Using emonPi to operate a room heater from surplus pv

Forgive such a seemingly stupid question, but here goes anyway…!

I’m looking to add an emonPi to my house with the initially idea of linking to Emoncms to plot solar pv production and energy consumption. The question is could a simplified version of the OpenEVSE be used to do the same utilisation to a 2kw electric heater (room heater, not an immersion heater), I already have solar thermal and my ev is out during the day most of the time. I’m after a relatively step less sequence of adjustment. Is the OpenEVSE quiet in operation as it does it’s job?

My interest is for my own use.

Electronics is a new area, household electrics and engineering are my background, but despite my years I’m a Newbie here, please be gentle.


Welcome to the OEM forum, Graeme.

I suppose the reference to an immersion heater means that you’ve studied the section in ‘Learn’ about the PV diverters. The appeal of the immersion heater is that it’s generally usable in the summer too, whereas room heating is, well, not quite as necessary then (hopefully). But there’s no reason not to do both - I think Robin Emley has a sketch (if not a published one) that would do that - I think it was for a storage heater (or heaters) and immersion heater, but it uses his own hardware, not an emonPi.

I’ll leave it to somebody who knows EVSE to comment on its suitability.

And don’t worry about being new - we all were, once upon a time.

Thanks Robert,
I have looked at the posts you are talking about, they might well be the eventual route. The problem (?) is the steps that they operate at are large enough to loose more than the OPENevse charger appears to loose. For the moment my car’s around in the summer so I can monitor the emonPi and stick it on to charge at an appropriate charge level via the type 2 charger, or the car’s own Eve’s granny cable. That gives me 5 different charging rates through the car’s charging rate, admittedly it’s a faff! As you suggest the summers no good for room heating, but up here in the High Peak that should leave a good 10 months when any assistance on the heating load would be welcome!

What sort of electric room heater do you have? Is it a basic heater element with no electronic controls?

IMO the best solution is the pv diverter. I have a 3kW electric heater in the hallway connected via a Mk2PVdiverter (Robins design, driven by an emonTx v2) and this works really well in all the non-summer months.

That doesn’t sound right to me

The main advantage of using a (mk2pv)diverter over an openEVSE (aside from the huge cost saving) is that it tracks the export much closer, the openEVSE as you say has different output levels and that may work fairly well, but you are still working in blocks, the heater won’t come on until a certain level of export is reached to enable the 1st level divert, by which time you have lost some export, likewise if once it reaches that level and is diverting, if a cloud passes or you boil a kettle you are going to import electric as the openEVSE will not necessarily drop a level instantly, so the tracking is blocky.

The mk2 approach will ensure you use pretty much all the surplus without importing as it works at individual Wh levels (by default). Both Trystan and Glyn (OEM and emonEVSE developers) have EV charging but they both also have PV diverters (although not the mk2) to heat their water, even with the EV charging, the DHW can also get heated as the PV generation and export never exactly match the EV charge levels, the diverter uses up what the openEVSE doesn’t, the fact they are heating water and not a room is no different.

What ever you do to use your PV eg use the washing machine when the suns out etc, the diverter will work around that and mop up any surplus energy without over shooting and costing you money.

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Thanks Paul,
I have been looking at the Mk2PVdiverter and the OPENevse. It looks as though the OPENevse is less fluid in its adjustment, while the Mk2PVdiverter is better. Are you aware of how big the steps are in the Mk2PVdiverter, I’d thought they went up in 500W steps? Part of the reason for doing so much reading around this is to do it at the lowest cost, but I’d like to ensure that the choice I made was as efficient as possible. Robin Emley’s design looks possible. Is your 3kW heater wired directly to the Mk2PVdiverter; to complicate things slightly my radiator location is a room away with no easy (hidden) means to pass new wiring. I was hoping to operate the device via WiFi from the mains via a spur, or even a 13amp plug as per my ADAX 2kW heater elsewhere.

IIRC the smallest setting is 1 joule, but I don’t know if that works in practice (@Robert.Wall might be able to confirm or expand on that) as I said above, the default setting is 1Wh, the aim is to match the granularity to the import meter’s “energy bucket” to swing to and forth within the metered granularity so as not to trigger a count.

You could go as low as ~£20 for the component parts for a mk2pvdiverter (load switching only) and drive that from the emonpi with some mods, ie new diverting firmware (derived from a MartinR’s “PLL” or Robin Emley’s “emonTx v3.4 as MK2PVdiverter” stretches) and physical access to a GPIO pin within the emonPi case.

Many years ago . . I set up the mk2 in a box by the heater “just to test” and simply ran a telephone wire back to the emonTx in the garage, this took it’s mains from a socket beside the heater and the heater just plugged into the diverter. Many, many moons later, it is still in that “just to test” setup. It has worked so well and the phone wire is so unobtrusive I’ve had no reason to prioritise moving it, although it is still on my do to list.

It is my intention to move the diverter to the garage and run a 13a spur from it to the heater location (I am able to run hidden mains cable, it’s just a bit of a chore to do so) . When I do do this, I intend to do away with the “other” diverter I had installed to the dhw prior to learning about the mk2pvdiverter as the mk2 is so much more efficient, it even mops up after the DHW diverter has taken what it can. I will also run another “diverter spur” outside so that when i eventually hack out hot tub heater circuit, I can have a second “diverter” supply to the hot tub to heat using only surplus PV in the summer months with just a changeover relay/contactor on the diverter output.

This should be possible, some of robin’s diverters have an RF capability, but with any RF/Wifi there is the issue of network speed and connections etc that could lower the effective resolution and/or leave the load on or off if there,s a connection issue etc, much of this can be catered for in software, depends how dirty you want get your hands. A receiver at the diverter end also obviously adds to the cost, but not by a lot.

I think the hardest part of the project would be adapting the emonPi firmware, there are examples you can draw from, but there is not currently any known diverting firmwares for the emonpi, a shame really as I feel there would be significant interest in one if it existed. IMO if you cracked that the rest would be pretty simple, cheap and very efficient.

Hi Paul

Thanks for that, it sounds really useful information. I’m sat here plodding back through Robin’s pages on the MK2PVdiverter as we speak; work keeps getting in the way as it does!

The bell wire approach is intriguing, I had dismissed that earlier, but it does simplify the system and, as you point out, potential problems could be avoided at a lower cost. I could take that around the skirting and doorframes to the location of the radiator. It wouldn’t be too difficult to remove the skirting and architrave even. Wow, I’d stopped seeing that as an option, thank you!

I think there is a need. I totally see the car (OPENevse) and the immersion as the logical first steps for most people, it’s just not the right solution to my needs.

The emonTx would be able to give consumption/export data to emoncms? While the MK2PVdiverter would enable control of operating modes, just in case I ever wanted to use the heater without any PV surplus as unlikely as that is.

The Mk2PVRouter most definitely does NOT work in 500 W steps. In fact, the steps are not in power at all, but energy. The power steps are whatever your heater rating is, the energy steps can be as small as you can realistically detect, down to 1 cycle of mains delivered into your heater. If you look at the videos on Robin’s Website on the old forum here, I think there’s one showing the disc of an analogue (Ferraris) meter oscillating to and fro over an arc of around 20°. It’s been a long time since I looked at it - if you can read the front of the meter, you should be able to roughly estimate the size of the energy steps in that demonstration.

(OK, you can never have less than 1 cycle with burst control - ½ cycle will result in d.c. which is bad, and you are not likely to be able to measure an amount of energy that small. The realistic ‘energy packet’ is around 1 Wh as Paul says, but making it larger whilst staying within the meter’s energy packet size reduces flicker.)

Paul’s “telephone wire” extension is perfectly valid. All you need between the ‘metering’ part and the ‘power switch’ part is enough voltage and current to light the LED inside the opto-isolator on the driver PCB. You use a mains supply local to the heater - which you presumably already have - for the power switch, and a supply local to the incoming power for the metering part.

Thanks Robert,

I think the MK2PVrouter is much more capable of the small levels of adjustment that I was after. I am going to go that route to control the heater. My confusion stems from a video which demonstrates the unit working in stages as a prototype. The more I have read the greater respect I have for Robin’s design.