Solar and battery storage solutions - your experience sought.

27 Jan 2010
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Buckinghamshire, Darn Sarf
United Kingdom
Dear all, first I must state that I'm not after an economic or environmental argument over the rights, wrongs, or costs of saving excess solar generation into battery packs. I've seen a friend's off-grid setup and my interest has been peeked in fitting battery storage onto an existing 4kWp solar array at my house. The project will be a hobby, so the economic argument (re: payback period, etc.) will not apply, although I don't want to waste money buying the wrong kit, which is where you come in.

The setup I saw is two off 2.4kWh Pylontech lithium battery packs (=4.8kWh) feeding a nominal 48vdc into a Victron off-grid inverter of around 3kVA capacity. It had a very attractive web based monitoring system, of which I'd like to have similar.
The main difference between the system I saw and the one for my house is that mine will be grid connected. I also have an existing Solar iBoost, a device which diverts excess solar production into the hot water immersion heater.

My initial thoughts are for a two-stage project:

Stage 1: Use a cobbled together lead-acid battery pack and a cheap low power inverter. Power requirements will be low, say 250W for 4 hours each night. The batteries will be charged only by excess solar production after the hot water reaches temperature - see Solar iBoost, sbove. Is there a recommended product of this nature? I should add that on completion of stage two this apparatus will be re-installed in my shed, and (if possible) work OFF-grid there. The goal here is to show that during the summer months (April to October) I can use almost (stressing the almost) nil grid power by careful management of the loads.

Stage 2: Buy application specific lithium battery packs, 2.4kWh to begin with (with an intention to increase to 4.8kWh) and an inverter that will produce 3kW so that I can have a free brew at night, and cook a late night pizza after the pub. Web connectivity will be essential here, after all what use is paying out all that cash if you can't show off the system at the pub as well? Something equivalent to a grid tied Victron inverter. Your recommendations of suitable products are invited.

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Charge and discharge rate is important, mine can charge at 2 kW and discharge at 3 kW so they smooth out the peaks and troughts in solar supply meaning in the main 8 am to between 7 and 9 PM this time of year I am not using grid power, the EV tariff means until 8 am grid electric is cheap, so 0:30 to 7:30 am batteries set to charge off grid, so starting with fully charged battery.

My battery is only 3.2 kWh 50 volt, and I stop discharge at 10% so there is enough for freezers and CH if we get a power cut, not been tested other than when smart meter fitted, CH continued to run.

Aim is not to run without using grid power, just to minimise use at peak times, by 7 PM our use has dropped, so around £1.50 off peak and a £1 on peak, batteries are not cheap, and they have a limited life around 7 years, my system will take 4, but can't see a second one paying for its self.

The one battery also gives me UPS for freezers and CH so worth while, don't think a second one would give enough extra to be worth it.

All fitted by solar firm, and the inverter auto connects and disconnects earth bond and neutral when grid power fails and returns. Clearly also need an earth rod, on power failure it moves from TN-C-S to TN-S and I have not needed to worry about compliance as did not do the work, the solar people did and issued the compliance certificate and the other certificates required, seem to remember a G99 or something like that.

They also set up the inverter software, and changed it when smart meter fitted. Today no real option but fit battery to get paid for export, that is real reason for fitting them, but by 8 am meter shows £1.38 used most days, and this remains most of day unless I take a shower.

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