# How many amps needed for my controller?

Posts: 18Registered Users ✭✭
Hi. I have a 37' fifth wheel and propose (7) 160 watt, Vmpp 18.5V, Impp 8.65A, Voc 22.2 V, 16% efficient, 12V panels or 1,120 watts. My goal is to select an MPPT controller that's big enough.

I've heard numerous calculations, with variations in the efficiency factors. The last one was 1120 x 80% efficiency/ 13.3 volts is 67.37 amps. Does this formula make any sense. If I use 12 volts, I get 74.67 amps.

I'm more interested in understanding the math first, then I'll consider various controllers.

Thanks for any help.

• Posts: 3,738Solar Expert ✭✭✭✭
Welcome to the forum,

Your math looks OK... I wouldn't use 80%, and I wouldn't call it efficiency.  Yes, there are losses do to inefficiency in the controller, but with proper design they can be minimized.  There are also losses in the cables, but these too can be minimized.  Your largest losses are in the panels which lose voltage when they get hot (84% of the energy that strikes your panels will heat them up).  For back-of-envelope estimates I usually say that 75% of the array nameplate wattage will be available to charge the battery in summer conditions.

How are you planning to configure the panels?  With 7 panels and one controller you have no choice but to put them all in parallel.  In that case you would probably be better off with a PWM controller... it depends in part on how high a voltage your batteries need.  It also depends on your climate.

MPPT controllers need headroom, that is they need to drop the incoming voltage down to the charging voltage.   12 volt panels can be a problem because when they are hot and their Vmp declines to 16 volts, and you need 15 volts to charge the battery... the MPPT doesn't do very well with only 1 volt of headroom.  A PWM controller will do better in those circumstances.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Posts: 356Solar Expert ✭✭✭
edited February 2016 #3
hi . as vtMaps says 7 is a difficult number . can you try 6 or 8 ? . then you can series / parallel them . Tray putting the panel specs into the midnight string calculator on line . Or the outback calculator.
2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . victron battery monitor . 24 volts 450 ah surette batterys . off grid  holiday home
• Posts: 3,738Solar Expert ✭✭✭✭
Try putting the panel specs into the midnight string calulator on line . Or the outback calculator.
BEWARE of those calculators... they allow designs that will technically work, but are very far from optimal.

For example, you could put your 7 panels in series with a Midnite Classic 200 charge controller.  That controller can handle over 5400 watts in some systems.  In your system (7 panels in series), if its in a cool room it will be able to handle about 1050 watts, and its fans will be screaming all the time.

The reason is that (virtually all) MPPT controllers need the input voltage to be higher than the battery charging voltage.... the headroom that I mentioned in a previous post.  BUT too much headroom is not good, the controllers become less and less efficient as the headroom grows.  In your system, with 7 panels in series, your input voltage (Vmp) would be 130 volts and you would be charging a 12 volt battery.  That's way too much headroom.

Inefficiency in the controller (or any other electronics) means heat production.   The Classic 200 controller will protect itself from melting by limiting the power it can handle.  In your system that would be about 1050 watts, less than 20% of what it could handle in a well designed system.  In your system it would have a somewhat shortened lifespan... it doesn't do your controller any good to spend hours a day on the verge of melting.

So beware of those calculators.  They allow some poor designs.  One thing about design and engineering... it's all about compromise... you can't optimize everything.   When you choose to run a moderate sized system at 12 volts (rather than 24), you make compromises... much higher currents, more voltage drop, more expensive wiring, breakers, lower limits on your electronics, etc.  When you find that the optimal use of space on your roof is seven 12 volt panels, you are compromising because 7 panels will limit your configuration choices to all series or all parallel... by optimizing space on roof you are un-optimizing the controller... forcing yourself to use a PWM (all panels in parallel) or an inefficently operated MPPT (all panels in series).

