Solar math makes my head hurt

[spudbudy]

Hello, fellow beginners and solar guru's alike. I'm just beginning to plan out my solar system and for some reason my ability to do simple math
is escaping me. The panels are tentative to be 160 watt X 12 for a system total of 1920 watts if my math work. Now here is where it goes south for whatever reason
the numbers don't seem to add up. Most likely I'm missing or skipping a step or 2.

160 watts
VOC 19.83
ISC 9.75
VMP 16.77
IMP 9.83
No panel fuse info
Max system Voltage 1000
Power Tolerance =-3%
I don't have any other temp info other then + or - some %

Now I'm trying to figure out the best configuration for these panels. I plan on using a Magnum Energy PT100 solar charge controller.
Going into a 400AH Lithium battery bank out to a Magnum Energy MSH3012RV to power both 50 amp legs in the fifth wheel.
I plan on using a 12 volt system. My numbers seem to be on the low side so I'm unsure what I'm doing wrong, this is what I get?
Possible configurations

2S/6P =39.66 Volts and 58.5 Amps
3S/4P= 59.49 Volts and 39 Amps
4S/3P= 79.32 Volts and 29.25 Amps
6S/2P= 118.98 Volts and 19.50 Amps
12S = 237.96 Volts and 9.75 Amps

Would someone kindly either point me to a calculator or explain to me what I'm doing wrong?
I've watched many video's on hooking up solar panels, so I know I'm missing something with the temperature affecting Volts and Amps.
Any help would be very much appreciated. My biggest fear is doing the math wrong and either overloading wires or components and starting a fire.
Plus, I would prefer to buy the right parts the first time and not have to replace undersized components. The Sunshine may be free but the parts are expensive.

Thank you to all of the DIYsolarforum groups for your assistance

 

[MisterSandals]

I plan on using a Magnum Energy PT100 solar charge controller.

Thats the PWM and not MPPT version?

Looking for the input requirements/limits.

 

[hankcurt]

From the data sheet:

The PT-100 is a Maximum Power Point Tracker (MPPT) charge controller
ELECTRICAL SPECIFICATIONS
Maximum PV input voltage (any condition) 200 VDC + battery voltage or 240 VDC - whichever is lower
Maximum PV operating voltage 187 VDC
Maximum PV array short circuit current 100 ADC
Nominal battery voltage range 12, 24, or 48 VDC
Battery charger output voltage range 10 to 66 VDC
Continuous charger output current 100 ADC (from -20° C to +40° C) with proportional power reduction up to 60° C ambient
Maximum output power 6600 watts
Efficiency 99% typical
Tare loss / nighttime power consumption <4 watts (fan off, display/LEDs off)

 

[hankcurt]

@spudbudy
Are you sure you have the panel specifications quoted correctly. I don't think that the short circuit current ISC should be less than the maximum power point current IMP. Also, the + and - % numbers are important, along with your expected operating temperature to make sure your solar panel output is within your charge controller limits.

Do you have the panel model number handy?

 

[Dzl]

Okay with the disclaimer (and admission) that I only skimmed your post (sorry) the basics of math are:

when you connect things in series, voltage increases
when you connect things in parallel, current increases

when you want to calculate charge controller size use Voc and Isc
when you want to calculate power use Vmp and Imp

So say you have panel with a Voc of 20V and a Isc of 10A
To figure out the max current and max voltage of the array:

12S =
Voc x 12, Isc x 1 = 240V, 10A

12P =
Voc x 1, Isc x 12 = 20V, 120A

6S2P =
Voc x 6, Isc x 2 = 120V, 20A

Then with Voc you need to account for low temperatures as well. You can use this table to figure out the factor to multiply by.

These figures (Voc and Isc) don't give you the accurate power output, even in ideal conditions, but they are the figures to use when sizing the charge controller, etc.

 

[spudbudy]

The solar panels I plan on using are
Newpowa NPA160S-12H
Our plan is to go fulltime, Max temp could be 100 or close and the expected low temp 20"s possible teens.
PMAX=160W
VMP=16.77V
IMP=9.30A
VOC=19.83V
ISC=9.75A
Temp coefficient of VOC -(80=/-10)mV/C
Temp coefficient of ISC -(0.5=/-0.05)%/C
NOCT 47+-2C
Operating temp -40 C to 85 C
Max system voltage 1000V
Power tolerance +/- 3%

 

[Hedges]

"expected low temp 20"s possible teens."
Look up record low temperature for your location (or where you will travel) and use that. If you guess wrong, it burns up the charge controller.

