Calculating Voc (STC) & Isc (STC) values

I have taken the Voc, Isc and Irradiance measurements for my strings and entered these on my paperwork, but I do not know what values I am supposed to be entering for the Voc (STC) and Isc (STC) values in the Array Parameters section of my paperwork.

Looking at the same paperwork from another sparky I cannot see how he has got his figures.

Can anyone help, please?

On the specs for the panels, usually on the back of the panel.

On the specs for the panels, usually on the back of the panel.

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That's going to be real inconvenient if he's got to take down what he's just put up, can you imagine the customers face!!!ROTFWLROTFWLROTFWLROTFWL

That's going to be real inconvenient if he's got to take down what he's just put up, can you imagine the customers face!!!ROTFWLROTFWLROTFWLROTFWL

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Not if you make a note of it beforehand, or think to check the manufacturer's website

---------- Post Auto-Merged at 20:11 ---------- Previous post was made at 20:06 ----------

On the specs for the panels, usually on the back of the panel.

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Thanks that's what I was thinking. Seems the other guy just made his figures up; which seems odd given how simple the solutions is.

---------- Post Auto-Merged at 20:12 ---------- Previous post was made at 20:11 ----------

On the panels data sheet available from the manufacturers (or suppliers) websites normally

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Thanks

Not if you make a note of it beforehand, or think to check the manufacturer's website ---------- Post Auto-Merged at 20:11 ---------- Previous post was made at 20:06 ----------

Thanks that's what I was thinking. Seems the other guy just made his figures up; which seems odd given how simple the solutions is.

---------- Post Auto-Merged at 20:12 ---------- Previous post was made at 20:11 ----------

Thanks

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Hi One rather rusty Ex GEC Avionics engineer with a question

Voc varies with temperature so it never just one number unless you take the value from the manufacturers specs at 25 degrees. Is that what is used for PV calcs?

How significant is excessively high voc in relation to the inverters max DC rating at say -5 degrees

I stumbled on this thread whilst pondering this question.

I am in the process of getting PV quotes left right and centre to try and get something installed before the FIT deadline

Most reps pump out figures at me using the sap formulas to show how much money I can expect to get back via Fit but as soon as I ask a technical question like this that has been bugging for months they go quiet.

Before getting quotes I played about with the Eversol Everplan tool

and each time I entered say 16x250w panels and picked their TL4000 Inverter all the numbers came up green except one

That being the total Voc at -5 degrees of 1071V which is way higher than the max DC input rating (680V) of the inverter at that low temperature

The tool says reduce the panels on the string but that seems to defeat the object and seeing as just about every installer seems to go for a string of 16-20 panels and a 4KW rating Inverter what is the true risk of anything going bang or being damaged in the colder weather?

what is the presumed minimum temperature to use for such calcs and is the maximum Voc every really likely to be reached?

any educational type links much appreciated

Thanks

Why would the panels be at 25 deg and the inverter at -5 deg?

Nah 25 degrees is the value given in the panel spec sheets for the given Voc I am not refering to the inverter itself being at -5 but the pamels on a cold day

the Everplan tool seems to be suggesting that the combined Voc of all the panels on a cold day say - 5 and frosty goes much higher - way over the inverters max input voltage

let take a Risen SYP250M as an example

spec sheet say Voc at 25 degrees is 37.5v so that x 16 panels equals 600V so within the Inverter spec - all is well

but what the spec sheet does not say is what the voltage is at lower temps i.e a nice frosty day here in sunny Shropshire.

Well to be precise it probably does say but I am not sure how to interpret it

The ever plan tool suggest that voltage is then way to high I suspect the Everplan tool is in error but perhaps not hence it has been bugging me a bit

I have done a google or two and this does seem to be a bit of a grey area

2 parallel strings would be the answer to that. 2 strings of 8 would give you around 280ish volts and a max current of around 16 amps.

Also I think it's referring to the cell temp at -5 degrees and not the inverter (as temp decreases voltage increases) that would explain how you get over 1000v on 16 panels

The spec sheet will give you a percentage voltage increase/decrease per degree varying outside stc.

The (STC) part is "Standard Test Conditions" - i.e. a defined irradiance, falling on a panel at a defined azimuth, with a defined Tamb, will produce a defined (and repeatable) Voc and Isc - It is simply a common standard.

HTH

KME