# Another Newbie trying to build his own system.....

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• Banned Posts: 17,615 ✭✭
Re: Another Newbie trying to build his own system.....
Hamfast wrote: »
Here are the numbers from the link provided here...http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/sum2/state.html

At a fixed 45deg angle, averaging the 12 month irradiance numbers (4.81) then dividing 4733WH by 4.81 yields an array of 984W.

An average of the lowest 6 months (much better I'm thinking) yields an array of 1364W.

10% battery bank (370Ah) - 37x48V=1776W x .77 derating yields an array of 1368W.

Wow! Pretty wild how those last 2 calcs work out. I don't think that's an accident. I'd feel very good about the 1440W array as I've proposed if not for Eric's observation regarding charging amps. Four strings of these panels provides ~20.5 amps to the CC.

Do MPPT controlers compensate for under-amperage from the array?

You did the derating backwards. If you want 37 Amps on a 48 Volt battery it's 1776 Watts before the derating loss, so that's 1776 / 0.77 = 2306 Watt array.

The MPPT controllers take the input power as "raw Watts" and determine an output current according to the battery Voltage. So whereas a PWM controller can only pass whatever current is available regardless of the Voltage, for the MPPT it is more like derated array Watts / system Voltage. In this case for a 1440 Watt array on 48 Volts:
1440 * 0.77 = 1108 / 48 = 23 Amps peak potential charge current.
That is not absolute as conditions vary and the MPPT controller may squeeze a bit more out if Voltage is available.
• Solar Expert Posts: 44
Re: Another Newbie trying to build his own system.....

Spank me!
That's alot of panels. 14 of the 180s will still only give my ~35.5W to the system. Also a lot of real estate.
In looking at other panels, I've noticed that the higher powered ones that I've been looking at (235 - 250W) like to be in strings of 3 on this CC, according to Xantrex's string sizing tool. Looking at several panels in this range I keep coming up with 12 panels in strings of three (round up ). The Astronergies (240W) in this config would provide 46 - 52 Amps to the system.

Off Grid System; 2175W array; 370ah @ 48v (8 L16 Scrubbers in series); XW MPPT60-150 CC; OutBack VFX3648, FlexNet and Mate

• Banned Posts: 17,615 ✭✭
Re: Another Newbie trying to build his own system.....

Well ... 14 * 180 = 2520 * 0.77 = 1940 / 48 = 40 Amps peak charge current.
12 * 240 = 2880 * 0.77 = 2217 / 48 = 46 Amps peak charge current.

Rough math there; not precise calculations.
But either would be more than the 37 Amps target for a 370 Amp hour battery bank.

Keep in mind that larger Watt panels are physically bigger, not significantly more efficient and that Voc becomes very important when you start putting panels in strings. You have to take the panel Voc and multiply it by the number of panels in the string and then factor the cold weather potential into it. The result must not exceed the charge controller's input max.

Fortunately most people don't live in a place where the Voc can go to 1.3 times the rating. Someplace like, say, the Cariboo. :roll:
• Solar Expert Posts: 44
Re: Another Newbie trying to build his own system.....

Thanks Coot.

I just multiplied the Amps of off the back of the panels by 7 ; using formulas for every step is still a bit new..

In the two examples you noted below, the results are still below the total 2306W.
You did the derating backwards. If you want 37 Amps on a 48 Volt battery it's 1776 Watts before the derating loss, so that's 1776 / 0.77 = 2306 Watt array.

When sizing the array, do total Watts (after derating) AND required Amps need to be satisfied?

Will either of these 2 arrays be adequate for charging the battery bank? Or do more panels need to be added until 2306W is reached?

I've noticed that the more powerful panels are bigger. Just trying to figure out the smallest footprint. 8)

Off Grid System; 2175W array; 370ah @ 48v (8 L16 Scrubbers in series); XW MPPT60-150 CC; OutBack VFX3648, FlexNet and Mate

• Banned Posts: 17,615 ✭✭
Re: Another Newbie trying to build his own system.....

Watts are a function of Amps: Amps * Volts = Watts (Power Factor excluded)

The derating affects the Watts, but since panels are a current source it doesn't have much bearing on the Amps. Rather it is the Volts that suffer from the effects of panel heating and the resistance in wiring & circuitry. This drops the over-all power as rated in Watts. A PWM type controller would pass the current available, but might suffer from too low a Voltage for proper charging. An MPPT controller will usually find the right power point as it can adjust Voltage and current to get the right charging based on the power available from the panels, again providing the Voltage is high enough to charge. Since arrays of this size nearly always use MPPT controllers and are usually arranged so that the panel nominal Voltage is higher than system Voltage so all is well.

Uh, a 2520 Watt array and a 2880 Watt array are both larger than the minimum required 2306 Watt array.
The smaller array @ 48 Amps peak is approaching the 13% upper limit usually recommended for a 370 Amp hour battery bank.

You could actually use twelve 180 Watt panels: 12 * 180 = 2160 * 0.77 = 1663 / 48 = 34 Amps peak current potential.
34 * 100 / 370 = 9% charge rate, which should work fine. (That formula is how you find percentages "backwards" btw.)
• Solar Expert Posts: 44
Re: Another Newbie trying to build his own system.....

Uh, I think you've noticed my head is spinning again...

Thanks a bunch, Mr Coot...8)

Off Grid System; 2175W array; 370ah @ 48v (8 L16 Scrubbers in series); XW MPPT60-150 CC; OutBack VFX3648, FlexNet and Mate