Voc 19.83V at 25 degrees C
I use -15 C for my location, so 40 degrees below nominal.
-80 mV x -40 degrees = 3.2V
19.83 + 3 = 23.03Voc at -15 degrees C.

I think your charge controller's spec says:
"Maximum PV input voltage (any condition) 200 VDC + battery voltage or 240 VDC - whichever is lower"

You can connect 8 panels in series for a charge controller rated 200Voc.
Maybe one more based on 12V battery, but I wouldn't do that - what if battery went dead, was zero volts?

You want to use 12, 160W panels. 6s2p, two strings of 6 in series would fit the voltage limit. And no worries at all about cold temperatures making voltage too high, no matter where you go.

"Output Current 100 amps"

160W x 12 = 1920W STC. (typically actual power is about 15% less in field conditions)

100 amps at 12V is 1200W. 100 amps at 15V is 1500W
Your 1920W array is somewhat over-paneled. Should be OK, just won't put more than 1200W to 1500W into battery.
Overpanelling will maintain full power longer during the day and off-season, when panels aren't aimed directly at the sun.

If you are able to tilt one string of 6 panels toward morning sun and the other string toward afternoon sun, then it won't clip output and power will be produced more hours. But, tilting that many panels may not be convenient.
If flat on the roof, they will be below peak output all times except summer, so rarely clipping.

 

[hankcurt]

Before going into the math on temperature coefficients for you solar panels, and deciding on possible arrangements for them, I wanted to point out the output current limitation on your charge controller.

Your charge controller output to the battery is limited to a maximum of 100A. So if you choose a 12 volt system, the most power you can get from the charge controller is 1200 watts (100a x 12v = 1200w). So no matter how many solar panels you add, you will only get 1200 watts. There's nothing wrong with over paneling your system, because the solar panels will rarely operate at perfect efficiency, but you should know that if you stick with a 12 volt system, 1200 watts is the power limit.

If you decide to use a 24 volt battery pack instead, your charge controller will be able to output 100a x 24v = 2400watts. This would let you use the full power from your panel output of 1920 watts under ideal conditions. So this is a decision you should make early in the process. It may affect how many panels you choose to install, or maybe even alter you choice of equipment.

 

[spudbudy]

"expected low temp 20"s possible teens."
Look up record low temperature for your location (or where you will travel) and use that. If you guess wrong, it burns up the charge controller.

Voc 19.83V at 25 degrees C
I use -15 C for my location, so 40 degrees below nominal.
-80 mV x -40 degrees = 3.2V
19.83 + 3 = 23.03Voc at -15 degrees C.

I think your charge controller's spec says:
"Maximum PV input voltage (any condition) 200 VDC + battery voltage or 240 VDC - whichever is lower"

You can connect 8 panels in series for a charge controller rated 200Voc.
Maybe one more based on 12V battery, but I wouldn't do that - what if battery went dead, was zero volts?

You want to use 12, 160W panels. 6s2p, two strings of 6 in series would fit the voltage limit. And no worries at all about cold temperatures making voltage too high, no matter where you go.

"Output Current 100 amps"

160W x 12 = 1920W STC. (typically actual power is about 15% less in field conditions)

100 amps at 12V is 1200W. 100 amps at 15V is 1500W
Your 1920W array is somewhat over-paneled. Should be OK, just won't put more than 1200W to 1500W into battery.
Overpanelling will maintain full power longer during the day and off-season, when panels aren't aimed directly at the sun.

If you are able to tilt one string of 6 panels toward morning sun and the other string toward afternoon sun, then it won't clip output and power will be produced more hours. But, tilting that many panels may not be convenient.
If flat on the roof, they will be below peak output all times except summer, so rarely clipping.

WOW, Now that's a detailed response. Temperature wise the coldest I've ever been is -26F and I no plans to camp anywhere it's that cold so let's say on the off chance down to 0F would be the absolute coldest. We plan on traveling across the country and wander into Canada and Alaska. But only during the warmer months.
I understand the overpaneling better now with the 100Amp limit. So it looks like I could go with 150W panels and drop it down somewhat.
And still charge with less overpaneling. Price is nearly identical. Watching some of Will's videos he says overpaneling is ok is there a rule of thumb as to how much over you should go. I certainly do not wish to throw away any power.

This is alot to take in as I'm still learning as I go. With the help of everyone here I am confident I will have a 100% safe and effective solar setup.

 

[spudbudy]

Before going into the math on temperature coefficients for you solar panels, and deciding on possible arrangements for them, I wanted to point out the output current limitation on your charge controller.