On an RV, partial shading is often a concern.  If that is the case for you, having your panels in series will be even less optimal still.  Perhaps you can find some higher voltage panels (30 volt panels are very common and inexpensive) that will fit on yourt roof.  They work very nicely in parallel with an MPPT controller on a 12 volt system.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Posts: 1,218Solar Expert ✭✭✭✭
edited February 2016 #5
Hi. I have a 37' fifth wheel and propose (7) 160 watt, Vmpp 18.5V, Impp 8.65A, Voc 22.2 V, 16% efficient, 12V panels or 1,120 watts.
So youve done your load budget? Whats the target Wh/d? People often (ok nearly always) come here with the specs all mapped out but often havent considered the design. Not really sure why that is exactly...
My goal is to select an MPPT controller that's big enough.
MPPT because youve done an analysis on the pros and cons in your specfic setup, or because "people say they are better"? Me I kinda like PWM for RVs. The cable runs are ultrashort, the environment is often hot (sometimes noisy fans and reduces the life of expensive MPPT controllers), they can be very efficient in some ways, their idle draw is a whole order of magnitude less, and for some other reasons which ill expand later if you want.
I've heard numerous calculations, with variations in the efficiency factors. The last one was 1120 x 80% efficiency/ 13.3 volts is 67.37 amps. Does this formula make any sense. If I use 12 volts, I get 74.67 amps.
Our std forum formula for MPPT is this:

Peak average charge rate at noon= 1120Wp /14.4V *0.77 =   60A.

Vt explained the reason for the derating. For PWM its slightly different and depends on the PV to controller voltage mismatch. But as a rule we use something like:
Peak average charge rate at noon =  Isc 0*.95 . You dont give an Isc but if we take a guess, then we get:
9.46A * 7 * 0.95=  63A.

So about the same (difference is in the noise floor). Where MPPT gains with its tracking, PWM semi makes up on idle, and switching effciency.

The reason i use Isc there is clear if look at a (typical) PV IV curve:

The current actually increases  (by only a tad) when hot. Its the voltage that takes a holiday. A PWM tends to operate to the left of the Vmp (knee of curves), and the current only falls ever so slightly from the Isc (far left) to the Vmp. [OK maybe more than you wanted to know. To fix that youll have to ask the big guy to heal my appendicitus]

The 0.95 derating is to allow for cable and controller resistance.

Make sense?

Now 1100W is out of the "small array" and into the nearly mid sized. That brings new challenges, cheif among them higher currents. We generally move up to 24V battery at or near this point, that will dial it back to 30A and thus puts in reach of a whole range of great PWM and MPPT controllers like the Prostar, Kid, Brat, Smart Harvest, Boghart etc.

1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar

• Posts: 102Solar Expert ✭✭

Outwestbound, why so few panels on a 37' RV?   I know there are probably A/Cs and vents and whatnots also up there, but it still seems like not much PV for that amount of roof space.   Remember that you can mount panels over vents and fans (except for the hot vents from fridges or heaters), which maximizes space usage and reduces possible shading problems.   FYI, I have eight big grid-tie panels, and there's still plenty of space for two 3' x 7' water-heating panels and a separate smaller PV system for the start batteries  -  I also have a 12"-wide walkway on the roof between my two rows of panels, and two large emergency exits (I may convert the rear one to a skylight so I can see the stars when in bed!).

John

40' Crown bus with 2kW of panels on the roof:

Eight tiltable Sharp 255W, two Morningstar TS-MPPT-60, Magnum MS2000, Champion C46540 generator converted to propane, eight golfcart batteries eventually, and maybe a smaller inverter for the fridger.

Southern California

• Posts: 18Registered Users ✭✭
vtmaps said:
Welcome to the forum,

Your math looks OK... I wouldn't use 80%, and I wouldn't call it efficiency.  Yes, there are losses do to inefficiency in the controller, but with proper design they can be minimized.  There are also losses in the cables, but these too can be minimized.  Your largest losses are in the panels which lose voltage when they get hot (84% of the energy that strikes your panels will heat them up).  For back-of-envelope estimates I usually say that 75% of the array nameplate wattage will be available to charge the battery in summer conditions.

