How many panels and what voltage panels do I need?

Hello,

With my goal to build a 5khw 48V system, I am looking into using 2 of these 24V LiFePO4 Powerwalls with an MPP Solar 5000w 48vdc 120v/208V/240V Split phase Solar inverter 80A Dual MPPT charger. The reason I am considering 48V is because of lower cost and better efficiency. Since, I will be living in a cloudy, rainy, snowy area with 157 days of sun per year, I am looking into the Monocrystalline higher-efficiency panels (from 20% to 22%).

However maybe I am missing something here. I see panels like any appliance are rated at specific voltages. So my question. Does this mean I need 48V solar panels? If yes, are these a lot more expensive than 12v or 24v panels? And can anyone recommend a good deal?

Thanks.

No, the battery voltage and panel voltage are not related when using a MPPT controller. The requirement is that the voltage given to the MPPT controller from the solar array is higher than the battery voltage otherwise the controller will not be able to charge the battery (there are a few exceptions to this but this is 99% of the way it will be). The MPP unit has an acceptable range of voltages for its PV input. If you post the exact model of MPP you are considering we can tell you what sort of panel configuration you can use.

'24v' panels often have a best power voltage of around 36v. Put two in series and you have solved the problem of keeping array voltage higher than 48v.

Thanks. The LV5048 is the exact model I was considering for this 5kwh system.

OK, max voltage on the input is 145V, but operating range is 60-115V. For a '24 volt' class panel this means you might be able to get away with 3 in series, and then parallel sets of those to get up to the desired power level but this depends a lot on the particular panel.

Say it's this one (not a recommendation, just illustration). Voc of the panel is 44.5 so 44.5 x 3 = 133.5, below the max 145V. Vmp of the panel is 36.7 so 3 x 36.7 = 110 volts, below the max operating voltage so the unit won't stop charging in the middle of the day. That yields about 600 watts per set so if we want 2400 watts of peak production we put 2 sets of those series connected panels in parallel and then connect one array to each of the MPPs solar inputs. I'm assuming the unit has two MPPT controllers in it since it has 4 terminals to connect to, but it could just be crashing them together in parallel. I'm not sure about that.

Temperature also comes into play with how you connect the panels. In very low temperatures the voltage of the panels goes up. Good brands will tell you what the temperature / voltage co-efficient is so you can work out what the max voltage would be in all conditions. For the panel arrangement above there is about 10v to play with so it's quite possible that under the right circumstances Voc would exceed the ratings of the MPP and damage it.

Below are specs of some specs of 2 different panels I am looking at. What do you think of getting 6 of either of these panels and with three in a series, paralleling those two sets as you illustrated? If both work, is one set of specs better than the other for the LV5048 ?

Panel 1 Specifications:

• Rated Power: 300W
• Open circuit voltage (VOC): 39.85V
• Max power voltage (VMP): 32.8 V
• Short circuit current (ISC): 9.71 A
• Max power current: 9.16 A
• Maximum system voltage: 1000V IEC
• Fuse Rating: 15 A
• Junction Box with bypass diodes and standard solar connectors
• Weight: 41
• Dimensions: 65" x 39" x 1.5"

Panel 2 Specifications:

• Rated Power: 310W
• Open circuit voltage (VOC): 40.0 V
• Max power voltage (VMP): 32.6 V
• Short circuit current (ISC): 9.83 A
• Max power current: 9.52 A
• Maximum system voltage: 1000VDC
• Fuse Rating: 20 A
• IP67 Junction Box with bypass diodes and standard solar connectors
• Weight: 40.34
• Dimensions: 64.57" x 39.00" x 1.38"

• Open circuit voltage (VOC): 39.85V
• Max power voltage (VMP): 32.8 V

• Open circuit voltage (VOC): 40.0 V
• Max power voltage (VMP): 32.6 V

No appreciable difference but they do have lower voltages than the ebay cheapie so they are better in that they give you more temperature headroom (if low temps are possible in your area) so thumbs up to either. 3 in series and parallel those sets to get more power would be the way to go unless you have very low temperatures there.

If you do have temperature problems that force you to run two in series rather than 3 they'd be getting marginal as the MPP you have selected recommends 60+ volts for MPPT operation. Below that it probably goes into some sort of PWM mode, but someone more familiar with that model MPP would know more about that. There used to be a MPP seller on the forum, maybe that person will see this and comment.

Hi. I received this information about the panels I was considering below. Does this tell us anything about potential cold temperature issues?

