PV comparison for $/w & minimum voltage output

[kepler]

Hi,  I was looking for feedback to make sure I'm not missing/messing something up.

I made a spreadsheet to compare $/w of different panels to find the best 'deal' that also calculated the expected maximum voltage from the panel under hot conditions to see if it could fully charge the battery to 27Volts.   Is this a correct way to look at the worst case output of the panels?   I think this link will work  PV comparisons

Any panel recommendations for cloudier conditions as we get a bit of clouds here in southwest Pennsylvania.   Anyone know how to make the most out of bifacial panels backside (light colored or reflective background) or what their cloudier condition output is (spec sheet says it is better than typical panel)?  Have not seen that performance data with the few I've ran across for sale.

Thanks   

[Dacian Todea]

Did a quick look at the table and the best option there is the fist Heliene panel 60m300.

Calculation of max power point voltage vs temperature seems correct except that panel will be around 35C above ambient so easily in summer panel will be at 60 to 70C and you also need to consider some voltage drop on wires depending on panels distance from battery.

[jim]

Just my 2 cents here.  (Edit:  DACIAN beat me to it and gave a better answer... ignore me...)

One important factor to consider is the space you have available.  If you absolutely need a space requirement, then it may make some sense to look at super efficient panels.  So, for example, a 60 cell solar panel could be 240w or 330+w for the same physical size. RVs may want to spend more per watt to get more energy per area.  However, if more than enough space is available then a used 260w panel may be 1/3rd or 1/5th the cost of a 320w panel.  Some "new old stock" 270w panels may be a steal.

Dacian always like to point out, perhaps the solution is to just add another panel.. I think it is worth some thought.  Again, harder to do with an RV or van setup.  If you have a roof or a backyard no big deal other than some more wiring.

I used to enjoy learning about all the solar advances to get another 0.1% efficiency out of a panel, but the reality is the wildly mass produced 60 cell and 72 panels are SOOO much cheaper it hurts my brain.  I still see people all the time adding 100w panels that are 2-3x what my used 250w panels cost..

Some solar panels can do better if they are partially shaded (cells split into two halves in some way), but the goal would be to have them never shaded other than by clouds. And I believe most would be rather equivalent when you lower down the solar intensity due to clouding.  The sbms0 can be different because you can add your own MPPT charger, but if we are considering the sbsms120 or the ssr20 (not sure on spelling) which pairs with the sbms0, I believe every 60 cell would have the appropriate voltage needed, and the amps change with more light.

It just seems to me any of the cool solar tech is 10x more than the inexpensive stuff, and therefore it makes little economic sense.  But then again, I'm sure some pay extra to get all black panels rather than some aluminum showing. 

[Casey & Gina]

I honestly didn't research or compare much at all when I bought my panels - I got Renogy 300W 60-cell panels off Amazon and they work ok.  But my roof space is a limiting factor - the panels already overhand the roof of my trailer by a few inches on the sides and rear.  Funny enough, they have black powder coating on the aluminum sides, but I could care less about that.  Probably would have been wiser to hunt for the most efficient 60-cell panels I could have found instead.  I intend to buy 6 more panels in the future to build an external array that I can have set up on some property that I can plug into the trailer during the winter and have tilted.  I could just buy more of the same, but alternatively, if the dimensions are identical, I could replace the ones on my roof with more efficient panels, then use the ones currently on the roof to build the external tilted array, so that I have the most efficiency possible on my primary panels mounted on the trailer.  From that angle, are 60-cell panels pretty standard in their dimensions, and what are the most efficient 60-cell panels currently available?

I did a similar spreadsheet but with LFP cells, to determine the cost, weight, and volume per Ah.  No surprise - less weight and volume efficient cells cost less, and would make the most sense for a stationary application.  But since I have limited space and need to keep things as lightweight as possible, it probably makes sense to spend some extra to be able to fit more battery capacity into my space.

I've come to learn over time, that pretty much anything optimized for mobility costs a lot more, and is a lot more of a pain to properly install to tolerate being moved.  It's a lot less expensive and often less work too if everything can be stationary.

