A typical domestic solar PV + battery system

Since November 2016, I have been monitoring a four bedroomed home in the North of England which has solar PV. The solar PV system is 3.1 kW which is close to the average size for a solar PV system in the UK. The house a fairly average annual electricity consumption.

On this page I chart, on a monthly basis, the energy balance of the house to see how much solar PV reduces the homeowners electricity bill.

I also model how much battery energy storage could reduce the homeowners electricity bill when installed with the solar panels. The 5.85kWh battery which I consider is around half the size of a the latest Tesla Powerwall.

Electricity generated (November 2016 to July 2017)

Solar PV with or without battery energy storage earns a feed in tariff for every kWh of electricity that is generated by the panels. The amount of subsidy earned depends on how many panels are installed, where they are located, their orientation (south facing, east-west facing etc.), any shading on the panels and the quality of equipment which is installed.

Solar PV earns money in three ways:

  1. A feed in tariff which is a payment from the Government for what is generated.
  2. An export tariff which is a payment from the Government for what is exported from the solar panels into the grid.
  3. A reduction in the electricity bill through using solar power instead of buying it from an electricity supplier.

During the day, any solar energy which is generated tries to feed appliances within the home. Any excess electricity is exported to the grid. With a battery, some of that excess electricity is stored in the battery to be used at night.

The total amount of electricity that can be saved is restricted by the amount of solar energy that is generated. In the chart below, I show the amount that the solar panels generated relative to the amount of electricity that was consumed in the home.

We see that in the winter, there is a shortfall between the amount of solar that is generated relative to the amount of electricity that is consumed. However, the solar is still able to reduce the electricity bill. In the spring, summer and autumn there is surplus of solar electricity so much is exported to the grid to be used by other people.

Overall supply mix (November 2016 to July 2017)

The amount of electricity that is actually saved depends on how good the homeowner is at using the solar energy. People who are in during the day tend to be better consumers of solar power. Similarly, people who are clever in how they time their washing machines to run also save more.

A battery makes it even easier to save electricity. Whether you are home or not, the battery stores any solar electricity which is not consumed in the house so that it can be used at the convenience of the occupier.

In this chart, I show how much the electricity bill was reduced between November 2016 and July 2017 for this house with solar and if they had had a battery.

Carbon emissions (electricity only) – November 2016 to July 2017

By reducing the amount of electricity consumed from the grid, the house reduces the amount of electricity which is generated by fossil fueled power stations. Therefore, by using solar panels and not carbon intensive grid electricity the owner of solar or solar and a battery will reduce their carbon emissions.

In this chart, I show the estimated carbon intensity of domestic electricity for the three scenarios. The battery shows the biggest improvement in carbon emissions because it reduces consumption from the electricity grid the most.

Source: www.mygridgb.co.uk

Monthly Electricity Supply

The charts below break down how much of the consumers electricity bill is being met by the solar panels each month with and without a battery.

1. Solar PV Only

Source: www.mygridgb.co.uk

2. Solar with Battery

Source: www.mygridgb.co.uk

The battery is very effective in the summer, meaning that most of the electricity in the home is being provided by solar. The energy bill during this time is very low.

During the winter however, the battery is less effective as there is little solar power being generated. However, during this time the battery can be charged off-peak using an economy 7 or economy 10 tariff. This provides further electricity savings.

Overall electricity supply – November 2016 to July 2017

Considering the electricity consumed from the grid and the electricity consumed in the house over the same period, the overall contribution of different electricity sources is shown below. In this chart, solar is only the solar from the house and storage includes grid as well as the battery in the house. This will climb over the summer as there is more sun to generate power from.

Electricity mix of the MyGridGB  home with PV and Battery Storage from March 2017 to 23 August 2017


8 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.

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