Yesterday provided a good example of my HEMS in action as the electricity price dropped quite low due to stormy weather conditions. Normally at this time of year the HEMS isn’t doing much with the storage battery as daytime solar output is enough to fully charge the battery, but yesterday low pricing was enough to automatically enable both battery charging and water heating overnight. Car charging was due to run anyway driven by the demand for an hour of charging, but battery charging and water heating was triggered by the low price rather than a needed to take power for a pre-defined period of time.
The screenshot above from my phone shows the HEMS’ plan for the the early hours of the 9th. The first price column shows one hour of car charging at the cheapest price. The second column shows half an hour of water heating as the electricity price has fallen below 3.5 p/kWh when it is assumed to be cheaper than gas. The third column shows four hours of battery charging when the electricity price is below 5 p/kWh.
The above image from the HAN side of my smart meter shows the energy consumption of the house varying through the night in response to these requests from the HEMS – battery charging at the widest point, car charging above that for an hour, and water heating above that for 30 minutes.
Finally this image shows the energy consumption versus price data for the same period shows how the action of the HEMS increases electricity demand as the price drops. Indeed on this day there was virtually no consumption at any other time.
For August 9th as a whole I paid 52 pence for 7.547 kWh of electricity. Taking off the 21 pence for the standing charge leaves 31 pence for the electricity kWhs alone, an average of 4.11 p/kWh.
Today I’ve further refined the wiring of the relays on the HEMS. At the time that I’d originally wired it I didn’t have small enough flex, or indeed multi core, which created an unnecessary number of cables (one per used relay) of over large size (and thus difficult to insert into the terminals). During the week I acquired some smaller gauge multi core allowing me to wire all three relays with a single cable containing one live feed and three switched live returns.
Of the 5 incoming / outgoing cables at the bottom (left to right):
Incoming mains (live / neutral / earth) from mains plug
Live and switched live to / from ImmerSUN output relay to activate car charger.
Live and switched lives to / from HEMS to activate car charger and water heating.
Red – live to contacts
Green – switched live for car charger direct – charge in response to price
Black – switched live for car charger indirect via ImmerSUN relay output – enable proportional charge in response to surplus PV
White – switched live for water heating – heat in response to price
Outgoing mains (switched live / neutral / earth) to RF solutions radio transmitter to activate car charger, and on the second cable clamp..
Outgoing switched live and neutral to ImmerSUN Boost relay input to enable immersion heater.
The revised wiring diagram looks like this..
All of this still leaves one unused relay on the HEMS (HAT #3) and one unused proportional output on the ImmerSUN (#2; available for future expansion.
Initially even my smallest boot lace ferrules would not fit into the terminals on the HAT. Fortunately, once the ferrules has been crimped around the new cables, and flattened by squeezing in pliers, then the ferrules could be persuaded into the terminals.
I’ve been on my dynamic smart tariff for some months now, so I thought it would be a good time to see what I’m actually saving. My actual tariff rate changes each half hour, but for the purposes the supplier calculates the weighted average of what I’ve actually paid for the invoice. In principal that should be monthly, but I’ve had some bills combined over more than one month.
6.29 p/kWh (part)
5.99 p/kWh (balance)
Over the course of the last few months my electricity price has reduced very significantly. I suspect that this is down to a combination of several factors including:
With the development of my HEMS (including its control of the battery storage) I’m getting slicker at optimising my purchase price
As we move into the summer the energy price is dropping with reduced demand and more renewable power available.
For reference the Energy Saving Trust reckons that the average UK price for electricity is 15.75 p/kWh on a flat rate tariff, or 19.0 on days and 9.1 p/kWh on nights for Economy 7. Thus my average electricity price in a month always beats their day rate and often beats their night rate. Of course the EST figures are the average market rates, so both better and worse deals will exist with different suppliers.
The 7.03 p/kWh for June 2019 seems to have been received with incredulity elsewhere so here’s the relevant part of the bill..
