In our article Flashing LED on Electricity Meter we looked at how the status of the LED on an electricity meter in a grid tied solar PV system can be used to decide when to turn on a water heating immersion heater to use surplus solar generated electricity rather than exporting it.
When there is a dedicated export meter with an LED which flashes at a rate proportional to the power currently being exported, things are relatively simple, but for one client recently we had to deal with a system including only a standard domestic import meter. This has an LED which flashes at a rate proportional to the amount of electricity currently being imported (i.e. purchased from the National Grid), and which is permanently on while electricity is being exported.
The client wanted a device which would turn on his immersion (standard 3kW element, but powered via a power reducer which halves the power consumption to 1.5kW), after a user programmable number of minutes of continuous electricity export. The exact number of minutes desired for efficient operation was unknown, so we made the device programmable – i.e. the user could themselves set the number of minutes of continuous export required before the immersion would be turned on.
The immersion would then remain on until 5 seconds of the import meter LED flashing. Therefore, if turning on the immersion results in electricity being imported, the immersion would be turned off within 5 seconds, so very little electricity would be imported to power it. While the amount of electricity taken by the immersion is insufficient to use up the full export surplus, the immersion would stay on, heating water.
Obviously this is not the most efficient system possible – something with a current sensing clamp to detect the exact level of import or export power is better, but the commercial options with this feature start at around £130.
This simple and easy to install system ensures that on a sunny day when no-one is at home using a high powered appliance such as a kettle or washing machine, surplus electricity from the solar panels will always be used to heat water rather than being exported (for which just a few pence would be paid), resulting in a payback period measurable in months.
The status of the LED on the meter (on or off) is detected using a simple light detecting resistor (LDR).
If you would like something like this device, please email email@example.com with details of your requirements.