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 May 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:
- A feed in tariff which is a payment from the Government for what is generated.
- An export tariff which is a payment from the Government for what is exported from the solar panels into the grid.
- 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 May 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 May 2017 for this house with solar and if they had had a battery.
Carbon emissions (electricity only) – November 2016 to May 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.
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
2. Solar with Battery
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 May 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.