discrepancy between total on generation meter and that on Tigo monitoring

[Nutmeg2]

a friend is concerned because the total generated value for her panels given on the Tigo website (optimisers were used as there are shading issues) is considerably higher than that shown on her ge. meter. The system was commissioned on 14th Jan and reading on gen meter is ~ 2/3 of the total given on Tigo monitoring site. 2 storey house. Her inverter is in attic; gen meter is in cupboard under stairs.
10 x sanyo 250s; sunnyboy 2500; facing almost due west.
Is this difference to be expected? Any comments?

 

[SolarCity]

2/3?

Seems very excessive. It would definitely be worth getting your installers out to take a look. Did they give you some predicted figures for month on month?

 

[Nutmeg2]

Thanks for answer - what difference might one expect? Have looked at the maths again - it is nearer 3/4 - ie about 1/4 seems to have been lost between panels and gen meter.

 

[SolarCity]

I think I've seen 10% but no more than that.

Could be a loose connection - best to get the team back out and gage their opinion

 

[Nutmeg2]

Thanks for your help. She has contacted installers who asked for reading from inverter. After climbing into attic, she found that the reading there agrees with the gen meter not the Tigo website. So loss of ~1/4 between panels and inverter - is this also likely to be a loose connection?

 

[SolarCity]

That's a difficult one. I'm not familiar with the Tigo system but if it is monitoring a certain power at the panel then it seems odd that the inverter and the meter should be registering less. I'd ask your friend to speak to Tigo. I'd be really interested to hear what they say.

 

[FB.]

I've noticed a few people whose monitoring device overstates the amount generated by several percent.
I've also heard mention (but have no proof) that some readings are taken from what's going into the inverter; not what's left after the inverter has lost several percent in the DC:AC conversion.

The definitive (and the one which determines your FiT payment) is the OFGEM-approved generation meter - it is calibrated for high accuracy (and that's why they often start with a reading around 0.5kWh on the display; it's due to the factory running power through them and tweaking carefully until the readings given are precise).

For the above reasons, I didn't bother with a monitoring device; I just glance at the generation meter, which happens to be in an easy-to-see location.


For that reason, I monitor my system by the generation meter.

 

[SolarCity]

A few percent loss in the inverter is one thing, but to be 25% out definitely points to a problem somewhere.

 

[FB.]

A few percent loss in the inverter is one thing, but to be 25% out definitely points to a problem somewhere.

In these dull days of winter, quite a few inverters could be suffering as much as 15-20% loss due to them not running efficiently at low input power. Most inverters see a big drop-off in DC:AC conversion efficiency the further they run below one-third maximum input power.

I can see it being theoretically possible if the monitoring device is quoting gross electricity generation and the OFGEM-meter is quoting net generation.

 

[Earthstore]

I think this has been covered before, your generation meter will/should be correct, purely by the way it reads the electricity, so you will either have some loses from the inverter to the generation meter through the AC cable (doubtful),
or your monitoring system is misreading. If it uses a clamp type fitting these can pick up stray AC current, giving you a higher reading, or it needs calibrating or setting up properly.
I hope this helps

 

[SolarCity]

In these dull days of winter, quite a few inverters could be suffering as much as 15-20% loss due to them not running efficiently at low input power. Most inverters see a big drop-off in DC:AC conversion efficiency the further they run below one-third maximum input power.

I can see it being theoretically possible if the monitoring device is quoting gross electricity generation and the OFGEM-meter is quoting net generation.

Are you sure about that? 15-20% seems unrealistically high.

When an inverter is on low power, it will perform less efficiently - but this is when the system is producing very little anyway. To be 15-20% out would mean a dramatic loss somewhere, even when producing a good amount.

 

[FB.]

Are you sure about that? 15-20% seems unrealistically high.

I'm fairly sure.

If we had a hypothetical 4kW inverter, it would most likely have efficiency something like this:

0.25kW DC -> 0.2kW AC (80%)
0.5kW DC -> 0.45kW AC (90%)
1kW DC -> 0.95kW AC (95%)
2.5kW DC -> 2.4kW AC (96%)
4kW DC -> 3.8kW AC (95%)

To some extent the conversion efficiency is also affected by the input voltage of the array; different panels have different voltage and has the most efficient conversion rate when it is somewhere in the middle of the inverters' preferred range.

Very large strings of panels (with high VOC), or very small strings of panels (with quite low VOC) may not be within the inverters' optimum range ("sweet spot") and may show an "unexpected" decline in performance relative to a string of panels with VOC somewhere in the middle of the inverters' preferred range.

So if we have a theoretical inverter of 4kW capacity and with a 200v to 500v operating range, it is likely to be at its absolute most efficient when running at 300v to 400v and 2kW to 3kW total input power.


.

