# Please check my understanding: # of panels and Series/Parallel for 24v system

Solar Expert Posts: 199 ✭✭✭
I think I want to go with a 24v system.

If I use the Kyocera KD315GX with the following specs:

Maximum power rating: 39.8 volts at 7.92 amps.

Specifications:
Pmp = 315 Watts
Vmp = 39.8 Volts DC
Imp = 7.92 Amps DC
Voc = 49.2 Volts DC
Isc = 8.5 Amps DC
Ptolerance = -3/+5%
Weight = 60.6 lbs.
Dimensions = 65.43" x 51.97" x 1.8"

Does this mean that I would need to wire all of these panels in parallel for a 24v bank, and just add up the amperages?

Not sure I understand what Voc vs Vmp are and how they factor in when connected to an MPPT controller. I know that I want the operating voltage of the panels to be as close to the charging voltage for the bank to minimize losses, as well as keeping the wire runs as short as possible.

I still have not yet decided how many AH I need at 24v, but I am convinced that 24V is the way I want to go. So, if I understand this right and I would connect these in parallel to the charge controller, once I know how many AH I need (and how much I need to harvest) I can just add additional panels to reach the desired charging amperage for the bank that I select?

• Banned Posts: 17,615 ✭✭
Re: Please check my understanding: # of panels and Series/Parallel for 24v system

You have an unusual situation with those panels: the Vmp is actually higher than normal for a 24 Volt system. So you could connect them in parallel on either a PWM type controller (and lose a little bit of power from each) or an MPPT controller. They could also go in series on an MPPT controller as it will down-convert the higher Voltage. The PWM would not.

So, all in parallel on a PWM controller the maximum current would be the Imp * the number of panels.
With the MPPT in any configuration it will be the Watts of the array * the efficiency / Voltage.

Now about putting them in series on an MPPT controller. Normally you'd go no more than 2X the "expected" system Vmp for the array. In this case a 24 Volt system would "expect" a Vmp of 35, so you wouldn't want to go above 70 Vmp for the array. At 39.8 these panels are close enough that two in series would not cause any drastic reduction in conversion efficiency.

Then there is the Voc consideration. When using an MPPT controller you do not want the Voc of the array to exceed the controller's input maximum, including adjustment for cold panels. In this case the Voc is 49.2 so two in series would be 98.4. Worst-case cold condition around 127 Volts, which is still below most MPPT controller's 150 Volt maximum.

So in short, depending on your wire runs from the panels to the controller and the total number involved, you could use these either way on an MPPT controller for a 24 Volt system without worry. I did this explanation of configurations as an example a while back: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller

24 Volts is a good system Voltage: it reduces the current needed for a given Wattage over 12 Volt and does not have some of the configuration/Voltage handling difficulties of a 48 Volt system.
• Solar Expert Posts: 199 ✭✭✭
Re: Please check my understanding: # of panels and Series/Parallel for 24v system

Cool.. Since I can use either controller type in this scenario, would
Ld you go mppt or pwm? Would the cost difference be worth it?
• Banned Posts: 17,615 ✭✭
Re: Please check my understanding: # of panels and Series/Parallel for 24v system
2twisty wrote: »
Cool.. Since I can use either controller type in this scenario, would
Ld you go mppt or pwm? Would the cost difference be worth it?

You can't really determine that without knowing the total size. The difference in price of the controllers is hundreds of dollars (TriStar 45 PWM version \$150, MPPT version \$400 for example). Obviously it wouldn't be worth it for one panel. At ~8 Imp per panel, you'd be at five panels before the current would be in the 40 Amp range (as per the cited controller) so let's look at six (1575 Watts):

5 * 7.92 = 39 Amps on PWM
1575 * .77 / 24 = 50 Amps on MPPT (approximately)

Now let's look at two (630 Watts):

2 * 7.92 = 15 Amps PWM
630 * .77 / 24 = 20 Amps on MPPT (approximately)

This is actually a bigger gain percentage-wise, but is 5 Amps worth \$300 to you? Is 11?

Other considerations include your wiring distance and how much control/logging/functions you want/need.
• Solar Expert Posts: 199 ✭✭✭
Re: Please check my understanding: # of panels and Series/Parallel for 24v system

So... are you suggesting parallel strings of 2 panels in series connected to mppt controller? That should be well below the 60a limit and well below the input voltage limit.

By going all parallel and pwm, it seems that I have to protect each panel, and due to amperage issues, even with a 60a pwm, I would be limited to 7 panels max...