I’ve never felt so engaged with my electricity supply, and am very pleased to have made the move from the dual-rate Economy 7 tariffs that I’ve used for around 30 years.
After a series of quite detailed posts, I think that the time has come for an updated high level overview of what we have.
We moved to our early 1970s house almost 4 years ago bringing with us our electric vehicle. The house had already been refurbished with new double-glazed windows, had cavity insulation (although that wasn’t recorded on EPC so must have predated the prior owners), and a token level of loft insulation. The existing gas boiler was arthritic, couldn’t heat the whole house, but was quite good at heating the header tanks in the loft! We had gravity-fed gas hot water (i.e. no thermostat or pump on the cylinder) which was completely obsolete, the cylinder dated back to the building of the house and had no immersion heater (although we had the wiring for one). So what did we do?
We substantially increased the loft insulation to reduce heat loss.
We had a modern condensing gas boiler installed to improve efficiency.
We updated to smart controls using eTRVs to set both temperature set points and schedules at room level. I built a smart interface to the boiler so that heating can be enabled remotely. I programmed a series of rules into Apple Home allowing the smart thermostats to enable the boiler when any thermostat wants heat and disable it when no thermostat wants heat. Some rooms also have additional rules linking heating to open windows or movement sensors. All of this reduces heat losses by only heating rooms that are (or will be shortly be) in use.
We installed our own solar panels given 4 kWp generation. (I also own a small share of a solar farm although there’s no contract that I’m aware of between that farm and my home energy supplier)
I invested in an immerSUN to maximise self-use of our own solar by enabling loads when surplus solar is available.
We switched to a green electricity supplier so when we need to buy electricity it comes from renewable sources.
We bought a small storage battery 4 kWh to store some of our solar production for use later in the day. Subsequently I can also use it in winter to buy when the electricity price is relatively low to avoid buying when the price is relatively high.
We chose a dynamic smart tariff to buy electricity at the lowest price based on market prices established the day before. The prices change each half hour and are established in the late afternoon on the day before.
We replaced the old hot water cylinder with a modern insulated one (to reduce heat loss) with a low immersion heater (to allow more of the water volume to be heated).
Our principal water heating is now by diverting surplus solar electricity proportionately to the immersion heater, that’s backed up by the gas boiler which is enabled briefly in the evening for water heating in case the water isn’t yet up to temperature, and when the electricity price falls below the gas price I can enable the immersion heater on full power.
All accessible hot water pipes are insulated.
Electric car charger:
I built my own electric car charger that takes an external radio signal to switch between four settings 0, 6, 10 and 16 Amps to help me adjust consumption to match to availability of output from my solar panels. (Subsequently such products were developed commercially with continuously variable current limits, but the limitations of my immersun and on/off radio signal don’t allow me to go quite that far. Having said that my car only does 0, 6, 10 and 14 Amps so I would gain no benefit from a continuously-variable charger paired with a 4-level car).
Smart electricity controls:
We have two systems for smart control of electricity:
The immersun to maximise self-use of our solar electricity by proportional control of loads.
A HEMS to manage the purchase of electricity (when necessary) at the lowest price by maximising consumption when the price is lowest.
When both systems want to enable loads (because the bought price is low and we have a surplus from our own panels) then cost is prioritised, so we’ll buy from the grid any demand not being met from our own panels.
Both systems are linked to 3 devices:
Battery storage. The immersun is configured to work alongside the battery storage with the battery storage as the top priority to receive surplus solar PV. The HEMS can switch the status of the battery as required to charge from the grid when the price is lowest, or to discharge when the price is highest, or indeed to revert to default behaviour.
Car charger. Second priority for the immersun after battery storage.
Immersion heater. Third priority for the immersun after car charging.
I have no firm plans for the future. I’m toying with adding to the HEMS various features including:
Making the display switch between GMS and BST as appropriate (it’s all UTC at the moment).
Edit configuration via the web interface rather than a virtual terminal.
