Double check panel math

[JASHOTZE]

I have the Renogy 400 watt solar kit and am pretty sure it can handle a few more panels than the four it came with. Can someone make sure I’m not missing something?

40 Amp Renogy MPPT
Max solar input - 530 watts / 50 amps
Panels VoC -21.6
Lmp- 5.72 amps

2s3p would be 43.2 volts and 17.16 amps right?
No that would make the watts over but haven’t seen anyone discuss the watts here in this forum regarding the panels. Only volts and amps. Thanks for the help!

 

[sunshine_eggo]

I have the Renogy 400 watt solar kit and am pretty sure it can handle a few more panels than the four it came with. Can someone make sure I’m not missing something?

40 Amp Renogy MPPT
Max solar input - 530 watts / 50 amps
Panels VoC -21.6
Lmp- 5.72 amps

2s3p would be 43.2 volts and 17.16 amps right?

Yes.

No that would make the watts over but haven’t seen anyone discuss the watts here in this forum regarding the panels. Only volts and amps. Thanks for the help!

Per specs, even if you produce more than 530W, it will only deliver 530W. You are over-paneled by a small amount and should be fine.

 

[Bud Martin]

The system will only pull the power from the panels up to the maximum of the charger limit and not more. You will be fine with over paneled the system, you can point each panel string in more than one direction so it will collect power through out the day.
What is the max PV Input Voltage of the SCC?

 

[JASHOTZE]

The system will only pull the power from the panels up to the maximum of the charger limit and not more. You will be fine with over paneled the system, you can point each panel string in more than one direction so it will collect power through out the day.
What is the max PV Input Voltage of the SCC?

Max input voltage is 100 watts.

 

[Bud Martin]

100V max, you are OK then.

 

[JASHOTZE]

Ok thanks!

 

[MichaelK]

Here is one additional point to think about. Panels almost never produce their rated output. That is because they are rated in a test chamber at a controlled temperature, and artificial sunlight at exactly 1000W/square meter. In the real-world you don't get that.

What I normally do is incorporate a fudgefactor of 0.85X to 0.90X in my panel calculations to compensate for this, which always bumps up the panel requirements a bit. So, let's say you went with a 3S2P configuration. That's better than 2S3P because 3 strings would require individual breakers. It also raises the voltage so voltage drop is decreased. Even three in series though would still be safely below the 100V limit of this controller.

So, the math, including the fudgefactor (FF) would be.... { (100W X 6 panels)/13V charging } X 0.85FF = 39.2A. So, in the real-world, you are not likely to ever see more than 39A. It's the amps that goes to the battery that is the true amp limit of the controller.

If you are really freaked out though about exceeding your controller's limit, you can position one array facing SouthEast, while the other is facing SouthWest. This will tend to dampen the noon peak to a lower level, but broaden the overall peak over a longer period of the day.

 

[JASHOTZE]

Here is one additional point to think about. Panels almost never produce their rated output. That is because they are rated in a test chamber at a controlled temperature, and artificial sunlight at exactly 1000W/square meter. In the real-world you don't get that.

What I normally do is incorporate a fudgefactor of 0.85X to 0.90X in my panel calculations to compensate for this, which always bumps up the panel requirements a bit. So, let's say you went with a 3S2P configuration. That's better than 2S3P because 3 strings would require individual breakers. It also raises the voltage so voltage drop is decreased. Even three in series though would still be safely below the 100V limit of this controller.

So, the math, including the fudgefactor (FF) would be.... { (100W X 6 panels)/13V charging } X 0.85FF = 39.2A. So, in the real-world, you are not likely to ever see more than 39A. It's the amps that goes to the battery that is the true amp limit of the controller.

If you are really freaked out though about exceeding your controller's limit, you can position one array facing SouthEast, while the other is facing SouthWest. This will tend to dampen the noon peak to a lower level, but broaden the overall peak over a longer period of the day.

Michael,

I think that makes a lot of sense. These are mounted flat on an RV roof, so i don't foresee ever being in perfect world scenario with these panels. I had it hooked up 2s2p for a few months and think the highest I ever saw was about 220 watts at 10 amps. If I go with three in series I am worried that shading or shadowing across one of the three would negatively affect the others more. That was another reason for the 2s3p. Thank you!