NOCT 45°C(±2°C)

Voltage Temperature Coefficient -0.29%/K
Current Temperature Coefficient +0.05%/K
Power Temperature Coefficient -0.39%/K

Can't say I've ever seen kelvin used in that spec before but variety is the spice of life. The voltage on the label on the back of the panel is with the cells at 25c. If we drop air temperature to 0c and the cells have time to fall to there too, voltage rises by about 7.5%.

Open circuit voltage (VOC): 39.85V x 1.075 x 3 = 128.5
Max power voltage (VMP): 32.8 V x 1.075 x 3 = 105.7

Open circuit voltage (VOC): 40.0 V x 1.075 x 3 = 129
Max power voltage (VMP): 32.6 V x 1.075 x 3 = 105.1

The model MPP you are thinking about has max Voc of 145, so good there, and max MPPT voltage of 115, so good there too. IMO you are good to go with 3 of either of those panels in series, unless it's gets under 0c where you are. If it does, plug in the new percentage (25c - local temp in c) * 0.0029 + 1 and recalculate.

It can get down in the low teens in December January and February, especially at night. Then there is wind chill that makes it even colder.

Can you help me do the calculation under these conditions so I get it right and don't damage anything?

Also is there an option just to turn off solar charging under these conditions, for example before we go to bed each night?

Teens F or C? If C, nothing to worry about and don't worry about wind chill. If F, let's say 13F, -10C (brr). 25 - -10 * 0.0029 + 1

Open circuit voltage (VOC): 39.85V x 1.1015 x 3 = 131.6
Max power voltage (VMP): 32.8 V x 1.1015 x 3 = 108.3

Open circuit voltage (VOC): 40.0 V x 1.1015 x 3 = 132.1
Max power voltage (VMP): 32.6 V x 1.1015 x 3 = 107.7

Still good

This is weird, the forum won't let me post the maths ... seems to object to the result of the calculation, questions are OK, just not answers

Change the periods to commas...

Accept internationally recognised decimal separators

Tip: it's probably an incorrectly configured IP address obfuscation prevention filter incorrectly identifying the value as an IP address.

Open circuit voltage (VOC): 39.85V x 1.1015 x 3 = 131.6
Max power voltage (VMP): 32.8 V x 1.1015 x 3 = 108.3

Open circuit voltage (VOC): 40.0 V x 1.1015 x 3 = 132.1
Max power voltage (VMP): 32.6 V x 1.1015 x 3 = 107.7

[This is what I see, devoid of commas, whew!]

Re: "LV 5KVA-48V
INVERTER / CHARGER"

My unit was shipped 21 March 2020 so might be the design (120-0-120v split-phase) settled upon for the time being.
Booklet is ver 1.1 and on pg 9:

Model - 5KVA
Typical Amperage - 80A
Cable Size - 8 AWG

PV Module Selection / Solar Charging Mode:
Solar Charger Type - MPPT
Number of MPPT - 2
Max PV Array Open Circuit Voltage - 145vdc
PV Array MPPT Voltage Range - 60~115vdc
Min battery voltage for PV charge - 34vdc

So these are the rules that I have in writing. I got some clues over the phone from distributor in UT (he should do a write up)
which were mostly about using the LCD Setting (pg 14 top):

"05 Li-Ion Batteries" you need to scribble this into the empty boxes - User-Defined "05 USE" <enter> and then go to 26,27,
and 29 on pg 18 & 19 where you must enter the parameters for your Li-Ion battery bank (you now have left the Lead-Acid world!).

33 thru 39 are about "equalization" which looks to be a whole lotta fun. 8^)


So why "60~115vdc" means what has been explained in this thread, and probably elsewhere as well, I find interesting (jargon alert!!)
And so forth. I will go thru all the other threads and take notes . . .

Hope this helps,
VR
--

WOW - I haven't seen something solar written in Kelvin since school ...

Don't worry about the commas - half my engineers are from out of country and thats just how the rest of the world does it ...

I stopped getting worked up over mono-poly when the prices for each fell -- and since neither are ever going to give anywhere near max (80% if your lucky) paying for the extra you won't see is hard to accept ... Since they are both virtually identical - go with the cheapest ... (on that note - and becuase you require so many - have you thought about buying used? We get our JA-SOLAR 355W panels for like $90 that are in perfect shape but taken off of hail damaged roofs (but the panels have no damage))...

LV 5KVA-48V is a good solid system to build off of .. I have seen allot of the commercial guys around here using them ... no issues ONCE they are setup ... BUT parameter setup apparently requires hitting a button all day like a little monkey and easily a 12-pack ...