How are you planning to configure the panels?  With 7 panels and one controller you have no choice but to put them all in parallel.  In that case you would probably be better off with a PWM controller... it depends in part on how high a voltage your batteries need.  It also depends on your climate.

MPPT controllers need headroom, that is they need to drop the incoming voltage down to the charging voltage.   12 volt panels can be a problem because when they are hot and their Vmp declines to 16 volts, and you need 15 volts to charge the battery... the MPPT doesn't do very well with only 1 volt of headroom.  A PWM controller will do better in those circumstances.

--vtMaps

Thanks. Not knowing anything, I'd originally taken the array nominal watts of 1120 divided by 12 and got 93.33 as a controller size, but with inefficiency and the denominator of 13.3, it seems like 63 - big difference.

I'm on an RV roof and trees are never certain, so I'm thinking parallel array. I'll have to research the batt voltage, as I'm not certain what you're asking. I'm planning Lifeline GPL-6CT 6V, 300AH together for 900 AH.

• Posts: 18Registered Users ✭✭

Outwestbound, why so few panels on a 37' RV?   I know there are probably A/Cs and vents and whatnots also up there, but it still seems like not much PV for that amount of roof space.   Remember that you can mount panels over vents and fans (except for the hot vents from fridges or heaters), which maximizes space usage and reduces possible shading problems.   FYI, I have eight big grid-tie panels, and there's still plenty of space for two 3' x 7' water-heating panels and a separate smaller PV system for the start batteries  -  I also have a 12"-wide walkway on the roof between my two rows of panels, and two large emergency exits (I may convert the rear one to a skylight so I can see the stars when in bed!).

John

A couple more panels would fit, but I don't need them. I threw in the 7th just because I'm too lazy to get up there and clean or tilt them. If I can get a reasonably decent harvest from 6 on paper, I'm good.

• Posts: 18Registered Users ✭✭
zoneblue said:
Hi. I have a 37' fifth wheel and propose (7) 160 watt, Vmpp 18.5V, Impp 8.65A, Voc 22.2 V, 16% efficient, 12V panels or 1,120 watts.
So youve done your load budget? Whats the target Wh/d? People often (ok nearly always) come here with the specs all mapped out but often havent considered the design. Not really sure why that is exactly...
My goal is to select an MPPT controller that's big enough.
MPPT because youve done an analysis on the pros and cons in your specfic setup, or because "people say they are better"? Me I kinda like PWM for RVs. The cable runs are ultrashort, the environment is often hot (sometimes noisy fans and reduces the life of expensive MPPT controllers), they can be very efficient in some ways, their idle draw is a whole order of magnitude less, and for some other reasons which ill expand later if you want.
I've heard numerous calculations, with variations in the efficiency factors. The last one was 1120 x 80% efficiency/ 13.3 volts is 67.37 amps. Does this formula make any sense. If I use 12 volts, I get 74.67 amps.
Our std forum formula for MPPT is this:

Peak average charge rate at noon= 1120Wp /14.4V *0.77 =   60A.

Vt explained the reason for the derating. For PWM its slightly different and depends on the PV to controller voltage mismatch. But as a rule we use something like:
Peak average charge rate at noon =  Isc 0*.95 . You dont give an Isc but if we take a guess, then we get:
9.46A * 7 * 0.95=  63A.

So about the same (difference is in the noise floor). Where MPPT gains with its tracking, PWM semi makes up on idle, and switching effciency.

The reason i use Isc there is clear if look at a (typical) PV IV curve:

The current actually increases  (by only a tad) when hot. Its the voltage that takes a holiday. A PWM tends to operate to the left of the Vmp (knee of curves), and the current only falls ever so slightly from the Isc (far left) to the Vmp. [OK maybe more than you wanted to know. To fix that youll have to ask the big guy to heal my appendicitus]

The 0.95 derating is to allow for cable and controller resistance.

Make sense?

