In Part 1 we explored the idea of using a Smart Plug as well as home solar monitoring to save money when charging a plug in hybrid car.
This post details a technical approach so the plug only turns on when excess solar is available.
See here for the code on GitHub:
- The code is experimental and proof of concept only – it has not been fully tested
- The code runs as a Linux service
- It features a web UI
- It checks home energy consumption and decides whether to turn the plug on or off based on a threshold
For each check interval the code checks the current state of the plug and decides whether to:
- Do nothing
- Leave on
- Leave off
- Turn on
- Turn off
Here’s a flowchart showing the decision-making process:
The Web UI
- Ability to disable / enable automatic control
- This is useful where the plug needs to be manually controlled via its physical button
- Configurable Min power threshold
- This is useful where it’s acceptable to use some grid power as well as solar (e.g. partly cloudy weekends with cheaper electricity rates)
- Minimum on / off buffer periods to reduce switching (e.g. for devices which do not benefit from being powered on and off continually)
- Monitoring messages to see how many times the switch has been controlled and its last state
- Overall net ( W )
- Useful for seeing current net household energy consumption
- Automatic recovery if the plug, solar monitoring API or Wifi network goes offline temporarily
So far this solution works great.
On a partially cloudy day, the plug automatically turns on or off once excess solar drops below the min power threshold. Similarly, the plug will turn off when household consumption is high – for example, during the heating cycle of a washing machine / dishwasher or when an electric kettle is used.
We got an interesting email from our electricity retailer after setting up this solution:
The message indicates we have successfully boosted our self-consumption – i.e. more solar energy is being self-consumed rather than being exported to the grid, giving the appearance to the retailer that the solar PV system is underperforming. Success!
This is not quite as good as having a home battery or a dedicated (and much more refined) device like the Zappi, however it comes close. It is a great way to boost self-consumption of excess solar PV energy using software and a low-cost smart plug. With around a year of weekly charging, this solution can pay for the cost of the smart plug by reducing the effective cost of electricity.