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.
The code
See here for the code on GitHub:
https://github.com/niftimus/SmartPlugAutomate
Notes:
- 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
The logic
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
The features
- 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
The result
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!
Conclusion
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.
Analytics Anvil 
What an awesome idea! How has it been holding up since? I’ve considering a solar PV plug for EV charging but I have to try this first. Thanks for sharing!
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So far it has been holding up great for us. The Solar PV system size and PHEV charger are reasonably evenly matched, so it’s not too hard to catch a stretch of good weather over a couple of days. For the most part the plug is just set-and-forget now – i.e. plug in and come back to find the car charged in a couple of days. It lacks the sophistication of some commercial solutions (i.e. being able to vary the rate of charge), but is effective so far for our charging / weather patterns. If the weather forecast is bad and we need an urgent charge it’s possible to disable the smart control and use the HS110’s out of the box timer function – e.g. to charge overnight on off-peak rates.
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