Hardware & Software options

I’m trying to determine the best combination of hardware\software to get going. Here’s my current situation:

  1. Live in the US
  2. Will be getting a PV system installed in the next few months (Sunpower with Microinverters)
  3. Will hopefully have the PV system link next to\near my main electric panel

My question is, what combination makes the most sense here?

  1. EmonCMS running on a Windows or Linux box along with a single EmonTX (with the Wifi module) monitoring both grid usage and PV system generation\usage?
  2. Raspberry Pi with the RFM69Pi installed on top of it running the EmonSD image along with a single EmonTX?
  3. Just the EmonPi - Solar PV bundle?
  4. Would a Pogoplug running Debian be sufficient to run EmonCMS? Is this a very resource intensive application? Or are there other things running on an EmonBase/EmonSD besides the CMS?

Do I need to monitor both the grid and PV system? Just the PV system? I couldn’t really tell what the difference was between the regular EmonPi and the EmonPi- Solar PV bundle. Sorry if these questions seem basic. I’ve been hunting around the website and community pages for the past few days and couldn’t find these answers - thanks in advance!


Welcome to the OEM forum, Chris.

The easy one first: The “Solar PV Bundle” is a bundle of standard components - mainly put together for the UK market, where we have (for most domestic customers) a single phase 240 V supply. This bit is important, because you have a split single phase supply, 2 lines and a neutral and you need 2 c.t’s where we only have one line and neutral and so only need one c.t.

Have you read our page about the emonTx in N.America? That’s probably worth reading if you haven’t. The basic assumption I’m making is you need to measure the current of both supply line conductors.

Unless you can measure the PV output (assuming you want to know that) by means of an optical pulse sensor, then the emonPi doesn’t have enough c.t. inputs. So that’s pretty much ruled out in that case.

This is probably the minimum you need. The emonTx will almost certainly require some minor modification to handle the larger c.t.'s that you’ll need on your cables - so beware, and don’t order our standard ones until you’ve checked. As part of that modification, you can convert it so that 2 channels are good for your grid connection, and one or two for your P.V. infeed. I say “or two” because if your inverter is connected across 240 V and doesn’t have a neutral connection (or the neutral only carries the current for the controls and electronics), you’ll only need one c.t. for that.

The difference between the Raspberry Pi and your Windows box + WiFi is collectively, we’re much more conversant with the RPi. So supporting you and the RPi will be easier. But if you’re an expert on your system, then go with that. You’ll need to set up a WAMP stack (emonCMS is all PHP, along with some Redis and Nodered) on Windows.

I don’t know Pogoplug, so I can’t comment. EmonCMS isn’t all that resource-hungry, as far as I’m aware, but I can’t be any more specific than that.

Make sure your installer gives you access to put the c.t’s on the cables. If you get the c.t’s in advance and they fit them, you can have ring-core rather than split-core, and the ring-core will be smaller, cheaper and more accurate. But you need the split-core if you can’t disconnect the cables to thread through a c.t.
(And beware, don’t leave a c.t. disconnected - short-circuit it for safety.)

You might be able to get emonCMS to run on a PogoPlug. The CPU (1.2 GHz ARM) should be adequate, but there’s a definite lack of internal disk space and RAM. (only 256 MB of RAM and 512 MB of internal flash storage) The unit has a USB port which should be usable as a connection point for an external disk drive, so that part is easily gotten around. The lack of RAM would be the bottleneck.
But the price is right at 10 bucks cheap! Sounds like a fun toy for a hardware hacker.



You’re a machine, I’ve been reading your replies to practically every thread on here - thank you for the response! Yes, I read the article about the emonTx in NA, a lot was over my head, but I did get that I would need to put CT sensors on both lines coming in. And it looks like I may need one or two sensors on the PV system, so the one emonTx should be able to handle all 4 sensors. The system size is about 16kw, so will I run into any issues using that 4th higher sensitivity input on the PV system if I need two sensors (I thought I read somewhere about more accurate readings on systems 4kw and below)?

I didn’t notice any ring-core sensors for sale on the website, but I do recall reading that using other sensors may require adjustment\calibration of the emonTx. Do I just find ring-core CT sensors with the proper (what looks like 3.5mm) plugs on the end and run through some kind of calibration routine?

The situation is a bit more dire than that on the RAM front - I believe the pogoplugs (at least the Series 4 ones I have) only have 128MB of RAM :slight_smile: Thankfully there are USB 3.0 ports on it, so I can potentially use one for disk space and one for dedicated virtual memory. Won’t be as fast as actual memory, but might still be usable.

Indeed. It’s one or two more than the emonPi - that is the point.

That comes from the UK’s 16 A limit (at 240 V) for domestic PV, so the 4th input is tailored specifically for that. If you’re not using our standard c.t’s (is your 16 kW at 120 or 240 V ? - I’ll assume 240, it’s 66 A and our standard c.t in input 3 would be OK for that.

The OEM shop doesn’t stock ring-core c.t’s, but the manufacturers listed on the N.A. page do equivalents to the split-core ones listed there. We can check the suitability if you let us know what you have in mind. The plug is the last thing to worry about - if you can solder a plug.
But it’s almost certain that you’ll need larger split-core c.t’s for your main incomers, and you will almost certainly need to modify the emonTx for those (as well as putting a plug on them).

Yeah, that would be a tight squeeze. :wink:

On another note…

If your house is of fairly new construction, and by that I mean early to mid 70s and newer,
then you’ve likely got AWG 4\0 Service Entrance Wires. (SEWs = main incomers) Their diameter is typically ~5/8ths of an inch. These are the wires Robert mentioned above, that are too large for the Shop-sold CTs.