So I guess that I'm just suggesting that the recent dull weather could account for some very odd occurrences, when coupled with calibration errors of monitoring devices and when coupled with comparing gross input against net output. I can see how weird stuff might happen - but be a "transient" problem once sunnier weather arrives and there's a decent amount of power going into everyone's inverter so that DC:AC conversion losses become relatively insignificant.

 

[SolarCity]

Interesting points. I'd still be surprised if this would account for 15% to 20% losses but I could well be wrong.

 

[danesol]

a friend is concerned because the total generated value for her panels given on the Tigo website (optimisers were used as there are shading issues) is considerably higher than that shown on her ge. meter. The system was commissioned on 14th Jan and reading on gen meter is ~ 2/3 of the total given on Tigo monitoring site. 2 storey house. Her inverter is in attic; gen meter is in cupboard under stairs.
10 x sanyo 250s; sunnyboy 2500; facing almost due west.
Is this difference to be expected? Any comments?

IMO Her and everyone else's setup is at the mercy of the weather - especially clouds, I have a similar setup and just recently I can go from generating 400w all day to a nice day like today @ 7.5KW !!

She needs to leave this for longer, but it wont harm reporting this to the installer just in case.............

 

[FB.]

IMO Her and everyone else's setup is at the mercy of the weather - especially clouds, I have a similar setup and just recently I can go from generating 400w all day to a nice day like today @ 7.5KW !!

She needs to leave this for longer, but it wont harm reporting this to the installer just in case.............

Yes, PV solar seems to be a very erratic generator of electricity in winter.
My own 3.75kW SE-facing array had a couple of 10kW days recently, but also some 1kW days too.
Direct sunlight on the panels makes a big difference - if your panels face SouthEast but the sun doesn't come out until 1.30pm, at this time of year you won't generate much power but those with SouthWest arrays will be well-placed to soak up that sun. Likewise: the SouthWest-facing arrays won't generate much if there's a sunny morning but it clouds over at lunchtime.

 

[series530]

Don't know with Tigo but with Enecsys the differences can be huge. As usual, I'm at the other end of the country and cannot access my spreadsheet: I've been compiling a comparison between the PV meter and the EnviR and the PV meter and the Enecsys website day by day. Provided the smart bridge doesn't freeze on the EnviR system the error between it and the PV meter is about 1 to 2% under all weather conditions. Contrast that with the Enecsys system whereby on a reasonably sunny winter day the error is about 3% and on a cloudy day with a low harvest it can be off by anything up to 150%. The problem is that the monitor seems to have poor performance at low power and absorbs most of the produced energy to drive the inverter. What ends up reaching the PV meter is piffling compared to what it claims to have produced. Overall though, so far, the Enecsys system is reading about 6% higher than the meter but I expect that gap to reduce as the summer comes. Tigo may suffer from a similar problem?

 

[SolarCity]

Sounds like the Tigo system may suffer from the same problem.

 

[Nutmeg2]

This is what Tigo had to say:
'Thank you very much form providing us with this information. The readings from the inverter and the generation meter are very similar and also reflect the expected amount of energy that your systems should have been generating.'
and:
'The information from the Tigo system is higher because this system is reading the energy generated directly from the Solar PV panels, however the system would have some losses on the cables and inverter. Therefore, the energy coming out of the inverter is the actual usable energy and the one being reading on your generation meter.'
Sounds to me like their usual fob off.
She is waiting to hear what her installers say.
Is a SMA 2500 invertor the right size for her 10 x sanyo250s?
I have a SMA2000 for 10 x sanyo 240s facing West with litle/no shading probs.

 

[fiardor]

I have done one tigo system and it was a while ago but i am sure certain parameters have to be inputted correctly when registering the system online.

Just a thought.

 

[SolarCity]

Both of those inverters seem fine for the systems you have installed.

Personally I would go for the HF models if I was to use the SMA range as they offer better efficiency. I very rarely use anything that isn't TL or HF at the very least.

 

[Nutmeg2]

That's a relief - thanks!
Mine is certainly HF; hers probably too as same firm installed both systems

 

[LiftMan]

HI. I have a tigo system installed end of last year. I take regular readings and initially the Tigo records were 13% above generation meter. The inverter was 2% higher.
I questioned the matter with Tigo and got a similar reply to the above. Tigo measures at source whereas meter is fact.
After several months the difference has now dropped to approx 8.5%. To be honest I was a little annoyed however I compare output with several freinds and despite me suffering a lot of shade, particularly early in the year, I am doing very well. At times even beating systems that appear to be better located.
Stick with it but would recommend MPPT scan on inverter is switched off (with installers agreement).
Hope this helps.

 

[Nutmeg2]

Friend's discrepancy now much lower - obviously related to season. Her panels doing very well overall, often better than mine (similar orientation but little shade).

 

[danesol]

Well I have 5kw more now on the OFGEM meter than the SMA Inverter :17: