Using a residential PV panel on RV

joelmyer wrote:
Thanks for the feedback.

Yes, 106 ah daily. I've put in LEDs so should be a little less.

I've been accused of speaking encrypted. By less amps coming down I meant from the panels on the roof to the controller down in the storage compartment behind the batteries. 250 watts @ 12 volts is a lot more amps than 250 watts at 24 volts. And the IsquaredR power lost in heating up the wire will be 1/4 with 24 volt panels. Yes from the controller to the batteries would be the same.

Hmmm... free power. I was mostly thinking getting the batteries fully charged. I spend $3/day on gasoline, $3/day on propane. Getting that second 250 watts on the roof will be hard.

LEDs save a lot. 2 double domes run 4 hours from winter sunset to bedtime is 24 AH before, or 5 AH after replacement. 20 AH less.

Amps from panel... The beauty of 24V and MPPT is that you don't lose amps - you lose watt-hours (or amp-hours, same thing). And very little at that. Daily resistance loss with 400W *24V array is ~2 AH or less. This is almost nothing. From 250W*24V panel the loss is not even worth talking about. But I would go for 400-450W, if roof space allows. For somebody who boondocks a lot and doesn't have unlimited budget, having not to not run a genny on most days and getting batteries fully charged would be nice. Or you'll run it half an hour for Bulk, and then solar will do the rest.

My batteries are full before 11 am in Baja winter, without a generator, though I use less than 40AH a day. For curiosity tried running all the loads other than MW, to draw ~90 AH and see how much I could harvest from the sun. Next day was sun with clouds and harvested ~70 AH, with my "regular" consumption pattern, i.e. not running the loads that I don't need. Then 100% full by 10 AM next morning. 490W array, 300 AH bank.

BoonHauler wrote:

...I still have to run a load test on a 12 VDC ceiling fan and decide on what roof top A/C I'll be running with but somewhere between 1.25 and 1.5kw, most likely the later.

As often with long threads, I don't know what we are talking about, anymore :)

And I fail to see the connection between solar and A/C. To me, these two things are unrelated until you reach solar wattage at least 1500W.

Which is something opposite to what the OP was asking about, as his setup was rather minimalistic: LED lights, water pump and some 12V fridge. This can be supported with 300-500W solar and 30A controller. With fridge being the biggest unknown. Lights, pump, radio, LP sensors and 2 hours of the wide wobbly web will draw 15-20AH. "2 hours" because this is how long a typical 4 AH laptop battery lasts when surfing the web, and in a minimalistic mode you need to charge the battery in daytime when the "big bank" is full and you have a surplus of energy, rather than running laptop through inverter. Unless the location is really bad, like PNW, a flat mounted 150W-200W panel will harvest 20A without problems. OTH, fridge draw depends on the volume and ambient. Well insulated, at temps under 90 a single-door upright unit with a freezer tray tends to work to ~10 AH per 1 cu.ft.

Thanks for the feedback.

Yes, 106 ah daily. I've put in LEDs so should be a little less.

I've been accused of speaking encrypted. By less amps coming down I meant from the panels on the roof to the controller down in the storage compartment behind the batteries. 250 watts @ 12 volts is a lot more amps than 250 watts at 24 volts. And the IsquaredR power lost in heating up the wire will be 1/4 with 24 volt panels. Yes from the controller to the batteries would be the same.

Hmmm... free power. I was mostly thinking getting the batteries fully charged. I spend $3/day on gasoline, $3/day on propane. Getting that second 250 watts on the roof will be hard.

pianotuna wrote:

The Rogue 3048 is state of the art. Just wish he had something bigger.

Yes. 50A would allow 750W array, enough for everything - or almost everything, at least in Southern regions.

Hi Almot,

I make it 106 amp-hours per day.

The Rogue 3048 is state of the art. Just wish he had something bigger.

joelmyer wrote:

westend wrote:

...

