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Low Temperature String Sizing, Portrait vs. Landscape, Sheathing Mounting and Temperatures


Hi Sean: I guess I have two questions: One is regarding construction and the other regarding string sizing. First you mentioned that shading of long edge is more desirable than shading of the short edge. Would this suggest that landscape mode is in fact preferable? Does a product like from QuickMount PV allow you to attach just to the sheathing versus the rafters?
My other questions is on string sizing. Should we be accounting for a derating factor when potential modules. And, should we use the coldest day of the year, or the coldest hour of the year?
We didn't really cover formulas for minimum voltage, but I found this one: Vmin = Vmp + (Thigh + Trise - STCT) x (TCVmp x Vmp x .01) athttps://www.civicsolar.com/resource/inverter-string-design-calculation.
I take it that this is correct?


Question about shading the long vs. short edges:

Yes, there can be benefits to mounting landscape if shadows are expected to come up from the bottom and shade the long edge. The main reason installers mount portrait in most of the residential systems in the US is because the rafters come up and down and then you put the rails perpendicular to the rafters, which makes it secure and requires less rail to attach the rails to the long edges of the modules and the modules will end up portrait.

Solar City uses Zep railless systems which are mounted landscape.

Quickmount PV has a railless system that can be used portrait and landscape.

The bottom line is, don't put your system where there is going to be shade and you will not worry about the landscape benefits from shadows that come from below.

You can also have shadows that come from the side, which would give portrait mounted systems a benefit from long edge vs. short edge shading.

Question about Quickmount attaching to the sheathing:

I used to work with the inventor of Quickmount when his invention was his part-time job. He is very concerned about quality and would not attach to the sheathing. There are engineered systems that do attach to the sheathing and most installers will not use these types of systems.

String Sizing Questions:

I went to your link and the first thing I see is a system mounted on a tile roof without the 3 foot space to the ridge, so I figure it might not be a recent article.

For the high temperature low string size, I do it a little different and would usually come up with the same answer.

For high temperature short string sizing, it is not a safety issue and is not covered by the NEC, so there is no absolute right way of doing this calculation. Usually designers will not use the shortest string, since there are benefits to having higher voltage.

I usually explain the string sizing a bit, rather than expressing it as a formula, since people usually get lost in formulas when they are learning.

Here is what I need for short string sizing:

1-High temperature (can get from solarabcs)
2-Temp Coefficient Vmp (usually same as temp coef power)
4-Inverter low MPP voltage

First Step-

I find the delta T from 25C

For this, I need the cell temperature and will add about 30C to the high temperature. If we get this wrong, the worst that will happen is the system will not produce well on the hottest days if we do use the shortest string.

If our high design temp is 35C, then 35C + 30C = 65C

65C - 25C STC = 40C increase in temp, which is going to make our voltage decrease, not increase.

Second Step-

Find decrease in voltage and I like to do it in decimal instead of percent, so if my temperature coefficient of Vmp is -0.45%/C, then in my head I will convert it to a decimal and make it .0045.

Note: Most datasheets do not give the temperature coefficient of Vmp. When they do give the temperature coefficient of Vmp, it is the same as the temperature coefficient of power and not the same as the temperature coefficient of Voc.

I will multiply the temperature coefficient by the delta T

40C x .0045 = 0.09

In my head I check what this means. 0.09 is the decimal for 9% and I will have a 9% decrease in voltage, not a 9% increase in voltage.

If I lose 9%, then I will keep 91% and I can express this mathematically by:

1-0.09 = 0.91

Third Step-

Now to derate my Vmp for a hot day:

If my Vmp is 30V

30V x 0.91 = 27.3V

Fourth Step-

If my inverter low end MPP voltage is 200V, then to determine the least number of modules in series:

200V/27.3V= 7.32 in series to keep it over 200V

If I rounded down to 7 in series, it would turn off, so I will always round up and in this case use 8 in series.

We will go over this in more detail in the HeatSpring Advanced PV Course!

Question about what cold temperature to use:

I recommend using the ASHRAE Mean Extreme Minimum Design Dry Bulb Temperature at solarabcs.org. This was put together by Bill Brooks and funded by the DOE. Half of the years on record, it will get colder than this. It is safe, because if it does get colder, it will be night, or the sunlight will heat up the PV above ambient, or the PV will be at Voc, sending power to the grid, which is about 20% lower than Voc.


Sean White

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