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Old 02-22-2015, 09:16 PM   #1
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How many solar panels on a 40-45 foot MH

So I'm a old/new RV'er. Owned both a brand new '87 Class C, then a brand new '89 Class A gas Moho, so things may have SLIGHTLY changed since then! Have not owned one since '95, so the old/new to rv'ing.

I plan on going with an all electric DSDP with 8 AGM batteries. I also plan on boon docking quite a bit so I want to add as much solar power as possible, but only on the roof as I do not want to deal with moving ground based panels around or getting them stolen etc. I'm also thinking of going with the newer ones that can be glued down and walked on, even though they are slightly less efficient than the framed units.

I also figure, like other things in life, you can never have too much... in this case solar power.

So, now to my question.

How many panels and/or watts has anyone been able to put on a 40'+ motorhome?

Thanks!
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Old 02-22-2015, 09:25 PM   #2
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I only have one on my 45UP. I might be able to get a couple more up there but with all the extra crap they put on the top these days, it's not much space left. I was thinking about going for an extendable tackle box design to hold 3 panels. fold them in for storage and fold them out when I get to a location.
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Old 02-22-2015, 09:27 PM   #3
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I have the same question and almost the exact same plan. I'd rather have extra panels to compensate on cloudy days and in shaded conditions and not have to fool with tilting them.

It's also unclear to me how much of the coach I could power off of batteries/solar. For example, can I run electric floor heat? How about the fridge? Induction cooktop? What's realistic for various system sizes?

I'm looking forward to any replies!
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Old 02-22-2015, 09:34 PM   #4
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Quote:
Originally Posted by IRVDoU View Post
I only have one on my 45UP. I might be able to get a couple more up there but with all the extra crap they put on the top these days, it's not much space left. I was thinking about going for an extendable tackle box design to hold 3 panels. fold them in for storage and fold them out when I get to a location.
"extendable tackle box design to hold 3 panels"

Is this something you will build yourself or know where to purchase existing product?
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Old 02-22-2015, 10:19 PM   #5
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The energy density of normal batteries is so far below the energy density of liquid propane or diesel that it is impractical to use battery power for any sort of comfort heating of the whole motorhome. Yes, you can run any load for a certain time, but then the problem is how to recharge the batteries.

I've run efficient inverter type air conditioners for a couple of hours on a hot day, and while I could have run it for a lot longer, I needed to remember to leave some power for normal use. Late in the afternoon is not a good time to be hoping to recharge seriously depleted batteries from solar panels.

Obviously running airconditioning when the solar panels are pumping in maximum current is a lot easier to manage than running underfloor heating on a cold day when solar input might be minimal.

As for fold out solar panels. Need to make sure one panel does not shade another panel as many panels are not very shade tolerant. Any design also needs to be strong enough to resist high wind velocities
Then there is the danger involved in climbing up on the roof once too often.
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Old 02-22-2015, 10:37 PM   #6
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Thanks for the posts so far.

I'm not planning to run AC off of the batteries and for heat, only to run the Oasis and not any of the other electric or heat pumps.

Just trying to add as much solar as possible to run the residential refrigerator, a little bit of the micro and/or induction cooktop if wired through the inverter, a laptop, and finally lights and TV in the evening without having to run the generator every day.

I'd like an approximate number of solar panels and wattage that I could possibly put on top without building a raised solar panel frame over the a/c's etc.

So how many panels have you been able to fit on your roof?

thanks!
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Old 02-22-2015, 10:53 PM   #7
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Check out and educate yourself at AM Solar's Educational Pages for RV Solar Systems
In addition, not all solar panels are the same size or voltage.
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Old 02-22-2015, 11:11 PM   #8
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I don't want to steal attention away from 4x4's original question, which is how many panels can fit on the roof of a typical DSDP. However, if someone can point to a place to read about which systems are wired through the inverter, or which can be if desired, that would be very helpful.

In theory there is ~8kW of energy hitting that roof at high noon on a cloudless day. It would be great to put a lot of it to work.

The sites by the RV system manufacturers are nice, but they don't get into enough detail about actual coach loads and operational tradeoffs. They also assume that most people aren't going to want to spend a lot on panels... but anyone can get panels right off Amazon for $1/w now, so the economics of last year or whatever don't necessarily apply anymore. More important questions seem like they might be:

* how many panels can we fit (original question)
* can we get an MH-reasonable MPPT charge controller that can handle the load
* how much storage do the batteries have, and how much could we upgrade to if desired (?)
* at what C can we safely recharge the batteries, and can we get thermal data
* do we have demand we can reasonably supply (during peak times) that is of value?

My guess is that we could come close to 2kW nominal and therefore come close to saturating the inverter.

Maybe I should start another thread to explore this further!
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Old 02-22-2015, 11:36 PM   #9
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In theory there is ~8kW of energy hitting that roof at high noon on a cloudless day. It would be great to put a lot of it to work.
How do you figure 8kW of energy??? I have 36 panels on my S&B roof that gives me 6.5kW of power. I can't imagine 36 panels on top of an RV nor having panels so much more efficient than mine to reduce it to fit (at least not at the current levels of technology in solar). My originally spec'd panels would've reduced the panel count to 27, but again, that's a lot of panels (not to mention weight).
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Old 02-22-2015, 11:51 PM   #10
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How do you figure 8kW of energy??? I have 36 panels on my S&B roof that gives me 6.5kW of power. I can't imagine 36 panels on top of an RV nor having panels so much more efficient than mine to reduce it to fit (at least not at the current levels of technology in solar). My originally spec'd panels would've reduced the panel count to 27, but again, that's a lot of panels (not to mention weight).
After the atmosphere does its stuff, it's about 1kW/m^2, or something like 8kW on the roof of a 40-footer. Typical panels are something like 15 or 17% efficient, so that's what's making your crinkle your eyebrows. I'll admit, I was being a little bit of a tease by choosing total radiated energy, but I was still right.

Speaking of weight, the thin-film cells essentially have no weight, which is why we probably want to be using them. And you can stand on 'em, and affix them to the roof with sticky tape, if you want. Seems like a no-brainer?
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Old 02-23-2015, 05:36 AM   #11
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Quote:
Originally Posted by 4x4 View Post
How many panels and/or watts has anyone been able to put on a 40'+ motorhome?
Here is a photo of my roof (40' DP with two 327 watt, high voltage PV panels).



There is space for one more panel behind the bedroom AC but then I am maxed out. Not that there isn't more open space - just none large enough to fit another similar (big) panel. So, in my case, I'd say about 3 panels/1kW will fit on my roof.

As mentioned, shading can be a problem. If one were creative enough, you could get a lot of capacity on the roof but it would take a very DIY approach.
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Old 02-23-2015, 05:56 AM   #12
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FWIW I have four 155 watt panels on my 43' Allegro Bus for 620 watts. On a sunny day in AZ in the winter, the solar will carry the normal load of the RR, some tv in the evening, lights, limited micro wave, etc. No ac and no use of the electric cook top. I have six 220 amp 6 volt wet cell batteries. By 4-5 am, the 10kw generator will start up because the voltage is down to 50% or 12.2 volts. What I have found to be more efficient is a 2kw Honda generator to run a 40 amp smart charger. With that small of a load, it will run for 12 hours on less than a gallon of gas. I set it off 50-75' chained to a tree or my trailer and start it up at 5 pm and let it run till bed time. With the batteries fully charged at 10 pm the noisy 10 kw gen does not need to run. I only use it to run the cook top or the ac. With an all electric coach with a freezer in the basement and normal use of the appliances requires 6-8 solar panels and a second six battery bank. For me, the 2 kw Honda made more sense. This doesn't answer the OP's question, but wanted to make him aware of how mauch is needed to be solar independent.
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Old 02-23-2015, 07:11 AM   #13
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Just thinking .... You may be able to conjure up something using powered slide trays on the roof.
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Old 02-23-2015, 08:35 AM   #14
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Some really good information! JFNM, that photo really helps. My idea is to go with the thin film bendable ones. Looks like it would be very easy to put 4 of them (they look to be smaller) where you have a single rigid panel because you can walk on them and get to the rest of the roof by not being limited where to walk, so looks like 8 panels without shadows and pushing it, 12 panels.

Just looking at the GoPower, their flexible panels are 100 watts each and measure approximately 42x21" so 4 would fit across the top of the roof pretty easily. In total anywhere from 800 to 1200 watts. Also found Renogy 100 watt bendable brand for about ⅓ the price of the GoPower ones!

Nebster... very good questions!

Crasher... good idea for another option of using a small generator, but if possible I'd prefer to overload on solar panels as much as possible instead of running a generator for 5 hours a day. If needed I'd probably just run the big generator for an hour to help with cooking dinner and to top off the batteries for the night.
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