Your charge controller output to the battery is limited to a maximum of 100A. So if you choose a 12 volt system, the most power you can get from the charge controller is 1200 watts (100a x 12v = 1200w). So no matter how many solar panels you add, you will only get 1200 watts. There's nothing wrong with over paneling your system, because the solar panels will rarely operate at perfect efficiency, but you should know that if you stick with a 12 volt system, 1200 watts is the power limit.

If you decide to use a 24 volt battery pack instead, your charge controller will be able to output 100a x 24v = 2400watts. This would let you use the full power from your panel output of 1920 watts under ideal conditions. So this is a decision you should make early in the process. It may affect how many panels you choose to install, or maybe even alter you choice of equipment.

I was trying to stick with 12 Volts but I see your point. I know I would need a 24 to 12 DC to DC added into the system. Change over the battery setup, What other items would be needed to convert over the system? Panel wise what configuration would be the best layout on 24 Volts?

I have not started to assemble the system yet, but I am trying to get all the correct information before ordering anything. All of this information is critical in making good sound decisions.

 

[Hedges]

Watching some of Will's videos he says overpaneling is ok is there a rule of thumb as to how much over you should go. I certainly do not wish to throw away any power.

Throwing away power is one of the most cost-effective things you can do. PV panels cost < $0.025/kWh (amortized over 10 years), a fraction of what batteries cost.

Enabling loads when batteries are full and orienting panels both SW and SE are two ways to reduce it.
Clipping production during summer noon because you put in too many panels means you don't under-produce morning, evening, winter.
Additional charge controllers (and batteries which can accept the current) also avoids missing out. But you pay full price for those components, vs. only missing out on PV generation part of the time (and the PV panels may cost less than the charge controller, especially for quality brands and lower voltage systems.)

By making a chart of cost vs. annual Wh, cost vs. lowest season Wh, you can engineer your financial decisions.

 

[paultn]

Correct me if i am wrong guys, but you can get the batteries setup in 24 volt by doing 2 -12v in series.
Then do Parallel rows of 2 batteries.

 

[spudbudy]

Throwing away power is one of the most cost-effective things you can do. PV panels cost < $0.025/kWh (amortized over 10 years), a fraction of what batteries cost.

Enabling loads when batteries are full and orienting panels both SW and SE are two ways to reduce it.
Clipping production during summer noon because you put in too many panels means you don't under-produce morning, evening, winter.
Additional charge controllers (and batteries which can accept the current) also avoids missing out. But you pay full price for those components, vs. only missing out on PV generation part of the time (and the PV panels may cost less than the charge controller, especially for quality brands and lower voltage systems.)

By making a chart of cost vs. annual Wh, cost vs. lowest season Wh, you can engineer your financial decisions.

I currently have 400AH any I could certainly add 200AH additional. Looking into it the solar charge controller is ok but, I would need to change out the inverter/charger to a 24 Volt model. This most likely will not what direction wanted to go in. The inverter/charger I have picked out will send power to both 50 Amp lines in the fifth wheel, so I don't need to have 2 inverters. I will look into the numbers some more and try to come to a consensus of the right amount of solar panels.

 

[MisterSandals]

Correct me if i am wrong guys, but you can get the batteries setup in 24 volt by doing 2 -12v in series.
Then do Parallel rows of 2 batteries.

Yes, 2 12v batteries in series creates a 24v battery bank.

The OP did not say (i did not see anyway) how many batteries he has or intends to have. But yes, paralleling the 24v banks keeps the voltage the same (the AH are summed).

I think the OP prefers 12v (i know i do!) because he/she probably has 12v appliances/lights/chargers and does not want to replace some of these and convert from 24v to 12v for the rest. If this is for an RV, a LOT of this stuff is built in at 12v and swapping to 24v is not easy or cheap.

 

[chrisski]

You will also need to balance the amps out of the CC to the battery bank, I have four six volt golf cart batteries 458 ah at the 20 hour rate Flooded lead acid battery bank with a max charge rate of 13% of the 20 hour rate, so I am limited to 60 amps of charging. I have a 1000 watts of panels which if fully used, I would be pushing 68 amps at full the aborb voltage of 14.7, in theory. However, the max amps I've seen pushed to my batteries are 45. If I only had two golf cart batteries with the same panels, I would exceed the max amperage rate.

I only bring this up, because if you are planning on the less expensive FLA batteries, you'll have the battery amp charging limit if you intend to use the full 100 amps of solar power. Too much could ruin the batteries.

Batteries other than FLA can be charged with more amps so that 13% is a limit for my particular batteries.

