A typical domestic solar PV + battery system

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14 thoughts on “A typical domestic solar PV + battery system

  1. What size battery are you using? The reason I ask is that the % of self consumption seems to be lower than I would have expected. Especially looking at April 17 where you are generating 366kwh and consuming 305kwh yet the percentage of consumption with battery is ~37%
    Thanks for taking to do this, it’s a fascinating project.

  2. Yeah this is great. Thanks.
    How does domestic solar PV show up in the production figures on other part of the website? Is it like the local wind, in that it doesn’t get measured?

  3. @Karl – a 5 kWh battery will charge in under 3 hours on a sunny day. The rest of the day, electricity is exported from the home, but must still be consumed from the grid at night after the battery discharges.

  4. Just wondering what the efficiency of the battery is? What % of electrcity put in can later be extracted.
    Also if excess solar is exported rather than stored it is used by someone else at that moment – displacing other types of generation. so given the efficiency rating above being <100% this needs to be considered in the overall impact on the system. But that said I think this type of analysis is useful in people gaining understanding of solar and battery systems

  5. My wife and I are currently completing a self build. We have installed 4kW of PV but use this with some power electronics to firstly heat a thermal store (for domestic hot water) and then satisfy load in the house and finally export to the grid. Electricity is our only means of heating and as such using PV into a thermal store represents a good compromise over batteries. Accepting that we would need some embodiment of a water tank the only hardware element of the system over and above a standard PV fit is the immersion and wiring which impacts upon creating carbon/eroding resource in its production. I believe this represents a more appropriate domestic embodiment of PV than utilising batteries and the burden of manufacture/risks of use/disposal.

  6. Really interesting comments. A couple of things to pick up. Battery round trip efficiency including the inverter is around 88%. I see what you mean about overall efficiency Julie, but equally the battery allows you to displace gas generation even when the sun isn’t shining and so the carbon argument is complicated.

    Gareth- I am myself a big proponent of heat storage and it was something that I considered in my PhD. Are you using a hot water tank or are you looking at other heat storage technologies? Would you be interested in writing a blog for the site on your eco-self build?

  7. I’d like to better understand the impact that tangible volumes of PV+ Battery installations might have on the daily peaks in demand. Are you able to give an indication of how much say 1m homes would help to reduce the peak by?

  8. Hm, I can see what you are trying to demonstrate here, but I think the analysis of carbon emissions is too simplistic

    At a simple level, the marginal plant on the grid system is almost always a gas-fired CCGT. So power exported displaces a CCGT in much the same way that power imported will take it from a CCGT. Therefore electricity exported has a negative carbon intensity, roughly equal to the carbon intensity of electricity imported from the grid. Put another way, for small producers, the grid effectively acts much like a battery but with lower losses. I think your anayisis ignores the carbon-positive effect of displacing gas generation elsewhere onthe system (do correct me if this is not the case).

    At more complex level, during the day CCGTs will generally be at a mid-high load and therefore have their best efficiency, around 55% LHV typically. At night many will be running at low loads, efficiency more like 40% LHV (I don’t have better figures to hand but this is about right). So marginal power imported at night has a higher carbon intensity than during daytime.
    You may want to reflect this in your figures. At present you seem to be overstating the carbon benefits of having a battery substantially. Of course, the cost benefits are different, and more closely resemble your analysis – exported power is paid at a low tariff, imported power is expensive.

  9. We have a 3.5kW solar panel installation on the S facing roof of a 4 bed detached house in East Lancashire rainfall about 50 inches, sun about 1300 hrs per annum. We also have an air source pump as the heat machine for central heating and domestic hot water. The output from the panels is controlled by an IMMERSUN device – power is allocated to the pump when running, to the rest of the house electrics and to an immersion heater in the 250 litre hot water tank. Our annual consumption from the grid is about 6000kwhrs per annum and the panels generate about 3200kwhrs per annum. We use about 500kwhrs of gas per annum for a gas hob and gas fire.

  10. What are installation costs and what is the current feed in tariff ? When I did the sums I’d be dead before payback.

  11. Isn’t this mickey mouse accounting? Surely, without a battery power not consumed in the home is being fed to the local distribution network, effectively supplying other local properties, and reducing not only generation for the grid, but also transmission and distribution losses. Add a battery, and those properties have to consume grid supplied power while the battery charges. The battery system has an overhead of 12% – rather greater than losses in transmission/distribution that run about 6% nationally (and less if you are close to a power station/distribution transformer, more if you are further away). Thus the battery system effectively simply adds the extra losses to total demand at the grid/national level.

    For the individual property things might be different. You do not mention that almost all small scale solar PV systems do not have an export meter. The FiT simply assumes that 50% of what it generates (which is metered) is exported, which is perhaps better thought of as 50% of the “export” tariff is simply added to the generation tariff. Clearly there is benefit in reducing power purchases offset by the cost of the battery system – whether that is worthwhile is more questionable. For those not at home during the day, there might be benefit in having an export meter fitted instead, so that they could be credited for their actual export. That might be a better option than the battery financially, depending on the FiT rates for the particular system. Fitting a export meter would of course be detrimental for those who ae at home all day and maximise their use of PV power. Export meters are only compulsory for systems above 30kWp.

    You do not explain how this particular system is monitored – do you have access to actual export data?

  12. I have detailed records for my own house which has PV. The PV saves about 3.6 kWh per day overall and cancels out precisely our annual use of electricity Рbrilliant. I have been tempted to fit a battery which would save me another 1.2 kWh per day to help offset my use of other fossil fuels. Unfortunately it would not really do so because my export of this electricity to the grid already benefits the world by reducing other forms of generation. If you want a purely financial argument the 4kw recycled Nissan battery is said to cost £4,000 and with my savings of about 24p per day at current electricity prices it would take me 46 years to break even. However the evening peak in carbon intensity that your graphs reveal would be reduced if large numbers of us had domestic batteries and pv. It seems to me that there is a good case here for the government to produce another fit-style scheme to support the adoption of domestic batteries. Although the government might be put off by the fact that solar-linked batteries would be of limited use when peaks are highest in winter.
    However I have resolved this for myself by buying an electric car for slightly different reasons. By charging that from my solar panels and, thanks to your graphs, when low carbon sources are lowest at night, I will minimise the carbon cost of using a car and also reduce my contribution to other pollutants such as NOx and particulates. (Of course it is better to walk where appropriate and I will continue to do so.)

  13. I have a 4.2KW PV and this also heat hot water when there is a surplus , Also I have 4 banks of 20 solar hot water evacuated tubes , combined this is enough to supply most haot water the year round except for end jan earl feb , any surplus hot water is pumped via the central heating system around the house.
    I think that the combination of PV and hot water is good as long as storage of hot water is large enough.
    Also i wonder if passive solar hot air heating with thermal storage in the form of a wax store or plaster and internal building materials that has phase change capabilities would be a a good way forward combined of course with far better insulation .
    I think also if any government intervention was required it could possibly make all insulation materials VAT free which is a very cheap and cost effective way of bringing all old and new building up scratch quickly either by DIY or installers.

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