Need Help Calculating Solar Panel System

Batteries supply the load, panels recharge the batteries.
To get 2400 Watt hours DC from 12 Volts you need 200 Amp hours used, or a minimum 400 Amp hour battery bank. This would be two parallel strings of GC2's for example.

Then to recharge that much the short cut is to look at the peak charging current of 10% of capacity, or 40 Amps. On a 12 Volt system with a PWM controller that's roughly 40 * 17.5 or 700 Watts. Note that the current is equivalent to the
combined Imp of the panels regardless of what Voltage they are. It is slightly different if an MPPT controller is used.

The Trojan T105 is often used in RE applications. Very good battery with only two drawbacks: it's expensive and its charging Voltage requirements can be too high for some controllers (14.8 Volts on a 12 Volt system) and inverters (they will high Voltage Fault under certain conditions).

Let's look at 12 Volts 450 Amp hours (four T105's). You want 45 Amps as the peak current at the minimum 12 Volts. So 45 * 12 / 0.77 (typical panel & controller derating) = 701 Watts. 700 would do, or you could round up if that fits the available panel selection better (for example to keep the number of panels even if that makes the array easier to plan).

On PWM it would be 45 * 17.5 Vmp = 787.5 Watts. As you can see there's about 11% advantage to the MPPT providing the panels with those specs are used. If you are looking at particular panels note that the Imp rating needs to add up to around 45 regardless of the Watts or Vmp. For example some 100 Watt panels have Vmp 18.9 and Imp 5.3: you'd need nine such panels in parallel to provide the 45 Amps. Put the same panels on an MPPT controller and you'd only need seven!

No. This is indeed the problem. Without having that 14.8 Volt Absorb the Trojans will never be fully charged and this decreases lifespan. You need to program the controller to the specs the batteries require. If this is not possible you either change the batteries or the controller.

Yes, for MPPT type controllers.

Those would be Renogy 100 Watt panels. I was just discussing these with someone else as a matter of fact. They will actually work better on MPPT than on PWM due to the unusually high (but still within range) Vmp of 18.9.

On a PWM controller one of these would produce its Imp of 5.3
On an MPPT controller one of these would produce 6.4 Amps. A 20% increase in current.
(NOTE: For comparison of performance of the two controller types for the panel under controlled conditions, not an actual measured output.)

These other panels will not match with the others in any configuration. Two of the 18.9 Vmp in series is 37.8 Volts, far above the 31.4 Vmp of the others. In fact it is over the Voc of the others meaning there would be significant power loss as the second panel type would have its Voltage 'pulled above' its Vmp by the first ones. And the current is too different between the two to run one each in series. A bad match. The only way to use these in combination without power loss is on separate controllers.

Crown and East Penn/Deka/US Battery as well as other makers have similar GC2's which do not require the high Absorb Voltage that Trojan does. They are also usually cheaper but just as good quality. Availability where you are is a matter you'll have to sort out yourself.

I feel your pain. I live in Canada and look at how cheap this stuff is in the USA ...

Yes, an MPPT controller can adjust the panels' 30 Vmp to proper Voltage for the system and turn the 'extra' Voltage into additional current. But of course they are not cheap. Nor will they adjust for panels that are incompatible with each other in the same array (a frequent mistake people make).

I would avoid it. There's better ones out there. MidNite makes the best available at the moment, although they are more expensive than Morningstar. But MS needs a separate meter to match features so that's $500 for the controller + $96 for the meter and suddenly $610 for the MidNite (which is also larger capacity) doesn't look so bad. Probably the best bargain in a 60 Amp MPPT right now is the Outback FM60 @ $500 with built-in display. Prices and availability where you are will affect the choice of course.

Frankly Morningstar needs to get its act together and update their controllers.

Actually they do. Which ones depend on where in Canada you are. We have GT here in British Columbia, but the utility power is so cheap the payback time is never. Tax credit? No. They even put the provincial sales tax back on all solar stuff the last time they played with taxes. We don't have much in outages here, but other parts of the country do. It's a big country; second largest in the world.

They don't make the 250 anymore, and you probably don't need it. You would not put all six panels in series. Not only does it put the Voc in 'danger land' it also decreases controller efficiency having the array Vmp that much higher than system Voltage.

Six 260 Watt panels is 1560 Watts, so the output would be (1560 * 0.77 / 12) 100 Amps. The Classic can not handle that much current, but will do about 96 Amps on 12 Volts. That is a lot of power - perhaps more than you need? It would be better on a 24 Volt system: same power, double the Voltage, half the current.

Cariboocoot wrote: »

Yes, the 12 Volt limitation of the existing inverter is an issue.

But those panels will work with a 12 Volt system through the Classic. At 30 Vmp the most efficient configuration is all in parallel. That means there is a need for combiner box & breakers/fuses for every panel.

Do you really need six 260 Watt panels? I can't remember what battery bank size we worked out. Think it was 450 Amp hours @ 12 Volts to provide up to 2400 Watt hours? Based on Trojan T105's which is what you ca get.

That would require 45 Amps of peak current which only needs about 700 Watts of PV. If you used four 240 Watt panels you'd get 61 Amps @ 12 Volts and 'max out' an FM60. Cheaper, meets the power needs by far (13% peak charge rate) and you could wire the array as two parallel strings of two in series (slightly less efficient due to the 60-ish Vmp but saves combiner box and fuses/breakers on array).

Plowman wrote: »

You don't need a breaker between the combiner box and charge controller, the individual panel breakers are sufficient.

BB. wrote: »

How long is the distance between array and the charge controller--And what AWG wire are you planning one.

Also, the same question between charge controller and battery bank wiring.

And you have a choice... 2 panels in series and a 2/4/6 panel array. Higher voltage array, smaller AWG wire needed from array to controller, and fewer circuit breakers in the combiner box.

If you are commited to 5 panels, then your wiring is the only choice.

-Bill

Cariboocoot wrote: »

The efficiency of an MPPT controller goes down as the array Voltage moves higher in relation to the system Voltage. It may take 150 Volts in, but if it has to convert that to 15 Volts charging then a lot of work has to be done to make the conversion. In that work power is lost to heat in the controller. We're not talking massive amounts, but sometimes enough to make a difference. In general you try to balance this with overcoming resistance loss in the wiring between array and controller.

Your diagram for the MPPT array is correct: two parallel strings of two in series. Array will produce 2X the Vmp and 2X the Imp of one panel.

Your diagram for the PWM controller is also correct, except that there should be fuses or breakers on each panel rated for their "maximum series fuse" current or if not listed 1.56 * Isc rounded up. Probably 15 Amps. This is to prevent problems should one panel short and receive the full current of the remaining panels.

You are correct that the PWM controller can not utilize panels in series without significant power loss. With the "12 Volt" panels on a 12 Volt system half the panels would be doing nothing for you.

30 cm from array to controller is unrealistically short. That would put the controller outside in the weather. With an output of <27 Amps @ 12 Volts 10 AWG (2.5 mm diameter) would allow a distance up to 2 metres. You will find 2 AWG or 1/0 impossible to fit in the connectors.