Increasing difference between Inverter & FIT meters

[Malcolm Morton]

Has anyone else noticed a gradual annualised change between the solar pv inverter meter and the (FiT) generation meter readings, and if so do you know the reason for this?

After my first year of generation  Jan 2013-14 the Fronius inverter was 25 kwh per annum higher than the FiT meter, and the difference has gradually increased since then. Currently the April 2020 annual difference between the inverter and FiT meters is 28.6 kwh/pa. The difference has increased generally consistently between those initial and later dates.

According to the literature the solar panels generation are expected to reduce at circa 1% per annum. (My 3.6 kwp system appears to be generally increasing generation to more than the previous year's averages!). Nowhere have I read anything about an expected increasing difference between inverter and FiT meter readings?

Has anyone any knowledge for the reason for this?

Malcolm Morton

[Melvin]

Hi Malcolm,
Do you expect the inverter reading be the energy taken in from the panels, or that delivered to your generation meter?
That difference is accounted for by losses within the inverter, and is usually small compared to the overall throughput.
The generation meter has to be certified accurate, whereas the inverter meter reading does not, it is just a close guide.
You should expect the difference to increase with time - like starting two clocks where one runs a bit slower than the other!
Melvin
 

--
You received this message because you are subscribed to the Google Groups "Microgen Database" group.
To unsubscribe from this group and stop receiving emails from it, send an email to microgen-database...@sheffield.ac.uk.
To view this discussion on the web, visit https://groups.google.com/a/sheffield.ac.uk/d/msgid/microgen-database-group/CABqtG1sUOwHjEpxbjDVW77s_BJXEh3Ed-7751yLvmeuOLX1iMg%40mail.gmail.com.

[Richard Griffin]

Hi Malcolm,

I fit helps, my own reading of export reading increase for April was 505.95 and the email received this morning from SMA (the inverter manufacturer) was 505 .919

The daily figures usually match within a tenth of a KW, although for a couple of days earlier this month they were well out, I think because they couldn't access the inverter when the Internet was struggling.

I can't access the local interface of the inverter as it has "tripped out" and requires a reset which I can't do without moving two cars out of the garage so it will wait.

(This has been an occasional problem with this inverter and indeed SMA swapped it twice to try and cure it.

It's very rare, except this month and it's happened five times so I suspect that's also related to Internet overload. What does anyone think. I guess I should start a new thread for this question, if anyone has input please reply to that).

Richard

--

[Ken Wakefield]

My 4Kw installation was installed on 31 Aug 2011. The inverter reading this morning is 32162 and the official Generation meter is 31845. This a loss of 317 Kwh = 1%. One major cause is the length of cable between the inverter and the meter. My inverter is in my loft at the side rear and meter at the front centre. Cable length is approx. 17M.

On Friday, 1 May 2020 08:43:15 UTC+1, Richard Griffin wrote:

Hi Malcolm,

I fit helps, my own reading of export reading increase for April was 505.95 and the email received this morning from SMA (the inverter manufacturer) was 505 .919

The daily figures usually match within a tenth of a KW, although for a couple of days earlier this month they were well out, I think because they couldn't access the inverter when the Internet was struggling.

I can't access the local interface of the inverter as it has "tripped out" and requires a reset which I can't do without moving two cars out of the garage so it will wait.

(This has been an occasional problem with this inverter and indeed SMA swapped it twice to try and cure it.

It's very rare, except this month and it's happened five times so I suspect that's also related to Internet overload. What does anyone think. I guess I should start a new thread for this question, if anyone has input please reply to that).

Richard

On Thu, Apr 30, 2020 at 11:13 PM Malcolm Morton <malcolm...@gmail.com> wrote:

Has anyone else noticed a gradual annualised change between the solar pv inverter meter and the (FiT) generation meter readings, and if so do you know the reason for this?

After my first year of generation  Jan 2013-14 the Fronius inverter was 25 kwh per annum higher than the FiT meter, and the difference has gradually increased since then. Currently the April 2020 annual difference between the inverter and FiT meters is 28.6 kwh/pa. The difference has increased generally consistently between those initial and later dates.

According to the literature the solar panels generation are expected to reduce at circa 1% per annum. (My 3.6 kwp system appears to be generally increasing generation to more than the previous year's averages!). Nowhere have I read anything about an expected increasing difference between inverter and FiT meter readings?

