My question is if I want a 12V solar panel that puts out 120 watts for charging my batteries and I buy instead a 24V panel will I have to get a 240 watt solar panel to equal the output of a 12V 120 watt solar panel? Thanks, Jeff.
Jeff & Lori
Fleetwood Tioga 22B - Chevy chassis
10' Wells cargo trailer
Gracie Mae - our 9 year old Papillon
Thanks for the info but I'm still confused (happens alot lately). So are you saying that if I use an MPPT controller on a 24V 240W panel that I'll get twice the usable amps into my batteries as using a PWM controller with a 12V 120w panel? Just trying to figure out what cost wise I need to buy. We mainly need it for recharging batteries for about 2 12V lights for 8 hours a day and maybe to run a TV for 5 hours a day but I want to size it at twice the size I think I'll need in case we are parked in partial shade during the day. We are in the position where we have to park on the street or parking lots or empty fields for 3-4 days at a time. Right now we have to run our generator about 2 hours a day to maintain our battery level. Thanks, Jeff.
smkettner wrote: You really should get an MPPT controller with a 24v panel and then you will get all 240w of power into your battery.
A 24v panel may have a Voc close to 40 volts that may not even be compatible with a low cost PWM controller.
Otherwise yes a 24v 240w panel may have about the same amps as a 12v 120w panel. But that is not really the point.
The problem is/was that MPPT controllers are/were so much more expensive than PWM controllers, that you could just get two 12v panels instead of going MPPT with one panel.
Of course you need roof real estate to have the option.
Now panel and controller prices are lower and options are more open within a budget, so there is a lot of calculating required to pick your optimum set-up.
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scroller95969 wrote: So are you saying that if I use an MPPT controller on a 24V 240W panel that I'll get twice the usable amps into my batteries as using a PWM controller with a 12V 120w panel?
Yes that is exactly what MPPT controller does. But they do cost substantially more.
To be MPPT cost effective you will want to size the panels to load the controller near max.
With low cost PWM controller you can over size the controller and keep adding panels if the system does not quite cover what you need.
It is a bit of a trade off. With PWM you will need a combiner box to parallel additional panels where as with MPPT you can run the panels in series and pull the wire direct down to the controller. The larger panels 200+ watts seem to be 24v but cost drops close to $1 per watt so you save some on panels. Then larger panels might be more shipping. It is plenty to make your head spin.
If cost is critical you will need to lay out both systems to compare.
I sometimes think a lot of confusion comes about because people don't have a background to understand electrical terms as applied to RV systems.
When you see a panel as being rated for so many watts, just remind yourself that watts are the way to tell how much power the panel generates. Watts tell you how much energy the panel produces when exposed to a 'standard illumination' or sunlight. Panels that are rated at the same power, ie.. Watts, can be rated at different voltage, because voltage is one of the two factors that make up the power that the panel can deliver.
The other factors is Amps. Again, panels that are rated at the same power, can be rated at different amps. At any given power or Wattage, you can look at the voltage, and determine the amps because volts x amps = watts.. They are always related in this way.
A 240 watt panel generates twice the power that a 120 watt panel does. Panels of the same wattage can be found at different voltages, and these panels would be rated at different amps depending on the voltage. To use an overly simple example: suppose the 240 watt panel was rated at 240 volts(they're not). Then you would know that that panel puts out 1 amp because 1x240=240. Now say you have a 120 watt panel that is rated at 240 volts, it would put out 1/2 amp because 240 x .5= 120. In reality, voltages can be all over the place, but the relationship is always the same to actual power produced.
Some confusion comes from the difference between PWM and MPPT. All solar controllers do just that, they control the power going into the batteries from the solar panels to extract the most power and get it to the batteries without overcharging and damaging them. PWM or Pulse Width Modulation needs to see a nominal 12 volt input from the panel, these are often rated as Low Voltage, or 17 volt panels. The PWM controller looks at the state of charge of the battery, and essentially turns the power from the solar panel on and off very quickly with the ratio of on to off changing as the battery approaches a full charge. They do not change the voltage from the panel to the battery, only the 'time' that the panel is on, going from full on to full off as the battery charges.
A MPPT controller does have the capability to change the voltage of the panels to match the battery voltage and varies the power going into the batteries by varying the voltage and amps as needed by the batteries. These controllers can use high voltage panels because they can actually change the voltage going into the batteries. Some can tolerate quite high voltage so you can put several high voltage panels in series (end to end, so to speak). This opens up another whole can of worms regarding wire size and voltage drop that you probably don't want to know about. But that's the difference between the two types of controllers in a nutshell, and why you need to choose the panel voltage that is correct for your controller.
* This post was
edited 03/12/12 10:41am by CallThisCamping?! *
Tony Tait
2011 Winnebago Adventurer 32H
Pulling 2002 Jeep Liberty
I don't think you would find an 80 watt panel and controller for $25.00. In fact I think you might pay more for an 80 watt panel than for a 120 watt panel--as some places are selling at $1.00 per watt.