 

[12VoltInstalls]

40 Amp Renogy MPPT
Max solar input - 530 watts / 50 amps
Panels VoC -21.6
Lmp- 5.72 amps

Max input voltage is 100 watts. VOLTS

I’ve been running 4 100W panels since April? and facing two directions, 2S2P. Just added two more panels in 3S2P and holy cow SCC output was 14.x volts and ~32A with the full sun hitting both arrays.

That shows how powerful an mppt can be with series panels in volts/amps even if less than ideal facings (55-56V from panels which are vertical).

Shading only effects the string in shadow btw

Try that- you’ll like it.

 

[fafrd]

I have the Renogy 400 watt solar kit and am pretty sure it can handle a few more panels than the four it came with. Can someone make sure I’m not missing something?

40 Amp Renogy MPPT
Max solar input - 530 watts / 50 amps
Panels VoC -21.6
Lmp- 5.72 amps

2s3p would be 43.2 volts and 17.16 amps right?
No that would make the watts over but haven’t seen anyone discuss the watts here in this forum regarding the panels. Only volts and amps. Thanks for the help!

You haven’t indicated what voltage you are charging at, but 530W @ 40A suggests this is for a 12V battery.

530W of input power or 40A of output current @ 12V is the peak output you can achieve and if actual input power exceeds that level, the SCC will ‘throttle-back’ input power by increasing voltage to reduce input current so that 530W is the maximum input power it will allow (moves from Vmp/Imp towards Voc/0A).

That ‘plateau’ where you are wasting potential solar power is only during the peak part of the day and only if there is sufficient solar radiation (not overcast). As others have already stated, panels rarely put out 100% of rating and 85-90% is a more typical peak output level.

So the short answer is yes, you can easily overpanel your SCC but the only real reason to do so is if you are not fully charging your battery every day.

If the ‘kit’ came with 4 100W panels you are probably rarely exceeding 340-360W of output, meaning 28-30A of output (70-75% of maximum rating).

If you add two more panels (either 2S3P or even 3S2P if your max input voltage is over 75V), you’ll get close to an additional 50% charge on a daily basis and during the peak part of the day, will rarely exceed 510-540W of input power, meaning your worst-case power loss due to saturation is unlikely to ever exceed 2% over the highest-intensity hour in the middle of the day.

As long as you have battery capacity to store the additional energy you’ll produce every day and some use for that additional stored energy overnight, I’d say it’s a no-brainer.

 

[JASHOTZE]

You haven’t indicated what voltage you are charging at, but 530W @ 40A suggests this is for a 12V battery.

530W of input power or 40A of output current @ 12V is the peak output you can achieve and if actual input power exceeds that level, the SCC will ‘throttle-back’ input power by increasing voltage to reduce input current so that 530W is the maximum input power it will allow (moves from Vmp/Imp towards Voc/0A).

That ‘plateau’ where you are wasting potential solar power is only during the peak part of the day and only if there is sufficient solar radiation (not overcast). As others have already stated, panels rarely put out 100% of rating and 85-90% is a more typical peak output level.

So the short answer is yes, you can easily overpanel your SCC but the only real reason to do so is if you are not fully charging your battery every day.

If the ‘kit’ came with 4 100W panels you are probably rarely exceeding 340-360W of output, meaning 28-30A of output (70-75% of maximum rating).

If you add two more panels (either 2S3P or even 3S2P if your max input voltage is over 75V), you’ll get close to an additional 50% charge on a daily basis and during the peak part of the day, will rarely exceed 510-540W of input power, meaning your worst-case power loss due to saturation is unlikely to ever exceed 2% over the highest-intensity hour in the middle of the day.

As long as you have battery capacity to store the additional energy you’ll produce every day and some use for that additional stored energy overnight, I’d say it’s a no-brainer.

Thanks! It is a 12v system. I dont have a large battery, (BB OWL 170AH) but also dont have a large draw. But I also havent started boondocking much yet and want to make sure i have as much capability as possible. Thanks! I think I'm going to go with the 3s2p format.

 

[fafrd]

Thanks! It is a 12v system. I dont have a large battery, (BB OWL 170AH) but also dont have a large draw. But I also havent started boondocking much yet and want to make sure i have as much capability as possible. Thanks! I think I'm going to go with the 3s2p format.