Now 1100W is out of the "small array" and into the nearly mid sized. That brings new challenges, cheif among them higher currents. We generally move up to 24V battery at or near this point, that will dial it back to 30A and thus puts in reach of a whole range of great PWM and MPPT controllers like the Prostar, Kid, Brat, Smart Harvest, Boghart etc.

Thanks man. I've had math through linear algebra, so I follow the graphs, etc. I'm a finance/ economics guy just trying to get this right. I clearly don't know much about solar and it's not immedialty obvious that I'm going to have the time resource to climb that hill. There is so much crap on my roof that series would be negated. IF I could line all the panels up along one side, I'd go series and just orient the camper accordingly, but they have to be on each side of the camper roof.

I didn't do a manual load yet, because I ended up with such a large system. Anywhere around 5 hours on those panels a day and I'm more than good. At this point, I'm looking at
7 grape gs-s-160-fab8 (may be 8 - plenty of room)
magnum msh3012
some kind of controller that makes sense (the midnite classics are insanely large.
trimetric meter
6 lifeline 6v, 300AH to total of 900

this is just what I'm thinking. I understand optimization problems subject to constraints, but am just out of my field.

• Posts: 18Registered Users ✭✭
vtMaps

Which panels do you think make sense? I looked around some, but found panels that where like 40" x 77" - too big. I do understand that higher voltage panels function better with MPPT. Astrenergy 305 watts were too big.

"On an RV, partial shading is often a concern.  If that is the case for you, having your panels in series will be even less optimal still.  Perhaps you can find some higher voltage panels (30 volt panels are very common and inexpensive) that will fit on yourt roof.  They work very nicely in parallel with an MPPT controller on a 12 volt system."

--vtMaps
• Posts: 3,738Solar Expert ✭✭✭✭
If you are willing to go 6 or 8 panels, a world of opportunities opens up.  You could wire some of the 12 volt panels in series/parallel.

When you research the battery voltage, you will end up at 24 volts.  When you research battery configuration you will discover that an optimal configuration of batteries is a single series string... avoid parallel batteries.  Especially avoid putting AGM batteries in parallel.  Remember, two 12 volt batteries in series (24 volt) has the same storage capacity as the same two batteries in parallel (12 volts).  The 24 volt system will be more stable and efficient.
some kind of controller that makes sense (the midnite classics are insanely large).
Controllers are rated by the output.   You design you wiring, fuses, PWM controller, etc to handle 125% of the nameplate power, even though typical power levels are much less.  If you use a MPPT controller, you can "over panel" it a bit, but that is not always optimal because you are spending a lot of time at the controller's limits.

With six 160 watt panels on a 12 volt system you need to think about a controller that can handle 960 watts ÷ 12 volts = 80 amps.  You will need an insanely large controller.  If you switch your battery to 24 volts, you reduce the current to 40 amps.

If you do decide to stay at 12 volts and 7 panels, I suggest that you use an MPPT and a PWM controller.  Put 3 panels in parallel on the PWM controller, and 4 panels in series/parallel on the MPPT controller.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Posts: 18Registered Users ✭✭
Question: Can two arrays, one serial (6 panels) and one parallel (2-4 panels), be run into a single controller? This is a general question. Trying to understand whether my phase 2 controller needs are part of the phase 1 implementation, or whether I'm looking at another controller down the road for the parallel array. Thanks.
• Posts: 1,218Solar Expert ✭✭✭✭
Hey, you need to anchor something here, youre kinda massaging a bowl of spaghetti on this. Put the shopping itch aside for a minute, you cant make a shopping list until you have a design.

Solar design is a linear process, that usually starts with a load budget, then battery, then charging comes LAST.
However sometimes in the case of an RV, its starts by maxing out the roof space.