 

[Hedges]

So maybe keep a small 12V AGM for 12V appliances, charged by 24/12V step-down from 24V system that powers heavy AC loads.

 

[hankcurt]

Throwing away power is one of the most cost-effective things you can do. PV panels cost < $0.025/kWh (amortized over 10 years), a fraction of what batteries cost.

I'm in total agreement here. Solar panels are not the expensive part of a PV system anymore.

When deciding on panels, don't forget to include the complications and expense of mounting them. A lot of low output panels may not be a better deal than a few high efficiency panels once you include those costs.

 

[spudbudy]

So after re watching several different videos, I'm even more confused based on what seems to be the correct formula to use.
I will post what I've found and let's see where it goes from here

Watts / Volts = Amps So 1200 Watts / 12 Volts = 100 Amps No problem easy to follow

1200 Watts X 90% / 12 Volts = 90 Amps Makes sense since panels don't always 100%

Then I have seen this one, 1200 W / 14.4V = 83.3 A This makes some sense based on Lithium battery charging profiles

Now this is where the confusion comes in if I take 12 100 Watt panels place them in series using these numbers not taking into account
the temp just for this example
Voc=19.83 19.83 X 12 = 237.96 Voc or 12 in Parallel 19.83 X 1 = 19.83Voc Yet 1200W / 12V =100A
Isc=6.56 6.26 X 1 =6.26 Imp 6.26 X 12 =75.12Imp
Vmp=16.77
Imp=6.26
Why don't all of these add up to 100 Amps? Does the panel Amps not actually add up due to inefficiencies? I'm so confused.
Sooner or later a light will go off in my head, either due to over voltage or over amperage

 

[chrisski]

Try doing VMP for voltage instead of 12 volts. 12 volt panels don’t put out 12 volts, closer to 18 volts, give or take depending on the manufacturers.

Since it’s close to 18 volts, that will be a ratio and proportion converted to 12 volts:

75.12 amps. X
====. =. ========
18 volts. 12

Solve for x and that should be close to 100 amps.

 

[Dzl]

So after re watching several different videos, I'm even more confused based on what seems to be the correct formula to use.
I will post what I've found and let's see where it goes from here

Watts / Volts = Amps So 1200 Watts / 12 Volts = 100 Amps No problem easy to follow

1200 Watts X 90% / 12 Volts = 90 Amps Makes sense since panels don't always 100%

Then I have seen this one, 1200 W / 14.4V = 83.3 A This makes some sense based on Lithium battery charging profiles

Now this is where the confusion comes in if I take 12 100 Watt panels place them in series using these numbers not taking into account
the temp just for this example
Voc=19.83 19.83 X 12 = 237.96 Voc or 12 in Parallel 19.83 X 1 = 19.83Voc Yet 1200W / 12V =100A
Isc=6.56 6.26 X 1 =6.26 Imp 6.26 X 12 =75.12Imp
Vmp=16.77
Imp=6.26
Why don't all of these add up to 100 Amps? Does the panel Amps not actually add up due to inefficiencies? I'm so confused.
Sooner or later a light will go off in my head, either due to over voltage or over amperage

I'm not sure I understand 100% what you are struggling with or what exactly you are trying to illustrate above with the math (I'm getting lost part way through). Below is some general info, and an example that may or may not be helpful.

Think of the PV side and the Battery side of your charge controller as separate. The only value that connects them (in terms of the math) is Power (Watts). It is not useful or relevant to compare Amps on the two separate sides of the controller because without considering Volts, comparing Amps is misleading. The Charge controller translates / converts between the two sides.

Say you have 10 panels with a Imp of 10A and a Vmp of 20V.
10A x 20V x 10 panels = 2000W (regardless of how you configure them)

Say you configure in 10S, that means your array is:
Imp: 10A
Vmp: 200V
Pmax: 2000W

These specs are what the controller will see on the input (200V, 10A)

On the output side the current is determined by the battery voltage and the total (input) power.
The equation to get power is Watts = Amps x Volts. We know Volts and we Know Watts so we need to tweak the equation to solve for Amps. That looks like Amps = Watts / Volts.

We know the input power is 2000W and the output voltage is (for this example) 13V.

So 2000W / 13V = 154A

*Because battery voltage changes this number will change somewhat depending on what voltage you use in your math (12v, 13v, 14v, etc)
**this example ignores any inefficiencies
***when trying to get all the numbers to work out on both the input and output side of the equation, use Imp and Vmp (not Voc, or Isc, which are used for calculating maximums and for safety).