Yeah, I see your maximum input voltage is 100V, so you are good to go for 3S with those panels.

600W of panels is likely to generate over 300Ah of charge current at 12V through the summer, so you’ll probably need to ~double your battery capacity to capture all the additional energy you’ll generate, but your wintertime production is likely to only be ~half that level, so where your current 400W of panels would probably never fully charge your 170Ah battery (assuming it started off empty), with the addition of another 200W of panels you should be able fully recharge your current battery throughout winter…

 

[MichaelK]

Michael,

I think that makes a lot of sense. These are mounted flat on an RV roof, so i don't foresee ever being in perfect world scenario with these panels. I had it hooked up 2s2p for a few months and think the highest I ever saw was about 220 watts at 10 amps. If I go with three in series I am worried that shading or shadowing across one of the three would negatively affect the others more. That was another reason for the 2s3p. Thank you!

One more thing. The 85% (0.85X) fudgefactor I stated above is for panels oriented directly at the sun. For panels laying flat though the fudgefactor is more like 60-75%, or multiplying by 0.60X to 0.75X. So, even 6 panels might not be close to max.

You can actually determine this empirically. Just put a heavy load on your system (>watts then all your panels) at about noon, and measure your output while under that load. Just divide your measured watts by your spec watts and you've got your own custom FF.

 

[JASHOTZE]

Yeah, I see your maximum input voltage is 100V, so you are good to go for 3S with those panels.

600W of panels is likely to generate over 300Ah of charge current at 12V through the summer, so you’ll probably need to ~double your battery capacity to capture all the additional energy you’ll generate, but your wintertime production is likely to only be ~half that level, so where your current 400W of panels would probably never fully charge your 170Ah battery (assuming it started off empty), with the addition of another 200W of panels you should be able fully recharge your current battery throughout winter…

That’s good to hear! Thanks for the help!

 

[JASHOTZE]

One more thing. The 85% (0.85X) fudgefactor I stated above is for panels oriented directly at the sun. For panels laying flat though the fudgefactor is more like 60-75%, or multiplying by 0.60X to 0.75X. So, even 6 panels might not be close to max.

You can actually determine this empirically. Just put a heavy load on your system (>watts then all your panels) at about noon, and measure your output while under that load. Just divide your measured watts by your spec watts and you've got your own custom FF.

I’ll give that a shot thanks!!

 

[fafrd]

I’ll give that a shot thanks!!

The other way to estimate power output is to use PVWATTS (setting incline to 0 degrees): https://pvwatts.nrel.gov/

 

[sunshine_eggo]

The other way to estimate power output is to use PVWATTS (setting incline to 0 degrees): https://pvwatts.nrel.gov/

Plus, if you have the balls to download and parse the hourly data, you can actually get average power for each hour of the day; however, it does factor the average weather influence on output, so it can be perfectly applicable.. and not.

 

[fafrd]

Plus, if you have the balls to download and parse the hourly data, you can actually get average power for each hour of the day; however, it does factor the average weather influence on output, so it can be perfectly applicable.. and not.

I find PVWATTS most useful for assessing the relative output levels I can expect from my 4 pitched roof orientations.

Of course, with a flat roof, there won’t be any pitch, but it should give some idea of the average daily output to expect from 400W per day and how much to expect winter to decline versus summer.

And as you say, if you are willing download and dig through the hourly data, you can determine the likelihood that 600W of panels will ever exceed 530W of output on a flat roof (my suspicion is that the answer will be no, especially if factoring in the 1-3% wiring losses from panels to SCC…

 

[12VoltInstalls]

I find PVWATTS most useful for assessing the relative output levels I can expect from my 4 pitched roof orientations.

I find anytime I have 12.9+ volts when I get home I’ve had good relative output levels

I tried PV Forecast on my phone but it is a) a trial and b) not very accurate from what I saw. It would be nice to look ahead now and again

 

[fafrd]

I find anytime I have 12.9+ volts when I get home I’ve had good relative output levels

I tried PV Forecast on my phone but it is a) a trial and b) not very accurate from what I saw. It would be nice to look ahead now and again

Is ‘PV Forecast’ related to PVWATTS?

PVWATTS is a pretty fantastic tool, at least for getting a good sense of seasonal variation in production levels as well as comparing different roof surfaces / mounting options…