Solar Excess And Energy Pricing

Paul Goulbourn

After creating the monitoring system for our solar installation earlier in the year I began to notice a few things about how it performed.

  • The solar output of the system is 9.6kWh and our house battery is around 10kWh.
  • The battery system has a maximum charge rate that the panels often exceed.
  • Whilst we get credit for energy exported back to the grid, the payment credit per kWh exported is approximately 8x less than the charge for 1kWh imported.

So as we rolled into summer, we got into the habit of waiting for peak sun to turn on dishwashers/washing machines and the car charger.

For home charging the Tesla app shows you the voltage being provided to the car and then allows you to tune the Amps being drawn. In my case in the UK, that typically means the charger is at 235V but I have the choice to charge anywhere from 5A to 32A.

So at a simple level if the car was charging at 16A and the solar array was exporting 1kWh, then I could add 4A to the charge rate and divert the energy to my car battery instead of back to the grid. (4*235=0.94kWh)

Assuming I would need to charge the car anyway, this diversion of energy represents a big cost saving.

To illustrate with the predicted Oct 22 UK energy pricing:

  • 5p for an exported unit
  • 40p for an imported unit

This would mean a saving of 35p per unit diverted.

NB. For clarity, I’m ignoring off-peak/variable tariffs etc. The general point is that consuming this energy locally in a useful manner should normally represent a cost saving.

So the ideal condition for us as a household producing solar energy is as follows :

  • The house battery is 100% full when the system stops generating energy for the day. i.e. this gives us the best chance to get through the night without importing any power from the grid.
  • We use any generated energy locally where possible. Exporting to the grid should be the last resort.

Given the car has a battery size of around 80kWh (8x the house battery) then it clearly makes sense for the excess generation to go to the car.

Whilst this is possible manually by watching the generation/home battery/grid import/export/house usage figures etc. It becomes very error-prone eg. the Sun goes behind a cloud for 10 minutes while you are charging at 32A etc.

Clearly, this is something ripe for automation, so I dusted down the IDE and dived in.