If we go with the latter then high efficiency cells (eg sunpower) should be given top priority, but it depends on the shape of your roof, how many interruptions there are on it. The bigger the panel the less overall roof penetrations. Make a scale drawing of the roof, and make a spreadsheet of the dimensions of all the modules you have access to, (start with NAWs shop), and then make a list of permutations that will physically fit. Can you see how this will then flow to the next step of how to string them, and what kind of controller/s youll need? As a Iceni said, also ponder the value of perhaps moving the one or two of the worst of the interruptions if it yields a large improvement in array size and geometry. Geometrically adjoining panels will make less roof penetrations, and increase output considerably.

We've all told you that a 24V system is the path to happiness here. We recently had a long thread about a large 12V system and the problems he was having with undersized cables. But if you choose to use 12V, then do so based on a reasoned compromise.  Valid reasons to stay with 12V might be:
- you have many recent and expensive purchases of fittings and appliances, all of which have untrivial 12V current demands; and some were custom fitted to cabinetry ie replacing them will make a huge mess.
- your alternator is 12V; and you move everyday without access to shore power; and your load budget is way more than your roof space will provide for; ie the design calls for heavy reliance on the alternator.

The reason not to stay with 12V are:
- because thats how weve always done it
- becasue thats how everyone else does it
- because i have quite a few (older) 12V appliances (with pretty minor current requirements)
- because i have 12v flouros
- because i have this one particular favorite gadget that needs 12V (TV, computer etc).
- because i have a 12v fridge
- because i have a 12v waterpump
- because i have a dearly beloved 12v inverter, that cost me my first born child, and has not once let me down (ok i confess this one might almost qualify to go above.. but not quite!)

All (but the last) of those are covered by a 24V to 12V DC converter. These used to be expensive, and are now not. \$25 bucks gets you a 20 amp model, and 20A ought to cover the bulk of it, maybe a second one for the fridge and waterpump. But, over time you take the next opportunity to swap out the pump with the 24V model, same price, same brand. Fridge ditto. Lights: led. Not much left, right? The odd furnace/ water heater etc.

Lets face it youre about to spend 5K+ on a solar system. You spent 100K+ on a RV, so budget isnt the limiting factor here is it, right?

So make a good decision, as Kelly Starret would say. Either way. When you have a decision tell us, and we can move unreservedly to the next step.

Feisty today huh
1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar

• Posts: 3,738Solar Expert ✭✭✭✭
Question: Can two arrays, one serial (6 panels) and one parallel (2-4 panels), be run into a single controller? This is a general question. Trying to understand whether my phase 2 controller needs are part of the phase 1 implementation, or whether I'm looking at another controller down the road for the parallel array. Thanks.
You are proposing to put the 6-panel string in parallel with the 2-4 single-panel strings.
That would only work if all strings have the same voltage.  So, that won't work.

There's nothing wrong with having several small controllers, and it can be an advantage when different panels have different exposures to the sun.  It also means the panels don't all have to match each other electrically... you could have some 30 volt panels on one controller and some 12 volt panels on another controller.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Posts: 1,218Solar Expert ✭✭✭✭
edited February 2016 #15
OK, so 12V it is. (btw which of my two criteria did you lean on?).

And you have autocad, its perfect for this. Make rectangles for all the PV modules you can practically find. Take all the suppliers you have willing to use, take all their monoC module sizes. Dont miss any, else youll go round in circles.

Then drag them around until you get a layout that works, then copy and paste the roof layout, right next to it, and start again with other panels to find another possibility. After a few hours of this, if like me youll end up 3 or 4 firm possibilities.

Then see what the vmps turn out to be. The rules for array strings are these:
- all modules on a single controller should have the same cell tech.
- for PWM , strings of one only 36 cell module, all in parallel. Imp may vary without issue.
- for MPPT, each string total combined vmp (of the string) must be nominally equal (as vt said +/-5%)
- for MPPT, each module in the string must be have an Imp that is nominally equal (ditto).