Has anyone any knowledge for the reason for this?

Malcolm Morton

--
You received this message because you are subscribed to the Google Groups "Microgen Database" group.

To unsubscribe from this group and stop receiving emails from it, send an email to microgen-database-group+unsub...@sheffield.ac.uk.

[Andy Fletcher]

On 01/05/2020 10:38, Ken Wakefield wrote:
> My 4Kw installation was installed on 31 Aug 2011. The inverter reading
> this morning is 32162 and the official Generation meter is 31845. This a
> loss of 317 Kwh = 1%. One major cause is the length of cable between the
> inverter and the meter. My inverter is in my loft at the side rear and
> meter at the front centre. Cable length is approx. 17M.

Isn't it more likely to be error and drift in the calibration of the
different meters?

GOV.UK has a document on meter accuracy/disputes[1] which states:

3.2 Electricity meters are considered “accurate” if the permitted
margins of error do not exceed +2.5% to -3.5% throughout the entire load
range at which the meter is designed to operate.

I think most meters are considerably better than this!


Andy




[1]
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/454144/Electricity_Determination_Report_2003-2014.pdf

--
Kids not coal
- what CO2 legacy are you going to leave? Use renewables.

Andy Fletcher
Email:an...@x31.com
PGP/GPG:0x7767E3C6
Skype:andy.fletcher
Freenode:andyfletcher

[Ken Wakefield]

Whatever the reason it is something I can not change so it is what it is. The guideline was always to install as close as possible to the official gen meter to avoid drops and I am sure the meters are different performance. It has always been a discussion point on this site but in the end it's not something to lose sleep. My cable is quite hefty to minimise but I don't know the tech details of it.

Ken

Email...@x31.com

[Geoffrey Evans]

Following on this theme :-

My 4Kw installation was installed on 22nd July 2011. 

The inverter reading this morning is 32068 and the official Generation meter is 31901.5. 

This is a loss of 166kWh over the period - approx. 0.4%.

My Inverter is less than one metre from the official Generation meter, which might help to explain the smaller difference.

Geoff 

P.S. It's encouraging to see that other people's installations are producing similar amounts of energy to mine.

--

You received this message because you are subscribed to the Google Groups "Microgen Database" group.

To unsubscribe from this group and stop receiving emails from it, send an email to microgen-database...@sheffield.ac.uk.
To view this discussion on the web, visit https://groups.google.com/a/sheffield.ac.uk/d/msgid/microgen-database-group/28acf23f-cdc8-4fce-98c6-00c061123914%40sheffield.ac.uk.

[Escape Man]

Sorry, Geoffrey, I replied rather than "Reply all"

For the rest of you:

Our installation was end of October 2011, about SSE facing, inclined at 45 degrees, 4 kWp. We have an SMA 4000TL rebadged inverter. Facing

So far, Generation = 30832 kWh, Export meter = 30750 kWh, a difference of 82 kWh, or about 0.3%

I am happy with that!

Bertil.

To view this discussion on the web, visit https://groups.google.com/a/sheffield.ac.uk/d/msgid/microgen-database-group/CA%2BO6dN-u1oJ-2L8PAy1HEqPZix4p6aWEYgi3PhpHKgjebjK6uA%40mail.gmail.com.

[Malcolm Morton]

Melvin, and others

thanks for your response.

I hadn't thought that the inverter meter would measure the DC power i.e. dc volts x dc amps, rather than ac power at the inverter output but I cannot otherwise explain it. The inverter must itself use some power to operate and I thought that power would come from the ac side i.e. was part of the house baseload, which is offset when the panels are generating above the inverter minimum threshold (circa 50w?)

The inverter efficiency i.e. in converting dc to ac power must also be diminishing over time, as evidenced by the increasing difference between the inverter and FiT generation meters, as well as the panels themselves deteriorating. I suppose that would make sense in that most appliances lose their initial efficiency

My total inverter reading, after 7.25 years, is 25862, and my generation meter reading is 25655, a difference of 207 i.e. 0.8% loss which I suppose is in an expected range.

Malcolm

To view this discussion on the web, visit https://groups.google.com/a/sheffield.ac.uk/d/msgid/microgen-database-group/177c6e11-3f30-d66e-acd6-5b53592579ce%40gmail.com.

[Kevin Morrison]

Just chipping in

Meter- 36696

Sma inverter - 36822

Difference of only 126 over 8 years!

