Solar panel question

[Guy]

I am new to this. I have been looking at 100W panels for my RV. In comparing panels for most output, do I take the Wattage and divide it by the VMP #to get max. amp output. If this is right, HQST, NewPowa and Renogy all have different VMP #'s which give me different amp #'s. My goal is to either get 2 100W or 1 200W and a MPPT controller. I will use it on the ground so as to maximize sun. How do I compare panels to make the most amps going to battery? Thanks.

 

[MichaelK]

You should be matching you panel wattage to the size of your battery. Since batteries are usually measured in amphours, you will want to multiply your amps by charging volts to get the watts of panels needed. A good rule of thumb is to divide the amphour capacity by 8 to get the charging rate. You also want to use the charging voltage, which for a 12V battery will start at 13V. I also suggest a fudgefactor of 85% because panels almost never put out rated output.

So, let's take an example of a 100Ah battery. 100Ah/8 = 12.5A of charging current. At 13V that works out to be 12.5A X 13V = 163W. Throw in the fudgefactor and you get 163W/0.85 = 191W. So, 200W of panels is a good choice for that battery.

Let's say instead you have 250Ah golf-cart batteries. Then you need { (250Ah/8) X 13V }/0.85 = 478W of panels.

You can wire your panels in series to reduce power loss due to voltage drop. Then you can run your panels from further distances if you can place them in optimal sun. The Voc of you panels goes up as the temperature goes down, so you don't want to exceed about 75-80Voc if you are working with a 100V MPPT controller.

Watch out though. There are fake MPPT controllers out there coming out of China. Don't get scammed. Look at the shipping weight. If the product weighs 8oz, it's a fake. If the shipping weight is 6lb or so, it is a real MPPT controller

 

[Rednecktek]

Watch out though. There are fake MPPT controllers out there coming out of China. Don't get scammed. Look at the shipping weight. If the product weighs 8oz, it's a fake. If the shipping weight is 6lb or so, it is a real MPPT controller

The other dead giveaway is that 99.9% of the time if it has USB ports, it's a PWM fake.

 

[Supervstech]

I am new to this. I have been looking at 100W panels for my RV. In comparing panels for most output, do I take the Wattage and divide it by the VMP #to get max. amp output. If this is right, HQST, NewPowa and Renogy all have different VMP #'s which give me different amp #'s. My goal is to either get 2 100W or 1 200W and a MPPT controller. I will use it on the ground so as to maximize sun. How do I compare panels to make the most amps going to battery? Thanks.

The above response is fantastic.

But your confusion is in the formula the panel information doesn’t apply to charging amps.
What you need to figure is the battery charging volts. You want the watts of the panels / by the battery charging voltage to show amps.

There will be some charger loss, that’s where the .85 comes in.

The VMP is useful to decide on the mppt voltage input.

 

[Guy]

You should be matching you panel wattage to the size of your battery. Since batteries are usually measured in amphours, you will want to multiply your amps by charging volts to get the watts of panels needed. A good rule of thumb is to divide the amphour capacity by 8 to get the charging rate. You also want to use the charging voltage, which for a 12V battery will start at 13V. I also suggest a fudgefactor of 85% because panels almost never put out rated output.

So, let's take an example of a 100Ah battery. 100Ah/8 = 12.5A of charging current. At 13V that works out to be 12.5A X 13V = 163W. Throw in the fudgefactor and you get 163W/0.85 = 191W. So, 200W of panels is a good choice for that battery.

Let's say instead you have 250Ah golf-cart batteries. Then you need { (250Ah/8) X 13V }/0.85 = 478W of panels.

You can wire your panels in series to reduce power loss due to voltage drop. Then you can run your panels from further distances if you can place them in optimal sun. The Voc of you panels goes up as the temperature goes down, so you don't want to exceed about 75-80Voc if you are working with a 100V MPPT controller.

Watch out though. There are fake MPPT controllers out there coming out of China. Don't get scammed. Look at the shipping weight. If the product weighs 8oz, it's a fake. If the shipping weight is 6lb or so, it is a real MPPT controller

I have at present 2 12V lead Acid at 67amphr each. They came with the rig. I just want to add some amps during the day. I don't live off the grid or anything. So if I'm doing the math right.....67amphr max drawdown /8 = 8.37. 8.37 x13 = 108.8 x .85 = 92.48watts needed on the best possible day.

 

[Abodyofscience]

Hopefully this doesn't contradict previous or future posts.

24v lead acid batts want between 26v (summer) & 31v (winter) current limited [probably 40A] charging, just like in a vehicle.
The tradeoff in a vehicle is batt life with varying underwood temps vs keeping a full charge.
Cabin temps will be lower than underwood temps.

 

[joeExotic]

Since you'll be using the panels on the ground maybe consider using a foldable / portable solar panel with alligator clips? What do you experts think about that idea. It will definitely save space.

 

[Abodyofscience]

Large alligator clips may pass current reliably.

 

[joeExotic]

I just can't imagine lugging around two big 100watt panels everyday. Easier to fold out a panel, preferably with kick stands built in, and charge the batteries. Then just fold them back up all nice and compact and place under the dresser. Just a suggestion ?

 

[justgary]

I have at present 2 12V lead Acid at 67amphr each. They came with the rig. I just want to add some amps during the day. I don't live off the grid or anything. So if I'm doing the math right.....67amphr max drawdown /8 = 8.37. 8.37 x13 = 108.8 x .85 = 92.48watts needed on the best possible day.

You didn't quite get the math right. You need to account for both batteries and you need to divide by the fudge factor. You didn't say whether it is a 12V (batteries in parallel) or 24V system (batteries in series). If they are in series, your assumed charge voltage is then 26V, not 13. This may necessitate using two panels in series to get the needed voltage.

So, for a 12V parallel battery setup, you have double the capacity: 67Ah * 2 = 134Ah at 13V. Now 134/8 = 16.75, and 16.75 * 13 / 0.85 = 256W.

For a 24V series battery setup, you have double the voltage: 67Ah at 26V. Now 67/8 = 8.37, and 8.37 * 26 / 0.85 = 256W.

The difference in the two is that you either put two 130W panels in series or parallel, depending on your battery setup.

 

[Guy]

You didn't quite get the math right. You need to account for both batteries and you need to divide by the fudge factor. You didn't say whether it is a 12V (batteries in parallel) or 24V system (batteries in series). If they are in series, your assumed charge voltage is then 26V, not 13. This may necessitate using two panels in series to get the needed voltage.

So, for a 12V parallel battery setup, you have double the capacity: 67Ah * 2 = 134Ah at 13V. Now 134/8 = 16.75, and 16.75 * 13 / 0.85 = 256W.

For a 24V series battery setup, you have double the voltage: 67Ah at 26V. Now 67/8 = 8.37, and 8.37 * 26 / 0.85 = 256W.

The difference in the two is that you either put two 130W panels in series or parallel, depending on your battery setup.

Thanks again. Yes, I have 2 12V lead acid batteries hooked in parallel. Each battery is 67 amphr. I only want to drain them to 50% of capacity. THat's why I used the 67 # in my calculations. Also 2 100W panels are only 40" x 40" and total wt. is 32# Would be using a 30amp mppt controller. My RV is all 12V. The controller says it's either 12v or 24V. It recognizes what you have. If I am using 12 volt batteries and my rv only uses 12V, must I hook up the panels in parallel only. Or can I hook them up in series, 24V, and then to controller, and then controller see's that I'm a 12V system and changes the output to battery to 12V+, not 24V. Or does controller only put out what's put in as far as DC volts is concerned?