FWIW, I built a system for my off-grid rig that has one 24V module, an MPPT controller, and I have 300 AH in two banks. I'm real happy with the performance of all of it.

Details please. Which controller?

I'm planning something similar. We winter 2-3 months in Key West.
I measured Feb 2014:
1-1.5 hours generator time to < 10 amps into 236 ah bank.
30 ah usage from generator off to tv time
3 hours generator/TV time, estimated 36 ah
overnight 30 ah

One 24 volt panel ~250watts versus two 12v panels.

One thing to mount on roof.
Less Amps coming down
More expensive controller.

Offtopic, but since you asked...
In the range 300-480W MPPT I wouldn't look any further than Rogue 3048. Simply the best, in terms of features for $. Good quality. With 250W panel this will also leave room for expansion.

About "one thing to mount" - anybody who did it, will tell you that after you've brought yourself to drilling the 1st hole in the roof, the rest 20 or 30 holes are easy, and then it doesn't matter how many :)...

Don't know why "less amps coming down". Amps at the controller output depend on panel wattage. You will likely get the same amps from 250W 24V panel as from 2*130W 12V panels, though less than from 2*150W 12V panels.

I'm a little confused about your consumption numbers, but it looks like you have 15+30+36+30=111 AH. On a good day a 400W array will collect that much, if not more. Makes a lot of sense getting 400-450W. 2*24V panels.

Hi fooddude,

I have no further information about the danfoss demand.


It does appear you are committed to 12 volt. In that case I'd still go with four batteries. I'd base the solar system on no less than 60 watts per 100 amp-hours of storage which would be 276 watts (300 for good measure).

Two 150 watt nominal 12 volt panels would, as BFL13, suggested give better flexibility to choose a controller (in series MPPT, in parallel pwm).

pianotuna, good info. In regards to the danfoss compressor - those will cycle to maintain temperature correct? So it would only be running a percentage of the day... regardless, even if it's only running 25% of the day, that's still almost 20 amp hours additional daily usage. Something to definitely think about...

Unfortunately I already have the PD-4000 installed, which I think only does 12-volt, and I already purchased the Shurflo 4008, which I think has a specific 24-volt option. Additionally, my LED lamps all say 12V... I think contrary to what I previously thought, I may be stuck :/ Guess I should have done more research, but after countless hours reading websites and such I started to go cross-eyed and just picked some stuff. Oh well, good info for my next conversion....

Hi,

I do understand your desire to be minimalist. I mostly boondock.

I love danfoss compressor units, but the makers are claiming 3 amps per hour @ 12 volts. That works out to 72 amp-hours per day.

Remember, if you want the bank to last you don't have 230 amp-hours--you really only have 115 except in dire emergencies where you trade off cycle life for convenience.

Due to the Peukert effect doubling the capacity more than doubles the resources at your disposal.

If you have purchased nothing so far, I'd definitely go to 24 volts and 4 batteries.

Do you plan on giving up the wobbly wide web? If not, how many hours per day will a laptop be running.

By going to 24 volts for the battery bank, you can use a larger higher voltage panel and a cheap controller (do get one that is temperature compensated). The savings may more than pay for the extra batteries. Voltage at 24 means line losses are cut in half on dc wiring.

Once you buy items on one voltage you are pretty much committed. Yes, you can, like 2oldman use a DC to DC converter to move 24 down to 12, but that is an extra cost, and causes some system parasitic losses. And DC to DC converters, as good as they are, are still not 100% efficient.

As batteries age, they loose capacity. Build in enough over capacity, and the battery bank lasts a lot longer.

Finally, it is always better to have too much capacity, than not quite enough.

Lesson learned by me is that two 12v panels give you the option of going MPPT in series or in parallel for PWM at similar daily AH haul.

IMO you could start with a $30 or less (ebay) PWM controller for the two 12s and then if you really "felt the need" you could go MPPT.