Energy yield figures

[Innocentbystander_01]

I have a 5kWp PV system with a SMA 5000-TL inverter. When I compare the energy yield figures from the inverter (via Bluetooth) per month with the values on the meter from the inverter to the mains, the value on the meter is only around 85% of the figure reported by the inverter. Why is this? I know there could be differences due to inverter efficiency and so on, but the difference is much larger than I expected. If I use the inverter figures, my CF is well over 11% for the past 5 years. Although I have a good south-facing site with very little shading I find this hard to believe in East Scotland! So where does the discrepancy between these two figures come from?

 

[rpdom]

I don't know the layout of your system, or which inverter you have, but on mine the readings are taken using a CT clamp around the main live wire to the utility meter. That meter is calibrated and guaranteed to be accurate (in theory!), and the CT clamp isn't.

Therefore you should believe your meter readings and not the inverter.

As an example, my inverter says I have exported 911kWh to date, but my main meter is showing 1035kWh and that what I get paid for. That's 88%.

 

[pollenface]

Is this a grid tied system? If so, some of the yield is absorbed by your house loads. You will always see less at the meter.

 

[Innocentbystander_01]

Thanks, that's very useful and clear, and makes sense. I don't have any external metering other then the export meter, as the project was planned to expand at some point to include storage and so on, at which point a full measurement system would have been designed. The PV roof + SMA 5000-TL inverter (3 phase) was accredited from June 2014 onwards so I've got a lot of monthly data from the inverter. The system has been managed from the FiT viewpoint by a second party and I don't have many export meter readings to go on, but those I do have, fit on a direct relation to the energy values taken from the inverter. Things are the opposite way from your readings - for every 1000 kWh on the inverter, only 835 kWh is being exported on the export meter. I'm hoping I can scale all my readings by this factor and still use the data. Do you or anyone have any advice on this?

 

[Innocentbystander_01]

Is this a grid tied system? If so, some of the yield is absorbed by your house loads. You will always see less at the meter.

Thanks! Yes it is grid-tied. I'm not sure about the details and will have to ask the electricians. The PV system is on a building separate from the main campus and the inverter and metering are in that building. There is a low-current load in the building (PCs, phones, lights, IT switch...) but I think this meter registers only solar generation as this is the meter reading sent in to the utility. I know the 3-phase ends up in a bigger switchgear room 30 metres away and I think a separate single-phase spur may come the other way. Sorry to be so vague but I've inherited the project and need to talk to a few more people to get it pieced together. For now, I'd like to know if I can use the inverter "yield" readings if I calibrate them against the generation meter. (I should say this is effectively a hobby project so there's no commercial element, the system is a demonstrator)