You didn’t mention if you wanted to do whole-house monitoring, or branch circuit monitoring.
If whole-house is monitoring is what your after, then you’ll need split core CTs vice solid
(what Robert called ring) core CTs anyway, as you won’t be able to install solid core CTs on
your SEWs.

If branch circuit monitoring is what you want to do, then solid core CTs make sense especially
if space in your load center (circuit breaker panel) is at a premium.

If you have access to the connection point where your PV system ties to your load center, and your PV system is a 240 V system, (and at 16kWp, I’m betting it is) then you’ll be able to get away with using only one CT to monitor your PV backfeed. That’s exactly how my system is set up.

@Bill.Thomson - Thanks for the responses!

I’ll start monkeying around with the pogo to see how far I can get it. Currently I have Arch Linux on there, but I think they dropped support for ARM awhile ago. I’ll slap Debian on there and see what happens.

Yes, my house is fairly new (~2017), so I’m guessing they’ve got fairly fat wires coming in. I’d like to do whole-house monitoring. So for the CTs, I’ll need clamp-style ones for the SEWs, and solid core on the PV system. Does it matter where I get these, or are there specific kinds that work with the emonTx? Are there suggested ones the fat-wire folk among us should get? I found this post:

So it seems like I should just follow those guidelines when shopping around? Looking for current transformer sensors, seems like the bulk of them all have 13mm x 13 mm openings.

Hmmm… Their https://archlinuxarm.org/ page shows:

We build optimized packages for soft-float ARMv5te, hard-float ARMv6 and ARMv7, and ARMv8 AArch64 instruction sets to use each platform to its full potential.

so maybe they’re still in the game.

Given a two year-old house it’s almost a sure-bet your SEWs are 4\0 Aluminum.

Magnelab, as well as others make a CT that has a 0.75 x 0.75 inch wire-window:

That particular “form-factor” - the physical size/shape - is in common use and is easy to find.
The downside is the majority of what’s available, is the 333mV variant. You want an unburdened CT, i.e one with no internal burden resistor. They are available, but aren’t as common as the millivolt variety.

Continental Control Systems is another supplier of CTs of that form factor and has a version
with a 1 Volt output. While not the best choice for the job, 1.0 Volts doesn’t miss Robert’s
recommendation of 1.1 Volts by much.

You haven’t said just how much of a hardware tinkerer you are, but from your description
of what you’ve done/are going to do with your PogoPlug, it sounds as if you like to hack on
the hardware too.

You might want to take a look at this:

It’s one helluva deal at 45 bucks. The two 100 Amp CTs included, sold for 47 smackers apiece,
and the unit itself normally retails for ~325 bucks. On top of that it’s new not used.
It uses RS-485 Modbus to deliver its data, which is nice because it’s a robust noise-immune protocol that can be put on common twisted pair copper (I use speaker wire with good results)
at distances up to 4000 feet. USB to RS-485 adapters can be had cheap too. We’re talkin’ like
4-5 bucks cheap.

To repeat myself - there is a big list on the “Using the emonTx in North America” page, along with details of what has to be done to make them work correctly.

@Robert.Wall - I saw that immediately after asking the question, sorry about that! All of the diagrams made my head spin and I completely forgot about those lists after the technical explanations.

@Bill.Thomson - I’ll double check Arch, I thought I remembered reading something about how they abandoned the ARM v5 architectures in the past few years. Either way, getting some form of LAMP running on it. I have another low power PC running windows 24/7, so I might piggy back on that if I can get the proper services running. Hacking the Pogo was a completely non-hardware experience for me, as I did it back when Pogo allowed you to enable SSH through their website. I did it for both of the ones I have (a Series 4 and a Series 4 Mobile - a more neutered version), and they’re both running old versions of ArchLinuxARM, some SMB services, and FTP service.

The Sunpower system you linked to looks really interesting, and for less than 50 bucks, sounds like a nice long-term project. I’m assuming that’s basically the same kind of setup: 2 CTs to the SEWs, 1 (possibly 2) CTs to the PV system lines, and then an AC-AC line coming into to provide the connection to neutral? Is there a way to test these kinds of systems before installing them in your electric panel? For example, by wrapping the CTs around the line and neutral of an extension cord with a simple light bulb at the end?

Not line AND neutral, line OR neutral. If you thread both through, you’ll get nothing.
If you use a tiny load like a light bulb, you’ll need to put many turns through the c.t. If you look at the test report on the SCT-013-000 (in the ‘Learn’ section), you’ll see that I use a 20 turn coil to make 5 A look like 100 A as far as the c.t. is concerned (and 50 turns to get up to 250 A). You don’t need a low voltage transformer - I only use that so that it costs very little power when you use 6.5 V rather than 240 V.

It’s actually a Continental Control Sytems WattNode Modbus.
CCS sells some of the CTs on the “suitable for use in North America list.”

Yes. One connection to each hot leg, and a single wire to neutral.

DISCLAIMER - For illustrative purposes only. This jig was built for a one-time test.

I threw this together one evening for just that pupose:

DSC02817 DSC02816
The wire gauge is quite small, but the “simulated” load was a 10 Watt incandescent lamp.
The CT has 60 turns wrapped through the wire-window, which made the lamp look like a
600 Watt load, i.e. right at the maximum rating of the 5 Amp CT. (120 Volts @ 5 Amps)