When you pro's talk, having solar panels sized to battery amp size, are you referring to total battery capacity or the 50% useable battery capacity? I am working on my Solar system, I have a total house bank of switch-able 410AH on Bank#1(GC2) and 170AH Bank#2 (Group24). I plan on installing 5 X 120watt panels. I figure this should work good. Is my plan good? Thanks Gary

For me, my planning is how much solar power I can get in the 5-6 hours of HIGH SUN to get my batteries back to at least their 90% charge state so I can make it through the next day/night run off my batteries. My camping off the batteries is planned to not drop my batteries below 50% charge by 8AM the next morning when I can either start recharging them by running my 2KW generator to use the on-board smart mode charging from my converter/charger unit or depend on the solar panels being in high sun all day long.To recharge my battery bank I see over 50AMP of DC charge at the initial re-charge state that starts tapering off to around 8AMPs of DC charge after an hour or so. Then i need to provide the 8AMPS charge state for an additional 2-3 hours to get my batteries back up to their 90% charge state. If I can't get to the 90% charge state I will not be able to make it through the night off my battery banks. My goal is to never drop below the 50% charge state over night.It will take alot of solar panels if no generator is planned to be used. Consider a 120WATT Solar setup will only produce 5-6AMPS of DC Charge in high sun.This is my game plan at any rate when camping off the power grid. Generator first for the short time running and then switch to solar for the long term charging. What ever it takes to get to 90% charge or more before the high sun goes away.Roy Ken

CT Wanderer"...When you pro's talk, having solar panels sized to battery amp size, are you referring to total battery capacity or the 50% useable battery capacity..."Not a pro but a delighted user of solar. I believe that people are referring to total battery capacity and not the 50% usable capacity. Unfortunately, a lot of folks do not know the difference. I guess that your 580 AH battery suite is composed of 12 V batteries or about 7 kW-h total or 3.5 kW-h usable. You do have a 36' motorhome so you could easily install over 1 kW of panels. I think that there seems to be a rule-of-thumb of about 8 to 10 times total battery capacity to maximum solar power. Your planned ratio is about 11 so you are in the ball paark, but another one or two panels would not be a bad idea. Your panels are 12 V (more or less) and probably will be in parallel so that you have 12 V to 12 V which simplifies things.We run 90 V from panels to battery suite since it reduces power loss, can use smaller cabling, and a smaller conroller (TriStar 45). Our battery bank is 48 V nominal.Roy BHow do you get 600 W of solar on your popup or is that 50 amp charge rate from the generator. Absolutely agree with running generator in early morning to get the battery suite up and then let solar top off. This is what we did with earlier systems; however, we have a 34' 5th wheel with sufficient solar to top off the battery suite fairly early in the day.Reed and Elaine

Watch out for shade. Four panels in full sun probably produce more than 5 panels with one or two in partial shade.

CT_WANDERER wrote:When you pro's talk, having solar panels sized to battery amp size, are you referring to total battery capacity or the 50% useable battery capacity? I am working on my Solar system, I have a total house bank of switch-able 410AH on Bank#1(GC2) and 170AH Bank#2 (Group24). I plan on installing 5 X 120watt panels. I figure this should work good. Is my plan good? Thanks Gary(I understand the question even though I do not agree with those who do relate their solar wattage to battery bank size as the way to do it.)The answer is they use the full rated AH capacity of the bank, not just half of it.Second question is whether 600w is enough for 580AH of bank. Answer is "It depends" on:-how many AH you need to replace daily with the solar (as a side note, if you do some of that with the generator/charger, that would be less solar needed)- how the panels are mounted flat or tilted-your latitude, and time of year for how much sun you can expect. "Just enough" solar for summer is "not enough" for winter.That AH replacement amount is the same no matter how many batteries you have. How many batteries you need is also related to your AH usage of course. If the solar is doing some of the AH usage as "extra" as well as recharging the batts, then you won't need as many batts to stay above 50% . But if some of that AH is high draw like microwave inverter then you might want more batteries to stay above the inverter's 11v shut off during the high draw times, even though for straight AH count, you don't need that many batteries to stay above 50% resting.Solar amount and battery amount requirements are two separate calculations, but each is based on your daily AH usage.

Solar Panel Size To Battery Bank Size | Good Sam Community

29 Replies

grizzzman
Explorer
May 25, 2015
BFL13 wrote:
JiminDenver wrote:
Can you over panel a PWM controller like you can some MPPT?

Not AFAIK. The PWM controllers I am familiar with just let the amps through above the controller's amp rating and the controller heats up. If left like that for long it will fry. How long? You will know how long once it fries and you note the time. :)

The buck converter in the MPPT controller has a limit so it "clips" the amps, same as any of those Mex type buck converter gizmos have a rated limit for output.