For example, you could on a single controller, at the extremes, have these sorts of things:

PWM: four 8 amp modules in parallel with four 10 amp modules
MPPT: 2 strings each comprising, [two 36 cell modules and two  60 cell modules] in series

And dont forget sunpower as their panels are approximately 20% smaller for the same output. They do cost more, and not the easiest to buy. They do this by putting the intercell connections on the reverse, hence the cell is completely black.
1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar

• Posts: 18Registered Users ✭✭
vtmaps said:
Question: Can two arrays, one serial (6 panels) and one parallel (2-4 panels), be run into a single controller? This is a general question. Trying to understand whether my phase 2 controller needs are part of the phase 1 implementation, or whether I'm looking at another controller down the road for the parallel array. Thanks.
You are proposing to put the 6-panel string in parallel with the 2-4 single-panel strings.
That would only work if all strings have the same voltage.  So, that won't work.

There's nothing wrong with having several small controllers, and it can be an advantage when different panels have different exposures to the sun.  It also means the panels don't all have to match each other electrically... you could have some 30 volt panels on one controller and some 12 volt panels on another controller.

--vtMaps

Thanks. My interest now is to implement a 6 panel 960 watt 12V panel array in series on the top of a fifth wheel travel trailer. This gets my voltage up to make better sense with an MPPT controller. Panels proposed are Grape GS-S-160-Fab8. Will an Outback FLEXmax 80 MPPT work? I'd think so.

The rest of my roof could hold numerous other panels, but in questionable shade areas. Hence, I'd think parallel would work on those, which motivated my earlier question. From what I gather, "phase 2" implementation should be planned for a parallel array of 2-4 panels in parallel on another controller.

I think this makes more sense, because trying to build capacity into phase 1's controller makes it too big.
• Posts: 1,218Solar Expert ✭✭✭✭
Also if considering a classic scale controller, please do your homework on charge controller tare loads. Id strongly advise Kid/Brat class idle tare for an RV.
1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar

• Posts: 3,738Solar Expert ✭✭✭✭
My interest now is to implement a 6 panel 960 watt 12V panel array in series on the top of a fifth wheel travel trailer. This gets my voltage up to make better sense with an MPPT controller. Panels proposed are Grape GS-S-160-Fab8. Will an Outback FLEXmax 80 MPPT work? I'd think so.
Poor design.  With 6 panels on 1 controller you should configure them as either two strings of 3 panels (Vmp=55.5) or three strings of 2 panels (Vmp=37).
I explained about too much headroom back here in this thread.  Also, 6 panels in series (Voc=133.2) is likely to damage any of the controllers mentioned in this thread except the Classic 200.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Posts: 18Registered Users ✭✭
vtmaps said:
My interest now is to implement a 6 panel 960 watt 12V panel array in series on the top of a fifth wheel travel trailer. This gets my voltage up to make better sense with an MPPT controller. Panels proposed are Grape GS-S-160-Fab8. Will an Outback FLEXmax 80 MPPT work? I'd think so.
Poor design.  With 6 panels on 1 controller you should configure them as either two strings of 3 panels (Vmp=55.5) or three strings of 2 panels (Vmp=37).
I explained about too much headroom back here in this thread.  Also, 6 panels in series (Voc=133.2) is likely to damage any of the controllers mentioned in this thread except the Classic 200.

--vtMaps
vtmaps said:
My interest now is to implement a 6 panel 960 watt 12V panel array in series on the top of a fifth wheel travel trailer. This gets my voltage up to make better sense with an MPPT controller. Panels proposed are Grape GS-S-160-Fab8. Will an Outback FLEXmax 80 MPPT work? I'd think so.
Poor design.  With 6 panels on 1 controller you should configure them as either two strings of 3 panels (Vmp=55.5) or three strings of 2 panels (Vmp=37).
I explained about too much headroom back here in this thread.  Also, 6 panels in series (Voc=133.2) is likely to damage any of the controllers mentioned in this thread except the Classic 200.

--vtMaps
Ahh my bad. I printed this thread and just studied it. I forgot to say serial/parallel. I suspected this configuration would be needed, and just realized the "serial/parallel" language means that. Thanks for offering the combinations. Looks like I can dial it in.

