A couple of times last week our dynamic electricity price excelled itself by going negative so we were actually being paid to use electricity. This situation typically arises when the weather is unusually windy causing a surplus of renewable power. Then, rather than the wind turbines being turned off to eliminate excess generation, the market price drops to encourage more consumption. Such additional consumption at the cheapest times will be a combination of genuinely increased consumption (such as my own shift from gas water heating to electric) and shifting electricity consumption from more expensive times to cheaper times (such as my own electric car charging and static battery charging).

The electricity price dropped as low as -4.85 p/kWh between 3:30 and 4:00 AM, with an average consumption-weighted unit price of 0.62 p/kWh. The red line shows the electricity price in p/kWh on the left-hand scale, the blue shows the average consumption in this billing month, and the bars show today’s consumption driven by today’s prices. (The right hand cost column is missing the leading ‘-‘ symbol where appropriate.)

The increasing electricity consumption as the price falls is driven by automated control of loads driven by my HEMS. The HEMS controls fixed battery charging (and discharging), electric car charging, and water heating in response to electricity price.

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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:

1. Use of multistand flex for wiring rather than solid twin-and-earth.
2. Use of bootlace ferrules to terminate the ends of the cables before insertion into the screw connectors.
3. 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).

My holiday project is to create a Home Energy Management System (HEMS) able to optimise the costs to charge my electric car.   I currently have a smart energy tariff where the electricity costs can vary each half hour and day-to-day as a function of market price variations,  The role of the HEMS is to select the cheapest time to charge the car.  It will potentially control other loads in future, although car charging is my priority.

The HEMs is creating using a small computer called a raspberry pi which can read the electricity prices from my supplier when they are updated each day.  Software of my own creation then decides when to turn a relay on or off to enable/disable the car charger,  A configuration file can be edited to influence its behaviour.  A charging schedule is built once daily and then a seperate process actions the schedule every half hour.  An existing radio link, which is already used to automatically enable my charger when the solar panels are in surplus, will signal that the charger should be enable or disabled.

I now have the HEMS running on my desk, although it’s not yet mounted in a case or wired to the transmitter.  I have also created a small webpage to show the status. 

The webpage shows the 48 half-hour blocks between 5:00 PM one day and 5:00 PM on the next day.  Each block shows the time, electricity price per kWh, and the resulting car charger schedule.  It should be noted that there’s often much more variation in price through the day than is illustrated here.  Indeed sometimes the price can go negative.

The green block indicates a period where the price is below a floor price.  The car charger is activated any time that the price drops below the floor price.

The yellow blocks indicate that the charger will be enabled for a total of 3 hours (2.5 yellow plus 0.5 existing green) within a user-defined time window to achieve an acceptable state of charge by next morning.  The user can define both the time window and the number of hours of overnight charging required.  A further block of yellow occurs outside the window in the afternoon where the price is within the range of these cheapest hours within the window, 

The orange blocks indicated when the charger could be enabled if more hours of charging were required than the 3 hours currently specified by the user.

Finally, the red blocks indicate where the price exceeds a user-defined ceiling price at which charging will not occur.

My next steps are to do a little more testing on the bench while I await delivery of the case, install the system into the case, and then install it in the airing cupboard.

In the longer term, my chosen hardware can switch up to four mains electrical loads.  I anticipate adding the immersion heater so that hot water can be made when the electricity price falls sufficiently to make that attractive versus solar or gas.  I could also enable some underfloor electric heating, but the costs to transmit a suitable control signal across the house would not be recovered by the small energy saving.