Help me size my solar panels.

[hugobenjegard]

Hi, I'm looking at this 60a MPPT from PowMr https://powmr.com/solar-charge-cont...12v-24v-36v-48v-mppt-solar-charge-controller/ and plan on using 4 of these https://www.amazon.de/enjoysolar®-M...d=1673441814&sprefix=180w+enjo,aps,130&sr=8-3 180w solar panels to reach 720w of PV current. The charger is rated for 720w so will this work or is it pushing it to close? Can the PV produce a higher current than its rated for? The vmp is 19.1v and the Voc is 23.2v for the panels so they'll work within the 20v -80v the panel is rated for I think? Thanks for all the help it's greatly appreciated!

 

[MichaelK]

Is this for a 12V system?

As a general rule of thumb, I like to de-rate the output of panels to 85% of nameplate. The current maximum is the OUTPUT current, which in this case is 60A. So, assuming you want to charge a 12V battery, and it starts bulk charging at ~13V, the math is... (60A X 13V)/85% = 917W.

Now the Volt limit of your controller is 160V. You have to remember to take cold weather voltage correction into account. At -40C, the correction factor is ~ 1.25X. That will get you close. For a really accurate voltage for your location's winter lows, use a string calculator like this one...

MidNite Solar - Classic Sizing Tool.

MidNite Solar is the industry leader and manufacture of quality Renewable Energy System electrical components and E-Panels.

www.midnitesolar.com

But, using the quicky formula, expect the real-world Voc to not exceed 160V/1/25X = 128Voc. So, the max number of panels you could put in series is 5 at 145V. Six could reach 174Voc at -40C.

It turns out that 5 in series would be making you 900W, which is just under that limit of 917W. As an alternative, if you want a bit more power than this, you could implement 6 panels, but wire them in a 3S2P configuration. The two strings would need to be pointing in two different directions so the noon peak does not exceed 60A. A string facing southeast, and another facing southwest would accomplish this.

 

[Mattb4]

Exceeding SCC watts is not a big problem within limits. Exceeding voltage max limit is. With your 4 panels wired 2S2P you should be 46.4Voc (well in max voltage limits of the SCC) at ~19a. I assume you are trying to charge a 12v nominal battery system. Though with a 2S2P setup you would be good for a 24v setup. If you go to a 24v battery you could increase panel wattage by double (1440w)

 

[hugobenjegard]

Is this for a 12V system?

As a general rule of thumb, I like to de-rate the output of panels to 85% of nameplate. The current maximum is the OUTPUT current, which in this case is 60A. So, assuming you want to charge a 12V battery, and it starts bulk charging at ~13V, the math is... (60A X 13V)/85% = 917W.

Now the Volt limit of your controller is 160V. You have to remember to take cold weather voltage correction into account. At -40C, the correction factor is ~ 1.25X. That will get you close. For a really accurate voltage for your location's winter lows, use a string calculator like this one...

MidNite Solar - Classic Sizing Tool.

MidNite Solar is the industry leader and manufacture of quality Renewable Energy System electrical components and E-Panels.

www.midnitesolar.com

But, using the quicky formula, expect the real-world Voc to not exceed 160V/1/25X = 128Voc. So, the max number of panels you could put in series is 5 at 145V. Six could reach 174Voc at -40C.

It turns out that 5 in series would be making you 900W, which is just under that limit of 917W. As an alternative, if you want a bit more power than this, you could implement 6 panels, but wire them in a 3S2P configuration. The two strings would need to be pointing in two different directions so the noon peak does not exceed 60A. A string facing southeast, and another facing southwest would accomplish this.

Thank you for the help, I'll probably go with 2 40a Renogy Mppts instead and connect 2 panels to each. This PowMr MPPT seems way to shady for my liking. This would keep me way under the 520w limit per mppt (360w for 2 panels) and I'd stay under the 100vdc PV max (46.4voc for 2 panels). It'll be a little more expensive but will give me room to buy more/bigger panels if I would like in the future. Does this sound like a good idea?

 

[hugobenjegard]

Exceeding SCC watts is not a big problem within limits. Exceeding voltage max limit is. With your 4 panels wired 2S2P you should be 46.4Voc (well in max voltage limits of the SCC) at ~19a. I assume you are trying to charge a 12v nominal battery system. Though with a 2S2P setup you would be good for a 24v setup. If you go to a 24v battery you could increase panel wattage by double (1440w)

Thank you, I'll probably go with a 12v system since I don't want to have to deal with downsizing current for my 12v appliances (will use it in a campervan). Or is there a cheap way of down regulating 24 -12v?

 

[MichaelK]

Yes, I would agree that PowMr is a bottom rung product. Epever is a step up.

Since you are mentioning this is for your campvan, can you go into more detail as to the physical positioning of the panels? I mentioned de-rating output to 85%. That's for panels directly facing the sun. If however they are laying flat on a camper roof, I'd suggest a bigger de-rating, to say 60-70%. With that in mind, the max you can likely use looks more like (40A X 13Vcharging)/60% =867W, to (40A X 13Vcharging)/70% =743W.

So, maybe as many as three 250W panels per 40A MPPT. Realisticly, how much roof space do you actually have? Maybe three might not even fit? Here is a situation where smaller 100W panels might possibly be a better choice, just trying to snuggle them in?

 

[hugobenjegard]

Yes, I would agree that PowMr is a bottom rung product. Epever is a step up.

