Electricity retailers sometimes give the choice of paying a flat rate for electricity, or so called Time of Use (ToU) rate. Time of use pricing usually has peak, off-peak and shoulder prices. This can also vary by time of year and also weekend or weekday.
For the consumer, Time of Use pricing may be beneficial if consumption can be shifted to off-peak hours, but this is potentially offset by more expensive rates during peak times.
Assuming a retailer gives the ability to choose – which one is cheaper?
This web calculator gives the ability to simulate costs based on historical meter data usage and configurable pricing and peak/off-peak definition:
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:
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.