Having begun the development of economyGREEN, a service to help you use electricity when it is at its greenest, and having an interest in home solar installations, I thought I would do some investigation into applying the same forecasting RenSMART uses for the UK to micro solar pv installations. The kind thousands of people have had installed on their roofs throughout the UK.
The idea would be to provide people with forecasts specifically for their location and installation. For example on a day with a sunny afternoon forecast, but cloudy morning, the economyGREEN service would suggest you delay your washing machine load until later in the day.
As a starting point I wanted to see what levels of excess are produced by solar pv in an average setting, and at what times of year.
I took a reasonably central point in the UK, Leicester, as the location of the installation, the electricity use profile of the average house, and a 4kW peak solar installation 
Key PointsThe key information I discovered was:
- 46% of generated electricity is excess to the average houses needs and is exported to the grid. This is close to the governments deemed value of 50% which was used to calculate the portion of the feed in tariff due for exporting to the grid.
- This is almost 2000 kWh per year with a value of about £350 to the home owner if they could make use of it instead of using electricity imported from the grid.
- This is saving of 1000kg of CO2 emissions per year. About the same as a return flight from the UK to New York.
What to do ?The next question is what to do with this excess as an alternative to exporting it to the grid. With the end of the feed in tariff and no reliable replacement payment mechanism, using it within the house would seem like a good idea.
One option is to delay the use of appliances until the points in the day when excess is being generated. This is what the economyGREEN tag and app do, telling you the delay required for an appliance to use the greenest electricity.
For the wider grid, this just as likely to be at night than in the day due to the large amount of wind energy we, as a country, generate. However micro generation is mostly solar, and most excess is generated at midday, when the sun is at its highest, as you would expect. If you are about to do a washing load at 6pm, it is not helpful to suggest you delay it 16 hours to use the next days excess.
Battery Storage is expensiveOne solution would be to install a battery storage solution. This would cost about £500 per kWh.
At peak generation in the summer months, there is an excess of 14kWh to store, the mean excess is 5.6kWh. That would require a £7,000 battery installation to store each days excess for use later or £3,000 to store the average days excess.
Based on the excess electricity value of £350, that would take 20 or 10 years respectively to recoup the cost of installation.
Heat Storage is cheaperA second solution would be to store the excess as heat and then release it throughout the cold parts of the day to keep the house warm and offset central heating.
Heat storage has a much lower cost than battery storage coming in at an estimated £20 per kW stored, That is almost 17 times cheaper than battery storage.
Also in the spreadsheet you will see that I have included average outside temperatures.
This shows that the average house wishing to keep an internal temperature of 19C is likely to need heating for all days on the year, as the outside temperature drops below this level. 14kWh of heat storage would cost £280 and £112 would store the excess energy as heat for most days.
economyGREEN for home solar PVBased on this, RenSMART will be creating a specific economyGREEN service to control storage heaters for anyone interested who has (or intends to have) solar PV installed.
For more information contact email@example.com
 4kW is the most popular installation size in the UK due to feed in tariff break points. The feed in tariff rate dropped significantly for larger installations.
 Based on price of a Lithium Ion battery backup installation
 Simple thermostatically controlled storage radiator storing for maximum of 12 hours
 Temperatures taken from the Metoffice weather station at Sutton Bonnington over the last 20 years.