Monthly Archives: April 2019

Different perspectives

The above images show four different perspectives on the same day of data (April 24th) from different sources within the home.

Smart Meter HAN

Firstly, the Smart Meter HAN image shows bought electricity to the home. Each smart meter sits on a Home Area Network (HAN) which is how the In-home display provided with the meter gets its data. The in-home display is an example of a Consumer Access Device (CAD). In my case I also have a Hildebrand Glow Stick as a CAD. The Glow Stick, which looks something like an oversized USB stick, also connects as a CAD to the smart meter allowing the meter to be read. An associated app, Hildebrand’s Bright, allows the Glow Stick to be read via the cloud. In principle the Bright app can display either energy in kWh or cost, but in my case can only display energy in kWh as Octopus don’t push the price data into the smart meter so energy cost always reports as zero. The data is presented by the minute.

Smart Meter WAN

Secondly, the Smart Meter WAN image shows the same data but from the perspective of the Wide Area Network (WAN) whch connects the smart meter to the energy retailer (Octopus for me). This half-hourly data is reported via the Octo Watchdog app. The data reported is cost per kWh (the blue line) and energy consumer / kWh (the red columns). The energy data in the red columns follows that of the red line in the prior illustration but in lower resolution (half-hourly versus minute-by-minute). You can clearly see most energy being bought when the price is lowest.


Thirdly, the Powervault image shows grid in/out and battery in/out. The green grid-in line mimics the red data from the above images. The battery in/out data is solely visible in this image. The resolution is good enough to see shorter events like kettle boil cycles.


The final image, from the Immerun, is probably the most useful although it lacks energy price and hides battery in/out within the House data (hence ‘House’ being zero at times). The immersun alone reports output data from the solar panels and diversion to the immersion heater. It also lumps the car charger energy within ‘House’, indeed none of these views can directly report the car charger behaviour although its the dominant energy consumer here.

I’m planning to construct my own view showing all the different prices of data together in one place. I already have access to:

  1. The Immersun data via the same API called by their app. I came across a blog post that described how to do this.
  2. The Powervault data API (I only have a control API at the moment) which should give me battery in/out (at least I’m on a promise of the API at the moment).
  3. The Hilbebrand data which duplicates the Powervault Import/Export at the moment, but has the potential to provide independent monitoring of my car charger.

In principle then that would leave me able to report 3 x energy sources (grid, panels and battery; of which grid and battery would be bi-directional) and report 3 x energy consumers (car, water heating and home).

Export metering and battery storage

A recent discussion centred on whether, as someone in receipt of UK feed-in tariff (FiT) and having a smart meter, I should be on metered export or deemed export. I had previously been advised that my smart meter did not have an export register, but playing with the buttons revealed an active export register.

The illustration above taken from OFGEM’s Guidance for generators: Co-location of electricity storage facilities with renewable generation supported under the Renewables Obligation or Feed-in Tariff schemes (Version 2) shows a configuration like mine, but rules out metered export since I can charge my battery from the grid and then export that power back to the grid. FiT terms are that only generated renewable power can be exported for FiT payment and thus, regardless of the capability of my meter, I’m only permitted to deem export. Deemed export does not incentivise purchase of power from the grid and then export.

There could be an opportunity to use the export meter as part of an export tariff instead of the FiT export component, which wouldn’t have a restriction on buying and exporting grid power, but my high levels of self-use make this unattractive compared to deemed export at 50% of generation.

Enhancing the HEMS

Over the last few months I’ve gradually been adding to the capability of my HEMS which determines when is the cheapest time at which to buy electricity for different purposes. After initially controlling my car charging only, I’ve now added both immersion heater control and battery storage control.

Desktop HEMS view with real-time Immersun status alongside

To the left and centre the picture shows the plan of the HEMS for two twelve hour blocks of time from when the cost data was last updated by my supplier. To the right is the real time display of my home. It’s a dull day so only 258 Watts is coming from the solar panels (maximum of 4,000 Watts), and so the HEMS has been buying energy from the grid to charge the battery when the price is low.

For each of the panels to left and centre, the electricity price is displayed for each half hour, with the colour-coded decision of the HEMS for what to do.

The EVSE (for charging the car) shows 3 of 4 possible statuses:

  • red – the energy price is above a configurable upper cost threshold, so the car will never charge.
  • green (not shown) – the energy price is below a configurable lower cost threshold, so the car will always charge.
  • yellow – these are the cheapest half-hours. The HEMS will enable charging for the required number of half hours to deliver a configurable number of total hours with a user-defined time window (typically overnight for the car). Overnight was configured for only 1 hour, although 5 would be more typical for a full charge.
  • orange – these half hours are more expensive. The HEMS will not enable charging in these hours unless the configuration is changed for more hours.

Water heating follows the same control strategy, but is configured to only heat water during plunge pricing events when the electricity price falls below the gas price.

Operating pattern for car charger and water heating

Configuration parameters allow customisation of:

  • Upper cost threshold – never charge when price above this threshold.
  • Lower cost threshold – always charge when price below this threshold.
  • Start of time window – arrival time for charging.
  • End of time window – departure time for charging.
  • Target hours – number of hours of charging/heating required within time window.

Battery control is configured similarly, but has more states than simply on and off, and correspondingly more colours. The colours reflect those used by the battery manufacturer. I currently use only 3 states:

  • blue – normal – the battery automatically either chargers or discharges either to absorb any surplus solar electricity or displace bought electricity when demand exceeds solar availability. Here used when the electricity price is highest.
  • light green – charge only – will charge proportionately to any solar excess as above, but will not discharge. Here used for mid-range electricity costs.
  • dark green – force charge – charge at maximum power drawing power from the grid as necessary. Here used for lowest electricity prices.