• Posts: 1,218Solar Expert ✭✭✭✭
And dont forget, a nice side effect of PWM and/or short strings is that its the best possible shading mitigation system. One panel shaded in a mppt string will kill the entire string.

Another little tidbit that they dont tell you that in the marketing blurb.
1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar

• Posts: 174Solar Expert ✭✭✭
For some reason as RVers we shy away from the the pita of doing an energy audit first to determine battery bank size which will determine solar watts needed as a starting point.
• Posts: 174Solar Expert ✭✭✭
zoneblue said:
Hey, you need to anchor something here, youre kinda massaging a bowl of spaghetti on this. Put the shopping itch aside for a minute, you cant make a shopping list until you have a design.

Solar design is a linear process, that usually starts with a load budget, then battery, then charging comes LAST.
However sometimes in the case of an RV, its starts by maxing out the roof space.

If we go with the latter then high efficiency cells (eg sunpower) should be given top priority, but it depends on the shape of your roof, how many interruptions there are on it. The bigger the panel the less overall roof penetrations. Make a scale drawing of the roof, and make a spreadsheet of the dimensions of all the modules you have access to, (start with NAWs shop), and then make a list of permutations that will physically fit. Can you see how this will then flow to the next step of how to string them, and what kind of controller/s youll need? As a Iceni said, also ponder the value of perhaps moving the one or two of the worst of the interruptions if it yields a large improvement in array size and geometry. Geometrically adjoining panels will make less roof penetrations, and increase output considerably.

We've all told you that a 24V system is the path to happiness here. We recently had a long thread about a large 12V system and the problems he was having with undersized cables. But if you choose to use 12V, then do so based on a reasoned compromise.  Valid reasons to stay with 12V might be:
- you have many recent and expensive purchases of fittings and appliances, all of which have untrivial 12V current demands; and some were custom fitted to cabinetry ie replacing them will make a huge mess.
- your alternator is 12V; and you move everyday without access to shore power; and your load budget is way more than your roof space will provide for; ie the design calls for heavy reliance on the alternator.

The reason not to stay with 12V are:
- because thats how weve always done it
- becasue thats how everyone else does it
- because i have quite a few (older) 12V appliances (with pretty minor current requirements)
- because i have 12v flouros
- because i have this one particular favorite gadget that needs 12V (TV, computer etc).
- because i have a 12v fridge
- because i have a 12v waterpump
- because i have a dearly beloved 12v inverter, that cost me my first born child, and has not once let me down (ok i confess this one might almost qualify to go above.. but not quite!)

All (but the last) of those are covered by a 24V to 12V DC converter. These used to be expensive, and are now not. \$25 bucks gets you a 20 amp model, and 20A ought to cover the bulk of it, maybe a second one for the fridge and waterpump. But, over time you take the next opportunity to swap out the pump with the 24V model, same price, same brand. Fridge ditto. Lights: led. Not much left, right? The odd furnace/ water heater etc.

Lets face it youre about to spend 5K+ on a solar system. You spent 100K+ on a RV, so budget isnt the limiting factor here is it, right?

So make a good decision, as Kelly Starret would say. Either way. When you have a decision tell us, and we can move unreservedly to the next step.

Feisty today huh
I think more in terms of the K.I.S.S. principal........the Tow Vehicle is 12 volt, a DC to DC system that works there from what I have read is \$250+. The camper emergency braking system is 12 and must feed from the batteries, ok that's a wiring thing. The on board converter can be changed out to an Iota or similar 24 volt or just thrown away like I did. You could change out the refrig to a AC/DC compressor model or just run on propane if a 3 way. Propane is what we do. When it's upper 90's out and your dry camping full or partial sun is not an option. The site we were in last summer near Tillamook Or last summer required me to make up a 30 foot extension for the portable panels.http://forum.solar-electric.com/discussion/24060/the-nest-and-its-solar-setup#latest  the link will let you see all the gyrations we went through and our system is only 360 watt.
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