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Daves CUMonitor gets an NRGDiverter

Dave’s been running with an CUMonitor for a while now and wanted to investigate driving his underfloor heating using a NRGDivert. On the 15th of May one was installed after some testing we found that the underfloor heater only consumed about 1.4 Amps when fully on.  After further investigations we found out that the under floor heating was controlled by WIFI already and the only convenient place to install the NRGDiverter was before the WIFI underfloor heating controller. This meant that the high speed switching of the supply could damage the WIFI switch. So A 100% on/off mode was added to NRGDivert.

And the WIFI Switch was controlled by a 3rd party App consequently this NRGDiverter was controlling nothing .

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CUMonitor Testing for Badger on RS485

We received and inquiry from the owner of a solar installation Nick Name “Badger” that has a wiring setup where the inverter has been wired to the consumer unit board using a 3 core cable that presents no opportunity to use a current clamp without cutting into the cable. Also there is no ready supply near the consumer unit.

So we have resolved to add Modbus functionality to read the current directly from the inverter using RS485. The functionality on the Inverter allows this.

Testing Results are here    in the badger folder.

We are also trialing Dual logging with second CUMonitor configured to record additional analog traces

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NRG Divert Algorithm

So NRG Divert needs to work as reliably as possible and what work for our use cases new use cases may not work ! So the CUMonitor will allow a record  of the results and a qualitative assessment to be made of those results.

We will publish on our website what Algorithms we are using and you should be able to see the results here


The NRG Divert relies on a CU Monitor to record the two current readings taken from the Consumer Unit feeds. It makes an assessment of the likelihood that the Grid connection is indicating that the CU is exporting to the Grid.

NRG Divert Residual (aggregate current consumption)

NRGDivert calculates the current consumption of the house simply by subtracting the Grid current from the PV generated. Obviously this is only worth doing if the CU is observed to be exporting.

This is then assumed to be a stable value and the remaining “spare” energy is calculated by subtracting the Residual from the total PV generated

The first cut of the NRGDivert software assumes the residual value does not change while it controls the load.

Spare energy and using it

So having calculated the “spare current” how to use it and only it is the next problem:

This is done be turning on a resistive load for some AC cycles every second.

Essentially every second the mains AC signal performs 50 cycles and by selectively turning the load on for these cycles you can consume this “spare energy”. Conveniently 50 cycles means that there are 100 half cycles to chose from every second.



Commanding the Load

Turning a load such as a Immersion heater on  without ensuring that zero current is flowing at the time would give rise to a very noisy electrical signal. So we can only do this as the voltage crosses the zero point to avoid this noise.

The Load is a known quantity (its Resistive load) so you know that the full current requirement will be presented when it is turn on.

This is a  power control solution known as cycle stealing. The consumed power is being monitored by CUMonitor  and we should be able to see this load appear as a change on the Grid Monitored value

Verifying the Load

The NRGDivert software  looks for the load appearing on the Grid monitored value and will only remain controlling the load if the expected value is seen and within acceptable tolerances of what it predicts.

This means if the expected Grid Value is out of the predicted range then NRGDivert will disengage the load and recalculate the residual value.


Proving that the algorithm works

On the electricity meter in Site one (ilog) there is a LED that illuminates when the the Meter is exporting to the Grid. This is recorded in the logs on the second plot as a thick yellow line

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What size House Battery

So I need an idea of the size of House Battery to get.  To Store the excess solar energy from our solar panels.


Well the CUMonitor plus the tel-tec web calculator lets you model the problem so you can try out different house battery capacities at the different times of the year. see example logging data here.

If you are interested in joining the CUMonitor user group fill out the form here. Note you will have to purchase a CUMonitor box to log your house it you want to take part as a member

The Battery size is entered at the top of the page and the first (light green) bar chart is of the Power remaining in the Battery and the second is the Power from the solar panels. The Assumption is made that the 24 hr period starts with the Battery full.

Some CU Meters have an LED that illuminates when the Meter is exporting the to Grid CUMonitor can optionally monitor this or use an internally calculated flow,

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Consumer Release of Web RD

Web RD is now ready for release. The consumer version is a kit just to build the display

The MAX7219 component comes with a edge connector and a connecting cable

This edge connector needs to be soldered onto the Web Rd  Module to connect the components

The ESP8266 01S Programmed for Niche-tec Web Connect is supplied preprogrammed with test software that displays a test pattern on the Dot Matrix display and is ready to connect to your LAN using one of the Free Web RD software packages available on the play store.

ESP8266 ESP01S

Assembling these three components will create the Consumer Web RD Display

Web RD   or Web Remote Display  is a stand alone display unit that has the ability to get data from the internet (LAN Required) and display it. This kit  is a minimalist set of hardware to support this functionality. We have reduced the cost so you only need to buy the Kit you intend to use. We offer links to other providers that sell the  components so you can source them at the lowest price. The Housing of the display is left to the user we have partnered with Xen Dragon 3d Printing to offer a 3d Printed housing:

We offer a free download from Cricut that creates a housing from cardboard that is free of any cost.

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CUMonitor Consumer Release

Development of the CUMonitor hardware is progressing and the first boards acceptable for consumer release have arrives and are going thru soak testing.