Since you are mentioning this is for your campvan, can you go into more detail as to the physical positioning of the panels? I mentioned de-rating output to 85%. That's for panels directly facing the sun. If however they are laying flat on a camper roof, I'd suggest a bigger de-rating, to say 60-70%. With that in mind, the max you can likely use looks more like (40A X 13Vcharging)/60% =867W, to (40A X 13Vcharging)/70% =743W.

So, maybe as many as three 250W panels per 40A MPPT. Realisticly, how much roof space do you actually have? Maybe three might not even fit? Here is a situation where smaller 100W panels might possibly be a better choice, just trying to snuggle them in?

I'll make a mechanism to tilt the PV but probably only about 30° or something like that and won't have it tilted 100% of the time. Regarding the panels as of now our best solution is to fit 4 180-200w panels on the roof. We have 180x310cm available but since we want to have 2 roof vents we can't fit any more than that.

 

[hugobenjegard]

Yes, I would agree that PowMr is a bottom rung product. Epever is a step up.

Since you are mentioning this is for your campvan, can you go into more detail as to the physical positioning of the panels? I mentioned de-rating output to 85%. That's for panels directly facing the sun. If however they are laying flat on a camper roof, I'd suggest a bigger de-rating, to say 60-70%. With that in mind, the max you can likely use looks more like (40A X 13Vcharging)/60% =867W, to (40A X 13Vcharging)/70% =743W.

So, maybe as many as three 250W panels per 40A MPPT. Realisticly, how much roof space do you actually have? Maybe three might not even fit? Here is a situation where smaller 100W panels might possibly be a better choice, just trying to snuggle them in?

Would it be beneficial to use these 200w panels instead? They have a Voc over 40v instead of 20v? https://www.amazon.de/enjoysolar®-M...sprefix=enjoy+solar,aps,136&sr=8-5&th=1&psc=1

 

[MichaelK]

Would it be beneficial to use these 200w panels instead? They have a Voc over 40v instead of 20v?

Up to you. Maybe I could suggest making cardboard cutout the shape of your perspectively panels and go up on the roof and see how they fit. Then plug the numbers into the string calculator above and see what works. Keep in mind that panels suspended up in the air are going to be slightly colder than stuff on the ground. Factor that into your winter low calculations.

 

[hugobenjegard]

Up to you. Maybe I could suggest making cardboard cutout the shape of your perspectively panels and go up on the roof and see how they fit. Then plug the numbers into the string calculator above and see what works. Keep in mind that panels suspended up in the air are going to be slightly colder than stuff on the ground. Factor that into your winter low calculations.

Thanks again! So there are no real benefits of going with higher voltage panels when using a 12v system? I'll still get the same current? We probably won't be using the panels below 15°C since that's about as cold as it gets in southern Spain during the days. Should I still use the numbers for -40°?

 

[MichaelK]

Thanks again! So there are no real benefits of going with higher voltage panels when using a 12v system? I'll still get the same current? We probably won't be using the panels below 15°C since that's about as cold as it gets in southern Spain during the days. Should I still use the numbers for -40°?

The benefit of high voltage really comes into play with running wiring for long distances. For the kinds of distances inside a van, voltage drop may not be that significant. Just throwing some numbers into a voltage drop calculator, this is what I got. Keep in mind that for a MPPT controller to work properly, you want the input voltage to be about 30-40% higher then the charging voltage. Play with the calculator yourself with your own numbers and see what you get.

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

 

[hugobenjegard]

The benefit of high voltage really comes into play with running wiring for long distances. For the kinds of distances inside a van, voltage drop may not be that significant. Just throwing some numbers into a voltage drop calculator, this is what I got. Keep in mind that for a MPPT controller to work properly, you want the input voltage to be about 30-40% higher then the charging voltage. Play with the calculator yourself with your own numbers and see what you get.

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

View attachment 129239

Thanks alot, I'll check it out right away! The wire will be a maximum of 5 meters so it's not that far I think. Would going any higher than 30-40% be negative or does it work even if it's 100% higher? Thanks!

 

[MichaelK]

Thanks alot, I'll check it out right away! The wire will be a maximum of 5 meters so it's not that far I think. Would going any higher than 30-40% be negative or does it work even if it's 100% higher? Thanks!

Actually yes. The big companies put out power curves based on how many charging amps you get at different input voltages. There typically a sweet-spot somewhere around 2-3X the battery voltage that gives peak performance. Going above 3X and typically you loose a bit of the power from extra heating of the controller. It's actually working a bit harder to bring the very high solar voltage down to battery voltage.

That being said, I've ran my own Midnight200 at 4X battery voltage, with no complaints. So, do as a say, not as I do!

 

[hugobenjegard]

Actually yes. The big companies put out power curves based on how many charging amps you get at different input voltages. There typically a sweet-spot somewhere around 2-3X the battery voltage that gives peak performance. Going above 3X and typically you loose a bit of the power from extra heating of the controller. It's actually working a bit harder to bring the very high solar voltage down to battery voltage.

That being said, I've ran my own Midnight200 at 4X battery voltage, with no complaints. So, do as a say, not as I do!

Thanks again man, putting 2x22.8voc panels in parallel should be great for a 12v system right? Since I'd like to put them in parallel to get better shade tolerance.

 

[MichaelK]

Thanks again man, putting 2x22.8voc panels in parallel should be great for a 12v system right? Since I'd like to put them in parallel to get better shade tolerance.

Remember, the Voc is only used for the determination of the limits your electronics can tolerate. The number that's important here is the Vmp, which needs to be 30-40% higher then battery voltage. I'll guess that the Vmp is ~18V, which is about 25% higher than a max charging voltage of around 14.4V. Most likely it is enough, but you'll have to determine that by trial and error.