Right?
• Solar Expert Posts: 199 ✭✭✭
Re: Please check my understanding: # of panels and Series/Parallel for 24v system

....and if I'm gonna go mppt, does it make sense to get one of the controllers that can handle 200v in and put 3 in series?
• Banned Posts: 17,615 ✭✭
Re: Please check my understanding: # of panels and Series/Parallel for 24v system
2twisty wrote: »
So... are you suggesting parallel strings of 2 panels in series connected to mppt controller? That should be well below the 60a limit and well below the input voltage limit.

On a charge controller the current rating refers to the output. So a 60 Amp MPPT controller on 24 Volts can basically handle 1900 Watts of array or six of these panels. With a 60 Amp PWM controller this would be seven, eight at most of these panels (60 / 7.92 = 7.5). So with the MPPT you get the same output current, but save buying two panels.
By going all parallel and pwm, it seems that I have to protect each panel, and due to amperage issues, even with a 60a pwm, I would be limited to 7 panels max...

Right?

Yep. All those parallel connections need to be protected by one fuse/breaker each. More wires to combine.
Six panels on the MPPT would require three parallel strings of two in series, so you'd need three fuses/breakers. You could do it the other way around but then your array Voltage would be very high and the Voc would be so close to 150 Volts you'd need a MidNite Classic 200 to be sure. You would also be moving away from an efficient conversion factor.
• Solar Expert Posts: 199 ✭✭✭
Re: Please check my understanding: # of panels and Series/Parallel for 24v system

So, as an exercise, if I selected 4 of the 390ah 6v crown batts, assuming a 50% DOD with no reserve, the a available power is 4680wh. Using those Kyocera panels, how many would I need with an avg sun hours of 5.1 to fully charge in one 5.1-hour period?
• Banned Posts: 17,615 ✭✭
Re: Please check my understanding: # of panels and Series/Parallel for 24v system
2twisty wrote: »
So, as an exercise, if I selected 4 of the 390ah 6v crown batts, assuming a 50% DOD with no reserve, the a available power is 4680wh. Using those Kyocera panels, how many would I need with an avg sun hours of 5.1 to fully charge in one 5.1-hour period?

Gosh I wish it were that simple.
Unfortunately it isn't just "used 200 Amp hours, charge rate 40 Amps, takes 5 hours to refill batteries". Darn Peukart anyway! Usually batteries need about 20% more put in than you'll get back out.

Let's say 390 Amp hours @ 24 Volts for the battery bank and figure what size array would normally be used to recharge it.
Ordinarily, then: 39 Amps * 24 Volts / 0.77 = 1215 Watts

Now here's how we check it:
390 Amp hours @ 50% DOD is 195 Amp hours @ 24 Volts = 4,680 Watt hours.
1215 Watt array @ 77% efficiency (not end-to-end here as we're talking only about the DC side at this time) over 5.1 hours = 4771 Watt hours.
Looks close but add the 20% factor for the Peukart effect and suddenly you need to generate 5616 Watt hours and need a 1430 Watt array.

That would work save one little problem: loads drawn while the batteries are charging. It also presumes a good day every day. But it is close.
Also remember that DC Watt hours at the battery are not AC Watt hours for loads: the inverter uses some power and some is lost in the conversion.
• Solar Expert Posts: 199 ✭✭✭
Re: Please check my understanding: # of panels and Series/Parallel for 24v system

Ok.... What happens if you have too much panel? Using the above example, if I were to have the same battery but say....12 panels. What happens when the batts are full? Where does the power generated go?
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Please check my understanding: # of panels and Series/Parallel for 24v system
2twisty wrote: »
Ok.... What happens if you have too much panel? Using the above example, if I were to have the same battery but say....12 panels. What happens when the batts are full? Where does the power generated go?

Well, step 1 is that the Charge Controller will not send any more current to the batteries.
Step 2 is that the CC will not draw any current from the panels.
Step 3 is that, just as if nothing were connected to the panels at all, the incoming solar energy that would have been converted to electrical energy is just converted to heat in the panel instead. No harm to the panel, and no need for somewhere else for the energy to go.

If your power source is a wind or hydro turbine, on the other hand, you need a different type of controller (shunt or dump-load controller) to keep the turbine from spinning too fast by continuing to draw current and wasting it in heating a resistance.
SMA SB 3000, old BP panels.
• Banned Posts: 17,615 ✭✭
Re: Please check my understanding: # of panels and Series/Parallel for 24v system

Yup: no connection to the panels = no electrical power produced.

But there is a caveat: if the controller is capable of passing all the current the array can produce and the battery is low while the panels are fully energized it can feed "too much" current to the batteries.

As in:
80 Amp charge controller with 2500 Watts of panel can produce 80 Amps @ 24 Volts. If the batteries are 220 Amp hours and low they can get 80 Amps put to them; a 36% charge rate.

You can program some controllers to limit current below the maximum capacity and then put the most array on so that when weather is less than ideal you could still get the charging you want.

Rather expensive way to do it, though.