A typical domestic solar PV + battery system

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25 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 – brilliant. 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.

  14. I’m currently planning a new build house that we are trying to get as close to off grid as possible (it will be grid tied but looking to limit grid usage to an absolute minimum). To do this we intend to have 16kW of solar PV and a 70kWh battery system. I’m faced with a couple of issues – grid supply limit – we can get a 6kW link in our area but we need to find a way to get the local electricity distributor to accept we will limit our input to the grid to below 26A.

    We will be using electricity + air source heat pump to heat the house and we have an electric car (Tesla) so our electricity demand will be quite high.

  15. If I buy a 3kWh battery (quoted £3720), am I right in assuming the best I could get would be it fully charging & discharging once a day?
    That would save me a maximum of 365 x 3 = 1095 kWh per annum. At 14p +vat per kWh, less 3.6p per kWh export payments that gives me a saving of approximately £120 per year? That’s a >30 year payback!
    Is my approach correct?

  16. Graham, no this is not the case, most battery systems are able to contribute by discharging as soon as they reach a preset minimum, (this is normally set at time of install) then as your load exceeds your generation any balance comes from your batteries.
    So in theory it would be based on your generation power/ battery charge level & what you are using during the day, if you have devices in use during the day you could subject to conditions charge & discharge the batteries 3 times dependent on size balance.

  17. How is this home heated and how can such intelligent distributed renewables utilize thermal storage to further shift grid consumption from high carbon intensity periods to lower ones? Hints: high efficiency air to water heat pumps combined with 10 day thermal storage served by PV/thermal hybrid modules would help.

  18. Hi James, regarding additional thermal storage to further shift grid consumption, I have implemented a thermal store that provides all of my hot water usage from April to September & assists in the winter months & this is heated with excess generation that is remaining after filling my battery bank, I have written a visitors blog which you will find under the historical blog on this site dated 13th July.
    I could not justify the additional cost of a wet tube or plate system & do not have suitable roof space & think most people would make more use of electricity & if the system is large enough excess can then be used for heating hot water even if you have a combi as long as it can take hot water input.
    I am able to use 90% of what I generate using the combination of batteries & thermal store although on odd days I still produce more than I can use or store & am considering adding a further battery.

  19. Solar thermal panels much more efficient than PV ones at heating water. Around 75% v 15%. Thermal store with proper stratification means we have reduced fossil gas usage by 66% since installing thermal store. PV reduced our electricity bill by 30%. We use much less energy than average for 3 bed detached house. Whilst we drive a BEV it isn’t much cop in winter. Our 17 year old demonised diesel is far better. Still capable of nigh on 100mpg real world after 1/4 million miles: http://www.autointell.com/News-2001/May-2001/May-2001-4/May-23-01-p8.htm If only the Govt would ban fossil diesel fuel from city gas stations and insist retailers offer HVO made from waste that works in ALL Diesel engines for higher MPG and vastly lower NOx and PMs, we’d be a less wasteful society, more like Finland.

  20. My question is: What make/model of thermal store are being used?
    I ask this because our existing Gledhill tthermal store is thirty years old and doesn’t as effective/efficient as i think a modern one would be.
    The information from different suppliers is very difficult to compare, and as far as I know there is no Which? type of comparison available. Installers tend to use one make so not as helpful as could be.
    So real life experience of makes/models would be appreciated.

  21. Does anyone have an idea of the Current C02 contribution from Hydrogen if supplied as pressurised in a cylinder. This is probably very subjective. My reason for asking is I am working on quite a large off grid home, with PV/battery /GSHP and am reviewing alternatives to the diesel generator, Have done careful demand modelling and got to c80% renewable and 20% diesel, and adding more PV/batteries helps but that is the economic sweet spot. I am assuming H2 production is quite energy intensive currently.

  22. Does anyone have an idea of the Current C02 contribution from Hydrogen if supplied as pressurised in a cylinder. This is probably very subjective. My reason for asking is I am working on quite a large off grid home, with PV/battery /GSHP and am reviewing alternatives to the diesel generator, Have done careful demand modelling and got to c80% renewable and 20% diesel, and adding more PV/batteries helps but that is the economic sweet spot. I am assuming H2 production is quite energy intensive currently.

  23. James,

    Most commercially available H2 is produced from CH4 with the carbon being converted to CO2, and yes it’s quite intensive. It’s possible to produce H2 via water electrolysis using zero carbon electricity; I believe there are one or two suppliers.

    But how would you use the hydrogen? Burning natural gas in a CHP system, or simply burning wood would be much better from a carbon stand point.

  24. Hi are you going to do a deal with air pump ,solar panels and water panels and a battery ? cost ?
    Or a air pump and a battery ? cost ?
    thank you for your time.

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