That means you can try to shove in more but it won't go through and heat the unit up and fry it. Advantage is you can "over-panel" for high noon and it clips that without overheating and the rest of the day with its lower insolation you can get more in, nearer the limit, so your daily average is higher over the whole time to make for a higher daily AH haul.

This is not magic, it just shows that with more solar, you get more amps. The rest is salesmanship.

The Bogert Engineerings SC2030 PWM controller clips amps when it hits 31 amps. And is capable of coping with 60 amps.
BFL13
Explorer II
May 24, 2015
JiminDenver wrote:
Can you over panel a PWM controller like you can some MPPT?

Not AFAIK. The PWM controllers I am familiar with just let the amps through above the controller's amp rating and the controller heats up. If left like that for long it will fry. How long? You will know how long once it fries and you note the time. :)

The buck converter in the MPPT controller has a limit so it "clips" the amps, same as any of those Mex type buck converter gizmos have a rated limit for output.

That means you can try to shove in more but it won't go through and heat the unit up and fry it. Advantage is you can "over-panel" for high noon and it clips that without overheating and the rest of the day with its lower insolation you can get more in, nearer the limit, so your daily average is higher over the whole time to make for a higher daily AH haul.

This is not magic, it just shows that with more solar, you get more amps. The rest is salesmanship.
JiminDenver
Explorer II
May 23, 2015
Can you over panel a PWM controller like you can some MPPT?
CT_WANDERER
Explorer
May 23, 2015
full_mosey wrote:
CT_WANDERER wrote:
2oldman wrote:
Plan is fine.

"Sizing" to me is "how many panels can I get on my roof?"

I like your thinking!!

Right now I have installed my Morning Star TS45 PWM controller and have all the wires for that done. With the roof box installed to get the wires to the controller. Next, I plan on ordering 3 X 120 watt panels and see how that goes. Than if I think, I am on the right track I will get two more 120 watt panels. I think I am limited to a max of around 720 watts to the TS45, before I start to max it out.

Have you run the Morningstar String Calculator?

HTH;
John

Yes when I did the string calculator, if I did it right, I would be good with 600 Watts. Which is the max I plan on going to. When getting up to 720 it starts pushing the limit. If I can run with the 600 Watts I would be happy and have to live with that.
reed_cundiff
Explorer
May 23, 2015
CT_WANDERER

720 W at 12 V is 60 amps.

If you go to higher voltages in run from panels and battery suite, you can use smaller wiring and smaller controller

Case in point, we have 90 V from panels (we did get to 1400 W to controller today when we ran fridge and hot water just for the heck of it) and 48 nominal battery bank. This would be 108 amps at 12 V but only 16 amps at 90 V. This goes to battery suite at 48 V nominal which is 29 amps. Our TriStar MPPT-45 controller handles things nicely.

We would have to go to two MPPT-60s to do this at 12 V. The 48 V (508 W) Mean Well power supply cuts the voltage to 12 V for the 12 V DC circuits and the inverter handles the 120 V AC circuit. 12 V to 12 V is a lot simpler process.
Reed and Elaine
full_mosey
Explorer
May 23, 2015
CT_WANDERER wrote:
2oldman wrote:
Plan is fine.

"Sizing" to me is "how many panels can I get on my roof?"

I like your thinking!!

Right now I have installed my Morning Star TS45 PWM controller and have all the wires for that done. With the roof box installed to get the wires to the controller. Next, I plan on ordering 3 X 120 watt panels and see how that goes. Than if I think, I am on the right track I will get two more 120 watt panels. I think I am limited to a max of around 720 watts to the TS45, before I start to max it out.

Have you run the Morningstar String Calculator?

HTH;
John
pianotuna
Nomad III
May 23, 2015
Hi,

I would go to a larger capacity controller if I were going to 720 watts of panels.
2oldman
Explorer II
May 23, 2015
landyacht318 wrote:
Fill the roof with solar, but the battery tray could still be too large if solar is to be the main/ only recharge source.
How much power you consume is a pretty important factor in your battery bank size.
landyacht318
Explorer
May 23, 2015
Being able to equalize Via solar does necessitate a higher wattage to capacity ratio too, and very light usage the night before, unless a generator is run for long enough powering a capable charger in the morning to do the bulk work.

Fill the roof with solar, but the battery tray could still be too large if solar is to be the main/ only recharge source.

As JID indicates, doing it right the first time is much cheaper than doing it right the second time, when upgrading requires different equipment or fatter wiring to accommodate the upgrade.
CT_WANDERER
Explorer
May 23, 2015
2oldman wrote:
Plan is fine.

"Sizing" to me is "how many panels can I get on my roof?"

I like your thinking!!

Right now I have installed my Morning Star TS45 PWM controller and have all the wires for that done. With the roof box installed to get the wires to the controller. Next, I plan on ordering 3 X 120 watt panels and see how that goes. Than if I think, I am on the right track I will get two more 120 watt panels. I think I am limited to a max of around 720 watts to the TS45, before I start to max it out.