Sizing each string

[calebc]

Need a little education on sizing each string. I bought a kit that has two stings of three panels each. How do you determine whats the best string size given a number of panels you have? In my case, two strings of three or three strings of two, or why not a single string of all size panels? Also, for future growth, I ASSUME the strings need to be the same size? Is this the same size in terms of watts produced, amps, number of panels, etc? I've tried searching, but haven't found what I'm looking for (I'm sure it's out there already, so apologize for what's probably a common question).

The "kit" I bought is 6 x 265 Kyocera panels, an Outback Flexpower unit, a combiner, and 4 batteries (and related cables and mounts for all this). I have the capacity with my Outback unit for growth, but it's easier to add a new string of two than it is a new string of three. Just trying to make sense of the benefits of doing three. Or for future growth, could I just add a panel to each string bringing it to two strings of 4?

[NEOH]

Strings in parallel need to be identical.
You need to size your string ( number of panels in each string ) to meet certain upper / lower limits per the Outback Flexpower unit.
Did you download and run the Outback String Sizing Tool?
Did you watch the Outback String Sizing Tool video?

[littleharbor]

Battery size? System nominal voltage? If 24 volt you can go 2x3 or 3x2. If 48 volt you need to go 3s2p.

[calebc]

Thanks guys (I know I'm slow to respond ). I have 4 Rolls Surrette S-550 batteries (6v 428 Amp hour @ 20hour rate). The system nominal voltage is 24v. So it sounds like I could do 2x3 or 3x2. However, how do you determine that? I tried the Outback string sizing tool but it doesn't have my panels and when I entered the specs of my panels manually, it says anything beyond 2x3 or 3x2 and the array wattage is more than the charge controller output (charge controller is a FlexMax 80). So is the FlexMax 80 really maxed out at 6 panels?

More than anything I'd like to understand HOW to figure this stuff out. My setup, as it is today, is more than I will need for my cabin, but I'd like to understand this stuff better

[J.P.M.]

More than anything I'd like to understand HOW to figure this stuff out. My setup, as it is today, is more than I will need for my cabin, but I'd like to understand this stuff better

To understand the basics better, there are many sources available. If the stuff you're referring to is an understanding of solar cell basics, one of the best primers for PV cells I know of is chapter 23 of Duffie & Beckman's, "Solar Engineering of Thermal Processes", even if it is a bit dated. 25 pages of meat & business on the basics of how PV cells work.

[sensij]

Thanks guys (I know I'm slow to respond ). I have 4 Rolls Surrette S-550 batteries (6v 428 Amp hour @ 20hour rate). The system nominal voltage is 24v. So it sounds like I could do 2x3 or 3x2. However, how do you determine that? I tried the Outback string sizing tool but it doesn't have my panels and when I entered the specs of my panels manually, it says anything beyond 2x3 or 3x2 and the array wattage is more than the charge controller output (charge controller is a FlexMax 80). So is the FlexMax 80 really maxed out at 6 panels?

More than anything I'd like to understand HOW to figure this stuff out. My setup, as it is today, is more than I will need for my cabin, but I'd like to understand this stuff better

The constraints you need to worry about most are the maximum input voltage and the maximum short circuit current.

Panels in series increase voltage. The controller will have a maximum allowed input voltage that needs to be evaluated at open circuit (Voc) in the coldest conditions the panels will see (there are places to look up good minimum design temperatures). You probably can't go more than 3 panels in series without exceeding the 150 Voc max input.

Panels or strings in parallel increase current. (Aside: once you have more than two strings, you need to fuse each one individually). Since input voltage is higher than output voltage, that means input current is less than output current. You will run into the output current limit before you run into an input current limit.

When it says you have more array wattage than the charge controller can output, it should be a warning, but still a valid design. Most of the time, the array won't be producing at maximum output. Therefore, most of the time, the extra array wattage is productively contributing to charge controller output. Under optimal conditions, if your array wattage is big enough, the charge controller would exceed it maximum allowed output of it converter all of the available power. What decent charge controllers like the Outback do in those conditions is force the array to produce something less than the maximum it is capable of, so that it can stay within the output limit. With that in mind, you could do 3x3, for example, with proper fusing, and most of the time all of the panels will be operating at the maximum. However, on the best days, you would artificially cap the output to stay at the 80 A rating.

[calebc]

Awesome, thanks. This helps a lot. The more I learn the more I see that some things (sun insolation for example) are spec'd on worse possible conditions (December generally) while other things are spec'd for best possible conditions so as not to oversize it. Confusing but it's starting to make more and more sense.

[inetdog]

Awesome, thanks. This helps a lot. The more I learn the more I see that some things (sun insolation for example) are spec'd on worse possible conditions (December generally) while other things are spec'd for best possible conditions so as not to oversize it. Confusing but it's starting to make more and more sense.

Some things. like battery charge maintenance, require you to consider worst case (typically winter) conditions to design a conservative system that will not require a lot of generator use.
Other situations, typically involving net metering, allow you to use the grid as an infinite battery and size your system based on the total annual use and total annual production.

Any device, either CC or GTI, with an MPPT input can typically withstand a large amount of excess power from the panels, which may allow you to get maximum production over a larger time window each day.
A device with PWM control is likely to be damaged by too much extra panel power and will typically have specifications limiting the allowed input power (current).