Solar PV Gross vs NET output

Freepost

I'm in the position of trying to sell my house and currently having an EPC certificate produced, the problem that I have here is that the MCS certificate for the solar array says I have a NET capacity (kW) of 3.82 and the EPC inspector is asking what my Gross capacity is which I don't know. What I have is 15 x 255W panels from LG (LG255S1C-G2), which for me = 3.825kW but if the 255W is NET then what is the Gross capacity. Why does this matter? ........ I really don't know what value a EPC certificate has for selling a house but where I am is with 80 points =  'C' but one more point will = 81 and this will get me a 'B'. The EPC inspector has told me that if the Gross capacity is 4kW then he will award the property a 'B'. ......................... I don't know enough about solar cells and whether the Gross output of the LG panels is above the 255W, I am more inclined to believe the the MSC Certificate has a typo using the word NET rather than Gross. Is anybody able to advise on this. 

Thanks F.

Netexporter

The gross capacity is usually called the peak (p) capacity. That is the stated capacity, when new, of the panels times the number of panels, so 15x225W = 3.83kWp. That is when the panels are new. I'm guessing from the capacity they are about 10-15 years old, so they would have lost some capacity. The limiting factor for what you actually get out is the inverter, which will be 3.68kW, assuming you have a standard G98 connection. 

QrizB

I agree with Netexporter.

Gross capacity is the total power of the PV panels; the theoretical maximum they can generate. 15 x 255W = 3825 watts.

Sorry, you don't have a 4kW solar PV system; can you persuade the EPC assessor that 3.825 rounds up to 4?

Reed_Richards

I think that one term describes the rated maximum output of the panels and the other describes that output after it has passed through the inverter.  By analogy with weighing, what the panels produce would be the gross output and what you get from the inverter would be the net output.

In my case I have 16 x 300 W panels so my gross output is 4.8 kW.  But the output from my inverter is limited to 3.68 kW (in order to comply with regulations on maximum export) so there is quite a big difference. 

Freepost

Thanks to everybody that has responded, I'm very grateful. I really like QrizB's idea of no decimal places and therefore rounding upto 4kW. Of course, as pointed out, the inverter restricts the power output to approx 3.6kW ... It's an Aurora Power One which renders the maximum output as quoted on the MCS Certificate somewhat misleading but I'm not about to tell the EPC Inspector his job:-)

Once again, thanks for you help.

F.

Bilivino

The EPC methodology manual I have, advises as follows (lifting just the relevent parts):

'The Gross kWp of the array' shoud be recorded.

'Each individual panel will have its own kWp value which when added together, provides the total Gross kWp value. Where possible, Net values should not be used for RDSAP data entry. Net values have already had losses applied to them which RDSAP will apply again, therefore the gross figure should always be used'.

I thinks this is probably where your assessor is reading from.

btw, there's no entry in the assessment to record the max power of the inverter or a DNO's limit, so I believe as much use as reasonably possible of the generated solar is assumed, so long (since 15th June 2025) that you have an export-capable meter.

There is also an RDSAP 10 conventions document on the BRE website, and Convention 9.05 is the one relevant to solar PV:

"...the peak power (kWp) of the PV array is required. Potential sources include the system specification documents or the schematic wiring diagram (possibly adjacent to the electricity meter or the consumer unit). If the peak power (kWp) cannot be found, the declared net capacity (DNC) stated on the MCS certificate may be used instead".

Link:

https://bregroup.com/documents/d/bre-group/rdsap_conventions_v12_15_june_2025

Oh, if you were still after a cheap way of getting one additional SAP point on the EPC and assuming you have a boiler, consider:

- changing TRVs to certain types of smart TRVs to enable you to qualify for TTZC

- confirming the heating pump age (if it is actually 'aged' 2013 or later)

- checking your existing heating controls and potentially changing to one that provides a form of 'weather' or 'load' compensation (and that is also listed in the BRE PCDB).

You'll need to confirm with your energy assessor that any of these will get you that extra point before committing yourself though.

Hope that helps.

Freepost

Thank you for the information, if I am understanding correctly then the NET wording was a mistake, typo if you want, the certificate should have read "Gross capacity (kW) of 3.82" ... this is what I assumed and clearly, we couldn't get anywhere near 4kW. However, with some changes to LED light bulbs I was able to achieve the 81% on the EPC and this resulted in a 'B' rating which I understand is very good ............ anybody want to buy a house?