CUMonitor PCB changes

The major changes have been to add the facility to power the board through a micro USB connector adding onboard voltage regulation of the 3.3v.  The new A to D circuits can run without the the addition of a burden resistor so these have been removed. This means that the CUMonitor consumer PCB assumes that the 30A 1V current clamps will be used. But this should be OK form most household use cases. The surface mount spaces for the burden resistor have been left in case 100A clamps are required. There was a missing resistor on the previous board that is also fixed.

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CU Monitor PCB Upgrades

The CU Monitor PCB upgrade has involved quite a few changes. Moving to the ESP32S2 processor has meant completely revamping  the build to use  the newer and faster processor. Aside from doing away with the with the ADS1115 chip and reading the A to D values across the I2C bus the board has simplified in a number of other ways.

I’m pleased with the upgrade process even though the PCB board will have to be recreated. The  porting meant using new libraries and did away with the I2C bus. There is lot more space on the chip for expansion and the number of samples  per second has risen to more than 9000 from the few hundreds we had with the old design. The current run of PCB’s have burden resistors that are now redundant and one missing resistor from channel 2. But otherwise appear to work great. I’m soak testing them now before ordering some production quality boards and to see if there are any “undocumented software features” in the new libraries.

One change to go in is adjusting the recess of the jack plug sockets these need to be less as when the jack plugs are inserted thru the case they experience an ejection force because the case pushes the jack plugs away from the PCB. Also I need to add pylon holes to allow rapid alignment when the PCB is inserted into the case

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Calculating the power

The CU monitor senses the current flow at the two points marked.  It’s important to realise that even if the voltage generated by the Grid tie inverter has exactly the same peak voltage as the National Grid the current draw into the house may vary throughout the waveform because the loads in the house are not perfectly resistive and the current consumption of devices in the house will not be linear with voltage. Practically, this may result in current flowing into the house at some parts of the waveform and out at others. This means that the current at the the solar panels and the Grid tie inverter input feed will have an ambiguity, adding a safe margin ensures that current flow in or out throughout the whole of the waveform can be accounted for.

The current will be in phase  with the voltage for a purely resistive load, will lead the voltage for a capacitive load and will trail for an inductive load. We need to measure the current at the sensors throughout a full cycle so this can be assessed and we can do this by sampling the current multiple times throughout the 50Hz cycle to  calculate the aggregate current.

Reasonable number of samples

Each current data point sample has to be individually processed (read, scaled and logged). To give a reasonable approximation of the current flowing throughout the cycle, I insisted that a minimum of 10 samples are taken each cycle

This means the sample frequency needs to be at least 500hz per channel to sample a 50Hz cycle. Both the output from the Grid tie inverter and Grid feed need to be sampled. My first design used the ESP8266 ESP01S card which was able to achieve this but the I2C bus connecting the ADS1115 to it was having problems keeping up switching between the two channels and all the other processing.

The Grid connection in my perception is the most dynamic link, potentially switching direction more times  than the other sample point. I decided to over sample this link while maintaining the 500hz minimum for the Grid Tie Inverter so I started using the ESP12

This was marginally more expensive than ESP01S but provided access to a 12bit on board ADC that allowed for this oversampling.

This combination worked well but meant only one channel of the 4 Channel ADS1115 was being used.

Having two components is more costly. The cost of ADS1115 plus the cost of the ESP8266 ESP12 is more expensive than the  ESP32-S2-WROOM  (which supports multiple ADCs). So I have redesigned the PCB to use this processor.

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Power and the grid

The power to the house is from the National Grid and our solar panels.

240V AC mains power

In the UK mains power is supplied from the National Grid. Alternating Current (AC) as voltage and current input is supplied as a nominal sinusoidal voltage of 240v RMS at 50 Hz with a peak to peak of about 338v. In reality this varies between 216 and 253 RMS. The voltage from the Grid goes from 0 volts to +338 volts down to -338 volts in 20 milliseconds. It repeats this 50 times a second (1000ms).

Resistive load

On the very simplest level a house is a resistive load. The voltage inside your house will follow a similar sinewave voltage at a slightly lower voltage because of the resistance in the wires of the Grid and inside your house.

Mains 240V AC ->
--> House Load AC

The voltage at the substation (on the Grid) will be more than that at the resistive load of the house and therefore the current will flow into the house from the Grid, through the electricity meter.

Grid tie inverter

Solar panels on the house generate electricity as Direct Current (DC). DC is fed in to a Grid tie inverter. The inverter inspects the sine wave voltage that is generated by the Grid as seen at the resistive load of the house and creates a matching sine wave voltage  slightly above that of the house. This will cause current to flow from the Grid tie inverter through to the house and if there is enough power from the solar panels, out onto the Grid.

Grid <--
<-- Grid tie inverter

So the power consumption of the house will approximate to 240Volts RMS multiplied by the current into the house from the Grid plus that  from the Grid tie inverter from the solar panels. Because the loads used in the house are not purely resistive this is not the whole store but it is a fairly good approximation.

Power management and current flow

To truly calculate the power usage, we would need to measure the voltage and the current profile throughout each cycle. In power management our aim is to manage the current supplied to the house so that we may use locally generated current rather than draw from the Grid. Power management rather than accurate power monitoring. Knowing the current flow is sufficient to achieve this if we also know the direction of flow. Both the Grid tie inverter and the National Grid will determine the voltage dynamically but use of the 240V RMS value will suffice to estimate power usage for our power management needs. Some electricity meters offer an LED that becomes solid lit when current is flowing out of the house and pulses when current is consumed by the house.