Questions Before Purchase

Recently had Solar PV installed on my property with a Hot Water “Doctor” that heats my immersion with excess generation the current monitor installed with my system is an Owl Intuition unit but it appears to be very inaccurate due to the Hot Water system in place, is this something that the EmonPi would be able to account for more accurately ?

If it is something the system can accuratly monitor what would be my requirements for the EmonPi and additional sensors.

What is this exactly? It appears to be similar to our diverter, but which switching principle does it use?

It’s most likely that the emonTx/emonPi will measure accurately, as both use very similar front-end hardware and software, but we need to know more about what you have to be certain.

What do you want to measure? The emonPi has only two current inputs, so it can measure Nett Grid and House consumption (for example) from which you infer PV generation. if you want to measure a third (diverted power, say), then you should be looking at an emonTx with either an emonBase or the emonPi, depending on where you can measure the currents.

If you have an emonPi with two current inputs, then you need the emonPi, 2 c.t’s, 1 a.c adapter, the 5 V PSU & lead. Plus of course a LAN & web browser.

The “Doctor” is a device that when excess energy is being produced it switches my immersion element instead of being sent to the grid, once the desired temperature is reached all excess is sent to the grid.

The current monitor has two “CT’s” one on the PV system cable cable and the other on the meter tail.

As it stand the current system can not distinguish between the PV generation being used for the hot water system so in turns produces high usage reasons and in accurate generation readings so I want to get the most accurate I can with an open energy system if at all possible.

I have a 4kW system (16 panels) with my inverter located in my attic, a voltage optimised next to my fuse board down stairs.

So The “Doctor” isn’t intelligent enough to proportionally control the power being consumed by the immersion heater to match your PV generation, and it’s a case of “all or nothing”?

Assuming that your immersion heater is not on your ring main, then if you want to measure the diverted power, you probably want an emonTx close to your consumer unit, so that you can put a c.t. on the meter tail, the PV infeed and the feed to the immersion heater. With that, you want an emonBase (Raspberry Pi with RFM69Pi). EmonTx and base communicate by radio at 433 MHz, so your Base can go wherever is convenient (assuming that you don’t have very thick stone walls).

If your immersion heater is fed off the ring main, then you’ll need an emonPi by your consumer unit to measure the nett grid power in the meter tail and the PV infeed; and the emonTx with one c.t. close by the immersion heater. Again, the emonTx and emonPi will communicate by radio.

In each case, you’ll need an a.c. adapter for the emonTx, and one for the emonPi if you go down that route. The emonTx won’t need a separate 5 V supply, but the emonPi will.

It an intelligent system, CT on the consumer tails triggers it when excess is being sent back to the grid and the immersion will power on to 200W or 2kW whatever excess is being produced.

The immersion is on a separate circuit within the house. The monitor of the hot water doctor tracks its usage separately to the Owl Intuition monitor that tracks production and usage my issue is its extremely inaccurate yesterday it is stating 8kWh of usage 4kWh of production and the “Doctor” used 1kWh and i know from meter reading in the morning before work and looking when i returned late evening that the house had only used around 5kWh.

As for the set up of the eMon system will this more accurately track my usage and generation and give me a close to true readings ?

There’s an analysis of the worst-case accuracy in ‘Learn’, normally of course you can hope that the components won’t all be at their worst-case values, and that is ‘out-of-the-box’ without calibration. In the past, users who have carefully calibrated the system have claimed that their energy readings over a period of time come within 1% of their supplier’s meter readings.

The reason for asking about ‘intelligence’ was the ‘Doctor’ website makes no mention (that I could find) of the method of control, whether it is proportional and if so, how the power is modulated. That could have a bearing on the measurement accuracy of any system, and might in fact be one of the causes of error in your ‘Owl’ monitor. The other source of error of course it it appears to only measure apparent power, whereas the emonTx/emonPi measures real power, which is the quantity that you pay for. We believe that, unless you have a burst-mode control, the standard emonTx sketch should give you accuracy within the limits that we specify. There’s a ‘continuous monitoring’ sketch under test that will also handle burst-mode diversion or, indeed, rapidly switched loads such as induction hobs.

So if i am to accurately monitor my Generation & Usage with the eMon system does your original parts list still stand ?

My inverter is located in the attic of a 3 story Town house (2006 plasterboard walls chipboard flooring) so from my understanding the eMonTX would be installed here with a CT on the supply cable to my installation.

Voltage optimiser is located adjacent to my Fuse Board on the ground floor under the stairs, eMonPI installed down here with one CT on the Live of my electricity meter and the other on the Live into my Fuse board correct ? Or can i get away with the eMonPi alone with Two CT’s ?

So what exactly do you want to monitor? And where does the inverter input join the rest of the mains wiring?

If you can measure the grid current and the PV current at the same place, you can have those two quantities and you can infer house consumption from the sum of those two. You need only an emonPi. But this won’t tell you diverted power - that will be buried inside the overall house consumption.
If you want any more powers separating out, and especially if your PV infeed doesn’t come down to your consumer unit separately, you’ll need to add the emonTx.
One possible alternative (which the domestic authority might not permit), would be to run a long cable from a c.t. in the attic down to the emonPi downstairs.

I would like to monitor my Generation, Consumption & Export.

I could measure both as Fuse Board & Voltage Optimiser are side by side under the stairs. By diverted power do you mean that used by the Hot Water Doctor (Immersion Heater) ?

I need your definitions!
I do mean power consumed by your immersion heater when I say ‘diverted power’ - that’s the term we’ve used here for a long time. The other common terms are “Grid Power (/Energy)” meaning the power/energy imported (+ve) or exported (-ve) on your connection to the electricity grid, and “House Consumption” or more simply “use”, which can include diverted power, depending on context.

If you have a voltage optimiser, you need to measure all the currents on the side of that where you measure the voltage, or the power will be wrong. (Or you need two units, one measuring voltage and current one side and one the other.)

If you can measure two quantities out of (1) Nett grid power, (2) P.V. infeed (generated) and (3) the rest of your house’s consumption, then you should be able to infer the third, given an a.c. adapter to measure the voltage and determine the direction of power flow. (For reasons of uncertainty in the measurements, it’s best to include the quantity that is generally the smallest in the set of measurements.)

Generation - What my PV install is producing and importing to my house
Consumption - What my house is currently using “House Consumption”
Exporting - What if any of my PV production is being exported back to the grid. “Grid Power” i believe you refer to this as

I will be installing a socket by the Fuse Board for the eMon Pi so the voltage readings it recives will be those of the Voltage optimised side

Then the part exported will show up as a negative import of Grid power, and the remainder will disappear into House consumption.