My inverter is probably 5m away from the meter.

I'm pretty impressed actually.

Kevin

To view this discussion on the web, visit https://groups.google.com/a/sheffield.ac.uk/d/msgid/microgen-database-group/CABqtG1t4F6Cdeq1D8yrL5zq3O9hgA-vLV4F1D-1KTh0VQz9pzw%40mail.gmail.com.

[John Haynes]

Hi Malcolm and all

In October last year we had a 4.5KWH battery installed and transitioned to Social Energy.

They provide a portal which shows generation amount from the Solar, Consumption amount from the grid, Household Load and Battery fill percentage. The direction of flow is also indicated.

This is updated in real time and I have an old I-Phone set up in the kitchen which displays this 24/7.

At dusk, for a short period of time, the generation figure goes negative. This is in the period where generation is virtually zero but the inverter has not been idle enough for long enough to actually shut down for the night.

This negative is usually around 20 - 30 W so I presume this is the amount of energy the inverter itself consumes when active.

We are still waiting for the upgrade of our Smart Meter from Smets1 to Smets2 after which the display will be upgraded to show earnings from Export to the grid. This was scheduled for March but has been postponed due to CoronaVirus lockdown.

After this, we will no longer be eligible for the deemed 50% grid export revenue from our FiT provider as we will be directly credited by Social Energy at a higher rate but based on actual export.

Hope this helps, screenshot attached of the system a few minutes ago.

John Haynes

To view this discussion on the web, visit https://groups.google.com/a/sheffield.ac.uk/d/msgid/microgen-database-group/CABqtG1t4F6Cdeq1D8yrL5zq3O9hgA-vLV4F1D-1KTh0VQz9pzw%40mail.gmail.com.

[Alun Hawkins]

> I think most meters are considerably better than this!

Maybe, maybe not. I once went for an interview at a UK electric meter manufacturer. During the inevitable facility tour I saw a bunker in the middle of the shop floor, "what's in there?". Turns out it was a test room where they took random samples from the production line to check their accuracy. The meters hurtling down the line were functionally checked, but not for accuracy. The random samples were used in statistical analysis to check that, on average, the meters were bang in the middle of the accuracy bell curve. This was a regulatory requirement.

So you might have a meter at one end of the distribution, your neighbor at the other end, and be paying 5%~ish different amounts for consuming the same energy. However, because, on average, the meters are spot on, then the supplier is not gaining or losing from the inherent inaccuracy of an individual meter.

Alun

[Beach comber]

On Friday, May 1, 2020 at 10:38:03 AM UTC+1, Ken Wakefield wrote:

My 4Kw installation was installed on 31 Aug 2011. The inverter reading this morning is 32162 and the official Generation meter is 31845. This a loss of 317 Kwh = 1%. One major cause is the length of cable between the inverter and the meter. My inverter is in my loft at the side rear and meter at the front centre. Cable length is approx. 17M.

You could check the cable size between the inverter and the grid connection point. In my case the distance was 15 meters and the installer was going to use 4mm cable.

The government guide states a voltage drop of less than 1% for this connection including isolator, breaker and meter. The problem is a lot of installers don't seem to know this.

Peak current for a 4kWh system is 16.66A at 240V and using an online calculator I found I needed a minimum of 6mm.

The power loss over this connection would add to the generation, export difference.

Voltage drops greater than 1% can cause inverter problems as the inverter needs to synchronize with the grid voltage and frequency.

Beachcomber 

  

[John Haynes]

Hi Geoffrey

The total installed cost was £3600 and according to the quotation I was presented with at the time, full payback in pure monetary terms is at the end of year 10.

For me, however, it wasn't just the financial payback that motivated me, it's also the environmental benefit of contributing to the reduction in CO2 emissions.

On average, a KWh of electricity results in just over 1/2 a lb of CO2 going into the atmosphere in the UK. (0.283Kg according to official sources)

With average consumption of 4000 KWh per household, that's over a ton per year.

Within reason (and £3600 is not a lot of money these days, particularly with a Bank Rate of 0.1%), the more I can do to reduce this the better for my children and grandchildren.

The battery is a 4.5KWh Solax and the savings include the projected cost of cell replacement at year 15.

Attached is an extract from the quotation

Hope this helps.