Control a domestic appliance. Our washing machine was replaced relatively recently, but the dishwasher is playing up a little and may be a candiadte for HEMS integration where the optimum start time is selected to deliver lowest energy price.
Late last year I started adding the ability to optimise my electricity price by shifting some electrical loads around in response to a dynamic electricity tariff. My electricity price changes half-hour-by-half-hour and day-to-day, with the prices for the day ahead published each afternoon. I already had the ability to manage the same electrical loads to maximise use of the output of my own solar panels for some 3 years. The first load smartly controlled to follow my electricity costs was my electric car charging, but I have subsequently added optimisation of water heating and storage battery behaviour.
However, time has revealed an occasional issue arising when both the bought electricity price was low and a solar suplus was available, so both sources sought to enable the car charger; but in practice the vehicle didn’t charge. The issue here is that the car charger does a sanity check on the radio signal indicating that it should be operating, which fails since the combination of two signals driving a common radio transmitter can lead to excessive duration of the ‘on’ signal which fails the sanity check.
My chosen solution is to disable one of the two signals sources when the other wants the car charger on. I’ve chosen to make the price signals via the HEMS the master, so when the HEMS wants to charge the car HAT #4 opens so that the ImmerSUN no-longer has influence over the car charger, and HAT #1 is used to control car charger behaviour. When the price is relatively high HAT #4 remains in the normally closed position and HAT #1 is open allowing the ImmerSUN to control charging behaviour via its output relay to use any surplus solar. (HAT refers to HArdware on Top – accessory circuit boards that mount on top of a Raspberry Pi. In my case board with 4 output delays. HAT #3 is currently unused.)
The above images show four different perspectives on the same day of data (April 24th) from different sources within the home.
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.
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:
The Immersun data via the same API called by their app. I came across a blog post that described how to do this.
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).
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).
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.
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.
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.
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.
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.
Here we have the whole assembly mounted on the side wall of the airing cupboard above the hot water cylinder. The whole assembly is mounted on a board which can be lifted off two screwheads. The cables are sufficiently long to allow the whole assembly to be placed on the floor should update be required.
Below the assembly can be seen a socket outlet which provides power for the low current controls only. The Raspberry Pi that provides the HEMS functionality is powered by a re-used USB mobile phone charger. All this control side can be isolated by the simple expedient of pulling out the plug. The high current power for the immersion heater is hardwired via the ImmerSUN. All power is provided via the fused spur with single pole switch that originally powered the immersion heater alone.
As an aside at this point, some ImmerSUN users have reported issues with power connectors melting as indeed have I in the past. In my installation the chances of this occurring are minimised by:
Use of multistand flex for wiring rather than solid twin-and-earth.
Use of bootlace ferrules to terminate the ends of the cables before insertion into the screw connectors.
Use of cable ties to secure the flex to the chassis of the ImmerSUN on the provided locations adjacent to the cable entry.
The illustration shows the components mounted on a board before installation on the side wall of the airing cupboard.
To the top left is the HEMS itself (less cover) with 4 black output relays visible on the HAT of which two are connected via the white cables to operate car charger and immersion heater. Immersion heater software isn’t written yet but will mirror the charger software but on a second channel. Not illustrated is the power cable to the HEMS – a standard mobile telephone charger and USB cable.
To the top right is a 10-way junction box typically used for boiler installations. It takes switched inputs from the HEMS (x2) and ImmerSUN and uses them to drive the two outputs (radio transmitter and ImmerSUN Boost).
To the middle right is the existing radio transmitter used to operate the car charger. In this new installation it can be enabled either by the HEMS when the bought electricity price is low or by the ImmerSUN when our solar panels are in surplus.
To the bottom is the ImmerSUN. In addition to power in and power out to the immersion heater, there are also two control signals: (i) a relay contact that closes when solar is in surplus to drive the car charger and (ii) a relay input that can be configured as ‘Boost’ enabling full power to the immersion heater on an external signal (in my case from the HEMS).