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This topic contains 24 replies, has 11 voices, and was last updated by  Trevor Larkum 9 months, 3 weeks ago.

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    Trevor Larkum

    There are a lot of promising battery technologies coming online – I was going to buy a battery storage system this year but I think I’ll wait another year before taking the plunge to see what comes along.



    lol yes its a good idea to wait as many standards are coming out


    The metal air batteries vs the flow batteries seem like a promising continuation, it might even be adapted for home use. I think by 2017 its likely we might see something, so yes I am waiting too… though I might mess around with a very potable battery for camping purposes…



    The Tesla Powerwall seems like it would fit the bill.



    We’ve got an off-grid small holding that I built a solar/wind mains power system for about £1200 using secondhand and recycled items. Been running 24/7 for about 5 years. The lighting and mains items it powers are all low wattage and were bought to fit the system not the other way around which is where most people go wrong.

    I had a budget so built the battery/inverter system to the highest power I could afford then made the other things fit. No kettle, we have one for the gas stove, no dishwasher/washing macine or iron (well a low power travel one) a small A++ fridge freezer, a small TV. The list goes on. The biggest drains are the central heating pump (70w) and the fridge/freezer (80w). I have enough in reserve to cope with about 9 days of no sun, no wind, in all that time it’s never happened, no wind on the west coast of Cumbria means it must be 30 degrees sunshine or you’re in doors.



    @notquitecumbrian ready for the Zombie apocalypse then?



    Ready? Looking at most of the ‘general public’ and the state the planets in, I sometimes think it’s already happened!

    Seriously, it was more the cost of connection, some £21,000 and that was 9 years a go so who knows how much it would be now?

    And before anyone asks the wife charges the Zoe at work for free as we would need our own power station to keep charging that.



    Using batteries to time-shift solar PV energy from day to night, or cheap grid energy from night to day is not so easy to analyse the cost benefits of.

    I started off with a Nedap PowerRouter. Lead Acid deep cycle batteries are expensive. They are rated per charge / discharge cycle, and need careful management to get the full life expectancy. It appears from 2 or 3 years of data that the batteries are actually earning their keep better than the solar panels, but only if they last longer than 7 years. Once panels and batteries start to degrade, there should come a point where you would be better off replacing them. Except once your original investment is paid off, in one sense you stand to make your greatest return since your electricity becomes nearly free.

    Trying to add additional battery capacity is also a hassle. Bigger batteries need more energy to charge them. Some battery inverters use an AC bus for charging their batteries which is easy to add to an existing PV system retaining the original string inverter. But self-use systems have to monitor AC current flowing to and from the Grid Supply so that they can use their stored battery energy to support local energy demand, topping up from the Grid as needed.

    This is fine as long as you have only one such device / current sensor per phase (usually just one for domestic supplies). With more than one device trying to supply and monitor current, they are quite likely to confuse eachother.

    If you add an electric vehicle into the mix, it gets even more tricky, since even a small car like a Zoe probably has 2 to 3 times the energy stored in its battery than in your home PV Self Use system batteries. In fact, using your vehicle charging as a source of predictable self use during sunny days is one way of putting your solar PV energy to good use. Except that this leaves you with no stored energy to carry through the night (since all your PV electricity will go into your EV). In any case, using cheap night time electricity to charge an electric car off the Grid probably makes most sense of all. Compared with normal domestic electricity use – even with a pretty inefficient house – the energy used by an electric vehicle is huge. So as a percentage, using cheap rate electricity to charge your EV is by far the most sensible thing to do. Compared with that, all the expense and hassle of batteries and solar PV for normal domestic needs is not that significant.

    Nevertheless, I have just setup a fairly elaborate system to use various batteries and inverters to bank enough energy purely to keep my Zoe charged. Exporting energy to the Grid seems like pure waste compared with storing and self-using. Even with hot water heating and batteries, I typically export at least a third of my solar PV energy to the Grid, so having a battery system capable of storing that energy to use for any legitimate self-use including the running of an EV would seem like a sensible mission.

    At 3 miles per kWh, there should be potential for extracting sufficient energy that would have otherwise just been exported without benefit to drive 9 miles a day.

    The problem here is that unless this ‘surplus PV energy’ is stored in a battery and used to power a suitable inverter, the charge rate will be far too low to charge an EV. The stored energy needs to charge the EV at 6kw for may be 4 hours. Many self use inverters limit the discharge rate of the batteries and also restrict their total power output. Compared with EV charging during daytime with simultaneous power from PV panels and batteries topped up from the Grid Supply that can charge at 7kw for a few hours, you might well end up paying more for the Grid power than the total cost of charging solely from the Grid on cheap rate night time electricity.

    The smugness you feel driving purely on sunlight can’t be denied, but when the real cost of batteries and other equipment is all factored in, it is not a clear case at all.



    Thanks for that interesting post. I look forward to the time I can complement my solar panels with storage. In the meantime I’ve gone for a half-way option of buying a slower charging cable (6-16 amps) in order to harness a greater percentage of the 2-3.5kW I’m managing to produce at the moment when the clouds are behaving themselves.

    I considered the Economy 7 option but given the car is at home most of the daylight hours, and we don’t do that many miles, it was not really an option economically, whereas £258 for the cable should see it pay for itself within 18 months, plus have the convenience of a granny cable. It’s horses for courses really – something that many people don’t seem to understand.

    I hope my next vehicle will have a two-way option so it will be able to function as a battery powering the house during the non sunlit hours.

    I look forward to hearing of any further upgrade you make to your home storage setup. Cheers.


    Trevor Larkum

    For anyone that’s been following this discussion, the price of home batteries has come down a lot recently so at FuelIncluded we’ve started to sell them:




    Hi Trevor, I’ve been doing solar/ battery storage for some years now and recently discussed keeping the Zoe battery if I can upgrade to the larger one at the end of my PCP.
    As a lot of my home use seems increasingly at the 5 volt DC USB charging end, its easy and very cheap to arrive at this Voltage with any old car battery as low as 10 volt o/p The local garage is happy for me to take them away.
    The guy in the computer store gives me Li on old laptop packs 90% of which have perfectly good cells, which can be charged and balanced with Turnigy chargers, again around £20 for a great charger.These packs are normally 11 volt, but I can reassemble them to any voltage o/p.
    Seems daft to start off at a power station, to transmit 44kv in order to end up with 5v at my house/when you can provide your own for almost nothing.?

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