycle battery that costs about 100 and has a useable capacity of 0.5KWH (avoiding deep discharge)
If the battery lasts for 400 cycles, then the total energy stored over the life of the battery is 200KWH.
That is 50 pence a KWH simply by battery deppreciation, you can buy power from the grid for less than that !
Even if the battery lasts for 800 cycles, that is still 25 pence a unit, still more than grid power.

Add to that the fact that the power put into the battery is not free, but could have been sold into the grid, admittedly for a low price.
Then add 10% battery charger losses, 15% battery losses, and 10% inverter losses, and the idea becomes even less attractive.

Remember also that grid power can be obtained very cheaply indeed for 7 hours at night on a suitable tarrif.

A battery bank is a very useful thing to have for power cuts etc, but the economics of daily cycling are seldom worth while.

It would make more sense to try and shift some of your consumption to the sunlight hours when you are producing, or to the off peak hours when it can be purchased cheaply.
"Installers and owners of emergency diesels must assume that they will have to run for a week or more"
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Joined: 29 Nov 2011
Posts: 2451
Location: Ross-shire, Scotland

PostPosted: Mon Dec 16, 2013 6:49 pm    Post subject: Reply with quote

It might make sense if consumption during the day was very low, so that excess electricity being generated by the solar PV was being stored in the battery bank rather than being sold back to the grid. IIRC, PV-generated electricity is sold to the grid at 3p per kWh, whereas it costs around 14p to buy electricity from the grid. Economy 7 gives much cheaper grid electricity, but doesn't kick in until quite late at night. Even so, it still costs twice as much as the amount you would sell your electricity to the grid for.

There would be an additional complicating factor, in that you would have to choose which items were going to run off the battery bank and switch them over in the evening (I assume?)
Engage in geo-engineering. Plant a tree today.
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Joined: 24 Nov 2005
Posts: 3086
Location: Rye, UK

PostPosted: Fri Jan 10, 2014 5:51 pm    Post subject: Reply with quote

I wonder if this question will be coming up a lot after the recent power cuts... Maybe if enough people are interested the cost of the hardware will come down! Currently thousands, e.g.

"Deal with reality or reality will deal with you"
Dr Colin Campbell

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