Category Archives: Living in the House

Myenergi Zappi EV Charge Point First Test

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Now the parking area is accessible to cars and with it being forecast sunny for most of the day, it seemed a good opportunity to put the electric car in the garage for the first time and test out charging it with the Myenergi Zappi EV charge point, using excess solar generation that probably wouldn’t fit in the PowerWall battery and would hence be curtailed.

BMW i3 EV in the Garage, connected to one of the two Myenergi Zappi EV Charge Points

The BMW i3 looks a bit lost when viewed from the CCTV camera that monitors the garage. The plan was always to park that car there, which influenced the location of the ‘primary’ Zappi EV charge point at the back of the Garage, between the first and second parking bays. (The ‘secondary’ Zappi is just visible at the bottom-right of the photo, where it can be used from the third parking bay and for cars parked outside.)

One of the key features of the Zappi unit is that it can harvest excess solar generation and charge a car at a variable rate to make best use of the solar generation that would otherwise be exported (or curtailed – depending on the configuration settings). While it’s useful to have this capability, with both a house battery and a decent export tariff it will typically be more economical to charge the car on the off-peak overnight rate and get paid to export solar generation during the daytime – except when there’s more solar power than can be exported or used to charge the house battery1.

With the PowerWall having a DC-connected battery integrated with the solar PV inverter, it’s not possible for the Zappi to tell whether power coming from the PowerWall (which it monitors using a CT clamp) is originating from the solar panels or from discharging the battery. It has a second CT clamp on the main grid connection, so it does know whether the site is Exporting or Importing overall – but the Zappi manual admits to the limitations in such circumstances.

With the Intelligent Octopus Go tariff, Octopus get to control when the car charges – and will either do this in the normal overnight off-peak period or they will add an ‘extra’ off-peak period if they decide to charge the car at other times. Before having the Zappi it was necessary to have Octopus control the car via the BMW API but now it is preferable to have them control the charger instead2. Right now – even when deleting and re-adding the Electric Car ‘device’ and when telling them there is a Zappi charger, they still want to control the car. I’ve submitted an online form which is meant to ask them to control the charger instead – for which they’ll ask for the Myenergi account details, not the BMW account details. The form said to allow up to one working day for that to take effect, so I’ll try re-adding the ‘device’ in a day or so.

The other configuration setting that needs attention before allowing Octopus free rein over overnight EV charging is the “Import Limit” for the PowerWall 3. Right now there’s no import limit set so there’s a risk of:

  • Octopus charging the electric car at 7.5 kW
  • Tesla charging the PowerWall battery at 8 kW
  • The House drawing 3.5 kW or more if both heat pumps are running

That would make a total of 19 kW or 82.6 A at 230V, which is more than the rated capacity of the DNO fuse on the grid connection and it would be A Bad Thing to have that fuse blow3. Given that the PowerWall is enforcing the grid export limit we might as well have it enforce the grid import limit too (by slightly reducing how aggressively it charges its batteries from the grid).

  1. It should be rare to have ‘too much’ solar generation like this once the charging algorithm for the house battery can be made to leave enough room for ‘tomorrow’s’ forecast solar generation ↩︎
  2. This should mean that any scheduled pre-conditioning ahead of a future journey won’t be affected, whereas that schedule gets wiped when Octopus control charge scheduling using the car API. It should also prevent the car charging for a few minutes whenever it’s plugged in. ↩︎
  3. There’s another 80 A fuse in the PowerWall Backup Gateway, which might blow first and is much easier to replace ↩︎

PIR Sensors for Lighting Control in the Outbuildings

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The house has PIR sensors to control lighting in the hallways / landings and bathrooms. These are Danlers PIR Occupancy Switches; model CEFL PIR in dry locations and CEFL PIR SEALED in bathrooms. These take a 230V live input and switch that with a relay for the output. They’re intended to directly switch 230V lighting (and are rated for 6A of resistive load) but they’re wired to a KNX ‘Binary Input’ bank which senses the switched 230V and is configured to control the relevant (DALI-dimmable) lights.

A key reason for adopting this approach was the limit of 20 KNX devices within an installation imposed by the (relatively) affordable KNX ‘ETS Lite’ software license used to program the home automation system – there’s only one KNX device (the Binary Input sensor), whereas there would have been 12 if these had been native KNX presence sensors. The Danlers sensors are about £45 each, whereas native KNX presence sensors are getting on for double that.

In the Ground Floor Hallway, which is relatively long and thin and is typically approached from the Front Door / Living Room (at one end) or the Kitchen / Family Room (at the other end) there are two sensors – about 1/4 the way from each end. These effectively operate ‘in parallel’ and the first one to trigger switches the lights on. This configuration works well.

While they function as intended, slight niggles with using the 230V Danlers PIR switches include:

  • Despite the Danlers units being a well-regarded, British-made product (with a 5-year warranty) roughly 1/3 of them have failed. Two were repaired under warranty – but failed again (and were then replaced under warranty) and two more failed outside warranty.
  • They operate with a very distinct ‘click’ from the 230V relay, which in some cases is A Good Thing – e.g. visitors using the windowless downstairs bathroom hear the ‘click’ as soon as they open the door, while they’re looking for the light switch – but it can be annoying in other rooms.
  • Since there’s no manual switch to choose to press (or not), the sensors always trigger when they sense movement – whether or not that’s intended or desirable
    • In en-suite bathrooms it’s annoying to have the lights come on at full brightness in the middle of the night – but they need to be bright at other times of day
    • The basic sensors can’t do anything different at night, so there’s an Automation rule in Home Assistant which reacts to the Binary Input sensor and does different things depending on whether it’s considered ‘night’ or not – which means some of the lighting is reliant on Home Assistant working correctly and is hence less robust than native KNX operation would be

For the Outbuildings, there’s a rather more modest requirement for automated lighting control than in the House – just the Shower Room and the Plant Room, both of which receive no natural light and which have Switched (rather than Dimmable) lights. Now the KNX device limit is higher (up to 64 devices in a single installation, using the ‘ETS Home’ license tier), using native KNX presence sensors seemed a better way to go – avoiding the need for a ‘Binary Input’ module (which have a high cost-per-input in small sizes). Since the Shower Room sensor needs to operate in a humid environment that needs to be ‘sealed’ to some extent, which limits the choice of MDT-brand sensors to pretty much just the MDT SCN-P360L2.03 which provides 360-degree coverage with two PIR sensors and is IP44-rated. (The Plant Room sensor is identical, even though it doesn’t need the IP44 rating.)

These sensors offer a range of additional flexibility:

  • They can take an input to specify Day versus Night, and do different things depending on the setting, without needing to rely on an Automation rule in Home Assistant
  • They can be adjusted in terms of their sensitivity threshold for presence / occupancy
  • They can provide logic to maintain overall lighting levels, taking advantage of natural light where that is available (and running artificial lighting at reduced brightness) then adding more artificial light when the natural light level reduces
  • They can provide an output to a HVAC system, with different configuration settings from the lighting outputs
    • For example, in the Shower Room, this could be used to change the fan speed on the MVHR system – either via a switched output or a ‘dimmable’ 0-10V controller

It remains to be seen how well the KNX PIR sensors operate in practice but so far I’m impressed by their specification, their small size, and the configuration options available. While it would be difficult to retro-fit these into the House (since the existing sensors use 230V Twin&Earth cabling, which would need replacing with KNX Bus cabling) I’d be inclined to use native KNX sensors in the House if starting again.