On Friday, September 18, 2020 at 8:55:47 PM UTC-6 jim wrote:

Just my 2 cents here.

[kepler]

Thanks Dacian,

I guess I was hoping having ground mounted panels would potentially have closer to the 'nmot' condition for the temp.  

I have voltage drop calculated above the pv panel table, it came out to 29.4V with the estimated distance and wire guage figuring the battery at 27v to get to full charge. 

  

[kepler]

Thanks for the response Jim 

No worry on space per say except to keep out of site of the wife. :)  Was hoping to have ground mounted, but might end up having to go on the roof (hopefully lighter color metal roof).  

My comparison seems to show the best 60cell panels for running directly to the battery, no mppt/pwm charge controllers, with highest amperage end up having voltage that is probably too low for my distance, unless I end up roof mounting the panels and larger wire which might be cost effective for the REC ones at the top of the list especially if I get around 20 panels, the shipping adder will be less per panel as well.   I'm also not concerned on getting the most efficiency, but do want to get the lowest life cycle cost and lowest maintenance plus Dacian's use of 24v avoids issues with some of the newer electric code requirements.  Yea, I don't get the 100w panels.  

I'm hoping to find ones with good cloudy performance.  From the irradiance to power graphs I've found, they look to be basically linearly proportional.  Not any that I've found for sale with substantially better output at the 200-400 range.    

Any recommendations on any never water leaking roof attachments?

 

[Dacian Todea]

Yes all panels will perform the same in low light as they all use the same type of cells and output is just directly proportional with the amount of light.

The max power point voltage is just a number and if at some point panel has max power point voltage at 29V due to panel temperature and say cable voltage drop and say at that max power point voltage current is 8A you will have 232W but when connected to the 27V battery the current will be slightly higher maybe 8.4 or 8.5A and thus x 27V = 227 to 229W slightly less than max power point power but not as significant as you may think. You can see that max power point power curve is fairly flat on the top so small variation in any direction from max power point power will not make that much of a difference.

[Electrons4me]

Hi Kepler,

Thanks for sharing. 

"Dacian's use of 24v avoids issues with some of the newer electric code requirements."

Would you be able to elaborate what has changed on the electric code regarding voltage higher than 24V?  

Thanks

E4M

[Dacian Todea]

E4M,

Not sure what Kepler is referring to but my projects are done to be compatible with world wide ultra low voltage standards. This standards differ a bit from country to country but most are limited to 60V and in case of Europe 75V dc so with a 24V battery ideal PV panels are 60 cells tho some use 72 cell panels and this have 40 to 50V open circuit voltage so just below the 60V limit in may places and with a 48V battery system you will need two of this panels in series so 80 to 100V dc thus well above the safe ultra low voltage standard.

[kepler]

The article referenced Section 690.12 update to the 2017 NEC (national electric code, usa, ?others) to require all conductors within an array’s 1-ft boundary have to be reduced to 80 V or less within 30 seconds of rapid shutdown initiation.  You can find more pretty fast on a web search or youtube.   

I was trying to look at other equipment that might be required to meet electric codes (for residential bldg) so I could get a better idea for comparing extra cost of more equipment & more total power output versus more cost of wiring Dacians 24v system configuration.  Dacian's 24v system wins hands down as far as system longevity making the system less fragile.    

Questions for others:

1) Any reason to not spend ~$0.3/w on a used(7-9yr old) panel versus new panel at around ~$0.41/w?  

2) I'm not sure a pv system with Dacian's DSSR20's and SBMS0 would be eligible for the solar tax credit in the US for either off-grid or grid-connected.  Is anyone familiar with this?

[Dacian Todea]

Kepler,

1) Yes the reason will be that old used panels are likely originally rated 230 to 260W (for 60 cell panels) while the new panels are probably above 300W so with groups of two panels in parallel you have around 500W (plus maybe some 5% degradation due to age 480W) vs two new panels above 600W and both sets will require a pair of 10AWG PV cable that may be a significant cost and so for same PV array size you may need a few of those 10AWG pairs and that alone may make up the difference in cost plus less solar panels mounting as you will have less panels for same power.