If you wish to stick with just 80 watts it would be possible to use just a diode and forget about the controller. I don't recommend it.
The rule of thumb is 60 to 150 watts of panels for every 100 amp-hours of battery bank.
example 120w panel rated at 7+ amps for 12v system , a 240w rated for a 24v system would also be 7+ amps
(2) 120w panel in parallel would be 240w but 14+ amps
(2) 120w paneks in series would be 240w at appox 28v and 7+ amps
when you use higher voltage whether series 12v system panels or 24v system panels you need a MPPT controller to convert the charge voltage and keep the watts by increasing the amps
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CallThisCamping?! wrote: I sometimes think a lot of confusion comes about because people don't have a background to understand electrical terms as applied to RV systems.
When you see a panel as being rated for so many watts, just remind yourself that watts are the way to tell how much power the panel generates. Watts tell you how much energy the panel produces when exposed to a 'standard illumination' or sunlight. Panels that are rated at the same power, ie.. Watts, can be rated at different voltage, because voltage is one of the two factors that make up the power that the panel can deliver.
The other factors is Amps. Again, panels that are rated at the same power, can be rated at different amps. At any given power or Wattage, you can look at the voltage, and determine the amps because volts x amps = watts.. They are always related in this way.
A 240 watt panel generates twice the power that a 120 watt panel does. Panels of the same wattage can be found at different voltages, and these panels would be rated at different amps depending on the voltage.
Some confusion comes from the difference between PWM and MPPT. All solar controllers do just that, they control the power going into the batteries from the solar panels to extract the most powerful and get it to the batteries without overcharging and damaging them. PWM or Pulse Width Modulation needs to see a nominal 12 volt input from the panel, these are often rated as Low Voltage, or 17 volt panels. The PWM controller looks at the state of charge of the battery, and essentially turns the power from the solar panel on and off very quickly with the ratio of on to off changing as the battery approaches a full charge. They do not change the voltage from the panel to the battery, only the 'time' that the panel is on, going from full on to full off as the battery charges.
A MPPT controller does have the capability to change the voltage of the panels to match the battery voltage and varies the power going into the batteries by varying the voltage and amps as needed by the batteries. These controllers can use high voltage panels because they can actually change the voltage going into the batteries. Some can tolerate quite high voltage so you can put several high voltage panels in series (end to end, so to speak). This opens up another whole can of worms regarding wire size and voltage drop that you probably don't want to know about. But that's the difference between the two types of controllers in a nutshell, and why you need to choose the panel voltage that is correct for your controller.
good explaination. Another way to think about it is that with a PWM controller, Until your near full charge Amps in (from the panel)= amps out (to the battery). Now suppose you have a 80W panel. you'll see it is rated at about 4.5A. but 12V x 4.5A = 54 watts. hummm. well the 80W comes from the panel voltage of 17.6V x 4.5A=80W at max output. So when using a PWM controller, you will only get 4.5A or about 55W from the panel into the battery.
Now hook the same panel to a MPPT controller. They are a DC to DC converter (more expensive) and power in (from the panel)= power out (to the battery). So for the same 80W panel, the power in is 80W with a voltage of 17.5V and 4.5A. The MPPT controller converts that 17V 4.5A supply to about a 13V (depending on battery state of charge) supply at 6.15A at about 97% efficiency for a net 6A to the battery until it is near full charge.
so as an example in a perfect sunny day under ideal conditions for 10 hours of sunlight, the PWM controller will give you 4.5A for 10 hours or 45AH into the battery. The MPPT controller will give you 6.0A for 10 hours or 60AH into the battery. So the MPPT controller will give 33% more amp/hr into the battery from the same panel. What a deal!!
Now back to the OP. Your 24v 240 watt panel. current for the 24V panel will be about 6A, hook it up to a 12V/24V autodetecting PWM controller than will autodetect the battery and charge a 12V battery. What you'll get is 6A into the battery. Hooked to a MPPT controller the same panel will give you about 18A as in the above case. so in one case the 240W panel is delivering about 72watts or 6AH to the battery, in the other it is delivering 240 watts or 18AH into the battery from the same panel!!! Now if the 240W panel was a 12V panel, you would get 12AH or about 144Watts from the panel.
Now comes the kicker. for that 80W panel a PWM controller might cost $25. a MPPT controller will be about $200-250.
* This post was
edited 03/12/12 10:53am by ktmrfs *
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2004.5 Silverado 4x4 CC/SB Duramax/Allison
Thanks for helping with all the math guys. You know what they say: there's three kinds of people in the world. One that's good at math and one that's not. This clears it up in my mind where I can do the math myself on the sytems that I put together on paper, Jeff.
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