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Old 03-04-2004, 01:18 PM   #15
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You calculations do make sense, but. You have not taken into account any incoming amperage form any type of solar array. I will admit that the array will not keep up with the demands that you outlined, but they will make a difference. I know on your 34 footer you have precious little roof room. On units without the skylights there are room for panels. On smaller units there is room. I had 4 panels on my 31 foot 77 that did not have as flat a roof as you do.

We installed 4-50 watt 3 AMP panels. I had 2 group 27 series batteries and a 1500 watt inverter. On a sunny day I could make bread in the bread machine (4 hour run time) and still have a full battery bank at the end of the day. Edie was able to run her Hair dryer for as long as was necessary, and we did not have a microwave. But others with the same setup did and they used them. I would not go with a bigger inverter because I could not achieve any additional run times without more batteries. It was a trade off.

I did not have to cart gasoline in the back of my truck, or load and unload even a light 64 LB genset. I could not run my AC. But I was able to do 85% of what I wanted without shore power, or making the guy next to mad because I am running a genset.

I understand the I want to use 100% of the coach's features attitude. Using the AC, Microwave, a hair dryer, etc. That is why I have a MH now. I have my genset at the flip of a switch. In th past I had 85% of it at the flick of a switch too.

Cost wise I think on a 3-5 year plan you are close to break even once you factor gas, and maintenance. Not to mention the possible theft. I also like the idea that whenever the sun shines I am getting power for free.

Bottom line. Some like gensets, some like Inverters, and some like solar + Inverters.

Brett G
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome

Wise men talk because they have something to say; fools, because they have to say something. -- Plato

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Old 03-05-2004, 09:57 AM   #16
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Since you seem to be superior at "new math", maybe you can help me to calculate my needs for a solar array. My original intention was to just have a solar battery maintanance program while my A/S was in storage, and not hooked up to shore power. I found some Siemans panels that are aluminum laminate...and completely shatter proof. The size on them is perfect (16" x 16"), and I figured I could put two in front of my Air Conditioner shroud, and one behind. Since they put out 10 watts each, I would have 30 watts peak. Keep in mind that I will not be using hair dryers, or any other heating elements with an inverter...infact I only have a tiny inverter for the 13" tv if the kids just have to watch it on a rainy day. I am only really concerned about having the juice for my interior lights, and the water pump, and possibly the heater (however in a pinch, I have a portable propane "flameless" heater that doesn't use batteries).

I have two 115 AH sealed batteries from Costco. My question is, will the 4-5 hours a day that those three panels are charging my batteries, make up for the basic 12v demand that I plan to put on them?

I don't want to waist my energy installing the panels if they will be worthless. I guess at the very least, they will be keeping the batteries healthy while in storage.

Any help is appriceatted.

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Old 03-05-2004, 11:42 AM   #17
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When looking at solar industry ratings, keep in mind that to acheive that rating, the panel has to be in the conditions they are rated (i.e. sun 90 degrees to the panel on a clear day, and a panel temperature no higher than 77F degrees). To get the rated output with a horizontally mounted panel, that's solar noon, on July 22nd, on a boat 1 degree (60 nautical miles) south of Key West Florida, with an ambient temperature in the 50's Farenheit. When it's that cool at that latitude at that time of year, we might say hell has frozen over.

The further north from that position you go, the further from July 22nd, and the further from solar noon, and the cloudier the day, the lower the panel output current. The further from temperatures in the 50's, the lower the panel output voltage. Watts=amps X voltage, so power goes down as either goes down.

I can't find the panel you're talking about to see the specs. While it isn't the same type panel you're talking about, here's a 10W monocrystaline Shell/Siemens ST10. I like Shell/Siemens because their flyers give you an idea of best real-world output in a section titled Typical Data at Nominal Cell Operating Temperatures. It says the ST10 10W panel is really about a 6.8W panel.

Here you can see that at 68 degrees F ambient, with a slight breeze, the ST10 output voltage is down to 13.7V (due to the panel temperature), .1V lower than the typical output of a Magnetek or Univolt. At higher ambient temperatures, it will be even lower, and by the time voltage drop across the cables is taken out, it will be even little lower than that.

I believe you'd be lucky to get a 13.2V "float" charge at the batteries from the ST10. Dividing the 6.8W by 13.7, you get about 1/2A real-world peak current, or only enough daily power to run a 1.4A 12V light bulb for 2 hours.

Two of these would be good for keeping the batteries up during storage, except perhaps in the winter, and even a third might be marginal then. And at this low of a power input into the batteries, you don't really need a controller.

For a small vintage Airstream, you already have a lot of battery capacity. If you run one two-bulb incandescent lamp (1.4A per bulb) for 5 hours in the evening (really four hours but an hour of bathroom light too), that's about 15AH, the water pump doesn't use much, maybe 1-2AH. Add an hour of bathroom vent fan and you're still under 19AH/day, with no radio, TV, or other electrical use like modern RVs have with all their circuit boards and modern radios.

If you left on a Saturday and came back on the following Sunday, that's 8 nights, taking your batteries from fully charged down to maybe 30% charge, with no solar or generator. No radio, no TV. Yeah, that depth of discharge is hard on the batteries. Even with 2 rainy days, the 3 ST-10 panels producing maybe 6-7AH/day total over 5 of the 7 camping days, would keep discharge to no lower than about the 50-60% level. So yes, on a much older Airstream, those 3 panels could make a significant difference, if nothing else, in your battery life.

I can't help you with the TV power calculation because I don't know what size or wattage it is. Do you use an amplified or unamplified antenna with it?

And if you can get me a link to a specs page for the panels you're looking at, I could give you a little more accurate picture.
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Old 03-05-2004, 12:27 PM   #18
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One thing I should add about solar is that you can't park in the shade to use it. If parking in the sun means having to run a Fantastic Vent on medium (2.5A) for 10 hours, that's more than doubling the daily electical usage I calculated above. It takes the best daily output of an 100W panel (17-25AH/day depending on time of year) just to offset that fan use.

So you may find times you'd rather park in the shade and skip the gain of the 3 little panels.
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Old 03-05-2004, 05:57 PM   #19
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I have four batteries, two in the standard boxes, and two in separate boxes in the front compartment, under the couch. The two extra are sealed batteries, so no fumes to worry about. I have not found out much about my solar panels.. I will try to get a close look, when I get the trailer, and a ladder in the same place..

I will be carrying a Coleman 3500 gen set as well, so I may be sufficiently powered..

Theo -- really enjoying his Long, Long Trailer..
'05 Cargo hauler gooseneck, carrying an '05 Jeep TJ, all hauled by an '05 C4500 Kodiak Truck
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Old 03-06-2004, 01:48 PM   #20
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Let me give you an example of TV use. Our 9" Philips color TV is rated at 50W, which at 120V should be about 420 milliamps (mA). It might draw that much with a scene of a snow covered field on a sunny day, with brightness and contrast turned all the way up, as well as the 1812 Overture playing with the volume turned all the way up! LOL! On average while watching most shows, it runs around 210mA, or about 25W. A small inverter powering it would be drawing about 2.5A on average from the batteries. So 4 hours use would be 10AH, 8 hours would be 20AH and so on.

Our satellite receiver is rated at 120VAC, 1A, but doesn't pull anywhere near that. Feeding our two dual-LNB antenna, it draws about 170mA normally, and goes up to 210-220mA when searching for a satellite. So it's normally running about 20W and an inverter powering it should be drawing 2A from the batteries.

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