I was thinking about this as I will also need to make a larger array and have looked at benefits of used vs new panels. Plus there is the risk of some damaged used panels.

I seen someone recently showed me used 250W panels at $50 each so maybe it is wort considering at those prices depending on how large the array is and how far from the battery but at 0.3/W used vs 0.41/W new is not worth it.  

2) I have no idea if that will be the case. My system is mostly targeted for off-grid as it will not make economic sense to have a battery in a grid connected system.

[michael clark]

n the USA, I purchased 40 250 watt panels used for $2340.80 so the total cost delivered to my off-grid project in South Dakota was $0.23408per watt. the 24-volt system is working great with the Electrodacus SBMS0 and the Electrodacus DSSR20's. The 3 decimal monitoring and battery protection for the LIFePO4 battery bank is the reason I went with the Electrodacus equipment. I have plans to use the heating part of the DSSR20's with diversion in the future. Slow but Sure in the snow. Today, we have about 6-8 inches so it slows things down a bit at times. I also like the long term stability of the Electrodacus equipment. I think the weak link in my off-grid system will be the inverter needing replacement due to capacitor and FET failure.

Sorry to chime in but used panels can be very inexpensive. and wire cost is minimal to make a very large array capable of heating and electrifying your homestead off-grid. 

Why would you want to be tied to the grid if you can do it yourself off-grid?

I use 10AWG PV wire for 2 panels in parallel. I bought 1000 feet of red PV wire and 1000 feet of black PV wire so all is color-coded. Easy to manage that way. cost 500 dollars including shipping. likely a lot more wire than needed but have more use for it in the future.

[ask first]

Michael,

Just so I'm not mistaken what you are saying. You paid $500 for each 1000' of red? Or $500 for 1000' red and 1000' black combined?

[Electrons4me]

@ Dacian,

Thanks for the input!

I have seen some pictures of your solar panels that you posted a few years and, since you have many installed, I was curious to ask what sort of operating voltage do you see in the summer (~27C ambient) out of your 60 Cell panels when your battery is in the final stages of being charge? I'm asking because the Vmp I consider as datasheet "ideal" spec but when the panels are charging a battery, are 35 C above ambient, maybe they are a little dirty, the conditions are non-ideal but their output voltage in these conditions do reflect real-world conditions and that's what I'm interested in.

I'm sure, as the chart posted by Kepler shows, panel Vout will depend on the panel specs but I'm curious about what you typically see regardless of the panel brand. I'm trying to gauge what voltage margin I have from the needed 3.55V x 8 = 28.4V for a full charge?

Thanks 
E4M

[Dacian Todea]

E4M

Battery is almost always around 26.5 to 27V and just over for a few seconds around the end of charge before charging is stopped. Battery will never get to 28.4V unless some very specific conditions with super small charge rates maybe in a very dark overcast day it can get close to 28V but in typical days with 0.1 to 0.2C charge rate cells will be at 3.4V close to fully charged so 27.2V then one of the cells will be first to get to full charge 3.55V so for a few seconds 27.5V or so and the charging will stop and voltage will drop below 27V almost instantly if you have any load. 

Of course panels voltage will be the same while charging the battery what will be affected with heat will be the max current and while in winter is 9 to even 10A in hot summer days (can get above 30C for a few days here even 35C in some summer days) so panels get hot and current will drop to around 7A maybe even slightly below but is irrelevant as in summer is so much sunnier and day is so much longer plus I need way less energy in summer since there is no heating that is completely irrelevant that I get less power with hot panels.

My panels are also relatively old now 260W panels the new panels are now around 300W+ and they also have a slightly higher max power point voltage 32 to 33V vs 30 to 31V for the older models thus there will not be this lower current in summer you will still get the same current like in winter tho you will have plenty more energy in summer so likely plenty of excess.

[Electrons4me]

@Dacian,

Great, thanks for the input!