How to calculate the Voc of the two PV strings

[praseto]

Hello,
After researching a lot I still have a question that I can

[Mike90250]

Strings in parallel, you add amps, volts are same

panels in series, you add volts, amps are the same.

Basic electricity. Don't build this stuff or second guess an electrictian till you know it.

To a carpenter, every problem can be solved with a hammer!

every grid tie inverter mfg, has a "String Sizing Tool" to caculate how many panels / configuration will work on their inverter. Use it.

[Antscran]

Hi,

It's also worth bearing in mind the calculators they provide are a good guide but it should always be checked by someone in the know, as long as you have the panel and inverter datasheet you can do this with some practice and adding a safe factor in based on the geographical location and panel mounting. The voltages on the panels will fluctuate with temperature changes as they are based on a standard test at 25C, in winter at lower temperatures the voltage will rise and this is dependent on the temperature coefficient of the panel used. I have seen a panel string voltage rise by 17% which was over and above what the software predicted for the location and the mean temperature averages.

Ant

[Mike90250]

Hi,

It's also worth bearing in mind the calculators they provide are a good guide but it should always be checked by someone in the know, as long as you have the panel and inverter datasheet you can do this with some practice and adding a safe factor in based on the geographical location and panel mounting. The voltages on the panels will fluctuate with temperature changes as they are based on a standard test at 25C, in winter at lower temperatures the voltage will rise and this is dependent on the temperature coefficient of the panel used. I have seen a panel string voltage rise by 17% which was over and above what the software predicted for the location and the mean temperature averages.

Ant

Unless a manufactures PV Sizing Tool is known to be broken, it really is the right tool for the job. They DO factor in, that panels will be 30F hotter than ambient, and it is up to the user, to properly input historical tempature data for their location. If you put garbage in, you get garbage out.
Of course, if you know more than the manufacture does, do as you see fit.

[Naptown]

Hi,

It's also worth bearing in mind the calculators they provide are a good guide but it should always be checked by someone in the know, as long as you have the panel and inverter datasheet you can do this with some practice and adding a safe factor in based on the geographical location and panel mounting. The voltages on the panels will fluctuate with temperature changes as they are based on a standard test at 25C, in winter at lower temperatures the voltage will rise and this is dependent on the temperature coefficient of the panel used. I have seen a panel string voltage rise by 17% which was over and above what the software predicted for the location and the mean temperature averages.

Ant

Average high temp is one inputted number. This will determine if the inverter will trigger on the hot days.
However more important is the low temperature number you use. DO NOT USE AVERAGE LOW TEMPERATURES! USE THE RECORD LOW TEMPERATURE FOR YOUR LOCATION.
This is important as low temperatures could potentially fry an inverter due to over voltage. High temps and an inverter not starting due to low voltage is not a good situation because of loss of production but at least the inverter is not at risk.
The manufacturers string calculators generally ( At least SMA does) will give you all the information you need if you go past the initial string sizing and read the information regarding voltages etc that is in the report section.

[Sunking]

However more important is the low temperature number you use. DO NOT USE AVERAGE LOW TEMPERATURES! USE THE RECORD LOW TEMPERATURE FOR YOUR LOCATION.

It is the most important data to enter, and the only one electrical codes are concerned with.

[praseto]

Thank you to everyone for the response. I think i got the answer to my question.The L1 and L2 for DC are connected and like Mike wrote it is Apms what we get. i didn't know until I opened the inverter.
I got another newbie questions:
All the string cal tools are trying to push for optimal use of the inverter. If they come up with two strings of 9 panels (BP 170w(Vmp=35.4,Voc=44.2,Imp=4.8,Isc=5) can I use only one string of 9 to start with and then I upgrade to 2 strings to get full power?

[Sunking]

If they come up with two strings of 9 panels (BP 170w(Vmp=35.4,Voc=44.2,Imp=4.8,Isc=5) can I use only one string of 9 to start with and then I upgrade to 2 strings to get full power?

Yes you can do that. Just be aware you will need the exact same panels for the future expansion.

[dogsbody]

Unless a manufactures PV Sizing Tool is known to be broken, it really is the right tool for the job. They DO factor in, that panels will be 30F hotter than ambient, and it is up to the user, to properly input historical tempature data for their location. If you put garbage in, you get garbage out.
Of course, if you know more than the manufacture does, do as you see fit.

Worried by this. Tested yesterday my single 80 watt panel on my boat to test out the temperature probe with my multimeter. In hazy sun, the ambient was 21 degree C, and the panel temperature was 46 degrees C. So in very aaverage Uk temperatures, the panels temperature is more than twice the air temperature.
What happens in the desert when the air temperature is 40 degrees C, and what effect on the outputs on panels in that situation.
Luckily in the UK the design panel temperature for air at 25 degrees is rarely exceeded, so I dont foresee problems here..........but what happens in hotter climes?

[Sunking]

but what happens in hotter climes?

The voltage output of the panels drop, current remains the same, thus lower power output.

[dogsbody]

So panels in temperate climates produce more power than in hot climates due to the temperature effect.
I am learning

[Sunking]

So panels in temperate climates produce more power than in hot climates due to the temperature effect.
I am learning

No way Jose. In the UK you only receive a few Sun Hours in summer. and almost none in winter. In the dessert southwest of the USA receives up to 7 hours in summer, and 4 to 5 in winter with almost no cloud cover year round.

To give you an example of extremes I will use Seattle WS which is just like your climate in UK, compared to Tuscon AZ.

A 1000 watt system in Seattle produces on average 2.6 Kwh /day
Tuscon = 5.4 Kwh/day

That is over 100% difference. Location means everything.

[dogsbody]

With the crap summer we have had here, I would love to be in Tucson. The last time I was there ToysrUs was open 24 hours a day,just in case at 4.00 am you want to buy some toys!

[praseto]

Worried by this. Tested yesterday my single 80 watt panel on my boat to test out the temperature probe with my multimeter. In hazy sun, the ambient was 21 degree C, and the panel temperature was 46 degrees C. So in very aaverage Uk temperatures, the panels temperature is more than twice the air temperature.
What happens in the desert when the air temperature is 40 degrees C, and what effect on the outputs on panels in that situation.
Luckily in the UK the design panel temperature for air at 25 degrees is rarely exceeded, so I dont foresee problems here..........but what happens in hotter climes?

The panel is going to accumulate more temperature to a point. After that is going to get cooled from the cooler air surrounding it. That is the reason in string sizing calculator tool you have a question about where you panel is located (roof mount or dif) so they include that factor in. The roof here in Florida gets easy to 160F (71C) and you will lose a lot if you don