John Haynes

On Fri, 1 May 2020 at 13:00, Geoffrey Evans <nbbc.trea...@gmail.com> wrote:

Hi John

Presumably you did a cost/benefit analysis before buying the battery.  What did the installation cost and what payback period do you expect ?  Our PV system paid for itself after 7 years and I’d like to see whether a battery system would benefit us.

Regards 

Geoffrey Evans

To view this discussion on the web, visit https://groups.google.com/a/sheffield.ac.uk/d/msgid/microgen-database-group/CAMnZ5xy4eY7%2BaxsmZMVb3E8pVVoBMiKHp%2Ba7CR93zr_0uM6_Mw%40mail.gmail.com.

--

Regards

Geoff

[Gary Coombs]

This is something I have noticed (and record) too.  I have two systems.  When the first was installed the TGM was about 5m from the inverter but when the second system was added it was moved to within 20cm of the inverter.  The second TGM is located the same distance from its inverter.  

The difference between inverter and TGM readings for the first system (3.5kWp and SMA 4000TL inverter) seemed to rise quite quickly to 1.2% and have stayed quite stable at around 1.2% ever since. The difference on the second system (1.75kWp and SMA 1600TL inverter) actually peaked at around 0.6% but have now dropped to about 0.5% and seem stable.  

Whether this is losses through the inverter or just that the inverter meter is not that accurate or calibrated is a good question.  If they were calibrated we wouldn't actually need a TGM, so I have always favoured the calibration reasoning.  Either way, the TGM is what we use and should be directly comparable with how your main electricity meter measures things... 

[Bart Hommels]

The meter in the inverter is not calibrated, and certified electricity meters such as the one used for export measuring are. My inverter manual states that the built-in meter should not be used for any commercial purposes. It basically is just a gadget, nothing more.

From my data I found that the discrepancy between what the inverter reports and what the export meter measures is larger for lower power levels, relatively speaking. It probably has to do with the power factor of the inverter output, which is closer to 1 for higher outputs, and gets further from 1 the lower the output goes.

Best Regards, Bart

[Neil Mitchell]

Thought I would check my official meter against that in the Inverter (a Solis 3). Installation is 3.25kwh with the inverter around 8mtr from the official meter. My inverter says 1305 kw total generation and the meter says 1304.40 kw. So to all intent and purpose they are exactly the same after 4+ years of use. Guess the inverter meter is calibrated.    

[Neil Mitchell]

Correction.. Inverter reading 13045 kw, official meter 13044.4. That looks better.

On Thursday, 30 April 2020 23:13:37 UTC+1, Malcolm M wrote:

[Mike Hurley]

Mine has always shown a difference of about 0.6% , which is close enough for me.  I would not expect to see an increasing or indeed a decreasing difference between the two meters unless there is some kind of fault such as a poor/corroded connection in the cabling between the inverter and the meter.   Could be a meter fault although I have not come across a meter whose readings changed steadily over a period of several years.

Inverter currently shows 24585 and export meter shows 24741. 

The output figures below suggest that there is no panel degradation as yet, although I haven't bothered to plot the values against the irradiance data.   Also I have never cleaned the panels.

2012    2782
2013    2802
2014    3013
2015    2894
2016    2887
2017    2802
2018    3014
2019    2944

Mike

--

You received this message because you are subscribed to the Google Groups "Microgen Database" group.
To unsubscribe from this group and stop receiving emails from it, send an email to microgen-database...@sheffield.ac.uk.

To view this discussion on the web, visit https://groups.google.com/a/sheffield.ac.uk/d/msgid/microgen-database-group/68e53941-5e5b-4972-b7a8-3e264cb0e065o%40sheffield.ac.uk.

[John Haynes]

Hi Mike

Doubtful if calibrated. Just accurate !!

John 

Sent from my iPhone

On 9 Jun 2020, at 19:00, Mike Hurley <mshha...@gmail.com> wrote:



To view this discussion on the web, visit https://groups.google.com/a/sheffield.ac.uk/d/msgid/microgen-database-group/f537964a-392f-1a2f-beff-4a5744db94dc%40gmail.com.

[Andy Fletcher]

On 09/06/2020 22:32, John Haynes wrote:
> Hi Mike
>
> Doubtful if calibrated. Just accurate !!

or equally inaccurate :p

(I'll get my coat)


/me waves

--
Kids not coal
- what CO2 legacy are you going to leave? Use renewables.

Andy Fletcher

Email:an...@x31.com