emonTx - where to put it?

I'm currently looking at solutions for dumping PV energy into water heating (like so many other forum members).

Noticing that the emonTx can measure the AC voltage and current (and therefore measure import/export of energy from the house) thats great!

I'm halfway through writing a nanode interface to my SMA SunnyBoy Inverter over Bluetooth so I can get the realtime power generation readings from that.  I'm doing this as I don't have a seperate fuse board for the PV circuit - its wired into the main house circuit.


But the problem I have, which I expect a lot of other UK people may have is:

1. My electric and gas meters are located on the outside wall of the house (behind the usual plastic cupboard doors!) like so many other houses built in the past 25 years.  Something like this..

The meter tails for connecting the current clamps to are also in this cupboard (obviously) so I can put the emonTx in there with a battery pack.  However, I now have the problem that I can't measure the AC voltage - theres no 240v socket in the cupboard or even outside!


Anyone else have this issue?  Solutions?




mharizanov's picture

Re: emonTx - where to put it?

 Don't you have a fuse box inside the house? I have similar setup for the meter, but also a fuse box inside the house. I wrapped the CT sensor around the live wire and am in business since.

mharizanov's picture

Re: emonTx - where to put it?

 Another thing that had me wondering is why you guys redirect excess power to water heating rather than charging batteries? Energy conversion is not as efficient. Where I live, it is typical to charge batteries during the day (when excess power is available) and use that energy later in the day when the sun is set.


We heat water using the vacuum-tube solar systems like this:



They work pretty well almost year-round as long as there is solar irradiation. If the sun is not enough, they simply pre-heat the water to some degree and electrical hearer kicks in afterwards. Some guys here even put these on solar tracking platforms and raise efficiency significantly.



glyn.hudson's picture

Re: emonTx - where to put it?

Hi Stuart,

I have the same problem as you, my solar system feeds into the same fuse board as the other loads in the house. It is however still possible to monitor it using the emonTx and CT sensors.

1. Generation CT - Remove the plastic trunking from below your generation meter and clip the CT round one of the wires going into the meter. It is possible to then replace the trunking with the CT inside.

2. Grid inport/export CT - Clip one CT round the live where where it comes into the house. This will monitoring gird import/export. This value with be positive when you are importing and negative when you are exporting.

3. Voltage sensor - in order to determine the direction flow of the current the emonTx needs to monitor voltage. This is done with an AC-AC adapter. The needs to be plugged into an AC outlet within a few meters of the emonTx. This can be tricky as often there is not a power socket near by. You might need to use an extension or put in a new connection into the fuse box. Get an electrician to help you if you’re confident doing this.  

On the emonTx it's possible to create another variable called consumption which can be calculated by: Consumption = grid + generation.

I plan to fully document my PV monitoring setup when I get a chance in the next few weeks.



stuart's picture

Re: emonTx - where to put it?

Yes obviously I have a fuse box however:

1. Its located flush with the ceiling and the mains/meter tails cables run in the ceiling and then drop into the top of the fuse box so I can't get a sensor around that

2. Its also located in the downstairs toilet so I can't put a 240v socket in there to measure the voltage or power the emonTx !!


If Open Energy Monitor were to become a successful "easy-to-use" product for the mass market, were going to have a lot of these issues!

You can't expect end users to open up a fuse box (and its probably against UK Part P regulations - although I'm not an electrician!)


I think what I would probably like is to use an emonTx unit in the outside meter box (just like Efergy + Current Cost meters do) and then do the voltage calculation on a base station in the house (perhaps the emonBase or the GLCD unit?)


stuart's picture

Re: emonTx - where to put it?

Most solar PV units in the UK are direct grid connected so don't have any batteries.  They won't power your house in the event of a power cut either (due to G83/1 regulations).  Although there are some inverters which can do this because they disconnection you from the grid.


The UK "FIT" generation payments dont reward for exporting your surplus energy to grid, so it would be beneficial to use the electric to heat water and therefore prevent consumption of Gas.


mharizanov's picture

Re: emonTx - where to put it?

 Thanks for explaining. It sounds quite odd, but I guess it is a matter of regulation. We have the so called hybrid grid tie inverters that let you charge batteries and use the stored electricity eventually, something like this:


mharizanov's picture

Re: emonTx - where to put it?

 I think this is not a mass product anyway - it does require some electrical and programming skills to set it up but it is quite rewarding.

 EmonTX can be battery powered and measuring voltage is optional (does improve accuracy, but still not a pre-requisite).

Here is my fuse box, I had the Arduino+power supply plug installed in ~30 minutes (did it myself, but yo can always get some professional to do it for you). Where I live, the fuse box is my property and I can open it freely. The metering devices are outside my house and I have no access there. Even If I had, I would imagine the electrical company guys' surprise to see that  thing blinking near their meter :) :) 

Mr. Sharkey's picture

Re: emonTx - where to put it?

...on any of the battery-based, grid-tie inverters with which I'm familiar. None of the inverters I've had experience with will export power to the grid (or a dump load) until the batteries are fully charged. The reason for this is that the inverter uses the grid as part of the charge control circuitry. Once the batteries have come up full, the "export" feature on the inverter is activated and any PV power in excess of what is required to maintain the batteries at float charge is sent to the grid.

In my own system, there are no loads on the inverter unless the grid has failed, so when the sun comes up in the morning, the batteries are essentially full already, and the excess is sent out to the grid. Unfortunately, although my "smart" utility meter can tell which direction the power is going (buying or selling), it continues to increment the kilowatt hours, and I get charged the normal unit rate for feeding the power back to the utility. In this case, it is imperative to make sure that none of the power reaches the utility's meter.

Although it pains me to burn up AC power in a water-heating dump load (solar direct water heating ~is~ much more efficient), at least I am offsetting the power bill a bit on days when I will need to run the water heater with grid power for domestic use.

(An aside: I have dissected a number of "smart" utility meters and hacked their circuitry. I can share details if anyone is interested. The utilities don't share their information with lowly commoners like us, so if you want to know how one works, you've got to DIY.)

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.