solar panel advantage of a bigger panel

[F3RR3T]

What are the advantages of having say a 240w panel vs a 85w panel if charging say 2 batteries.

I assume for instance if you only have a hour of sun and have a 240w panel you got the same energy from 3 hours of a 85w panel being in the sun correct?

Personally i will be living in my mega-tear and will be hooked into power. But solar is a nice backup for if i go off grid for a weekend. Battery wise i have 4 Odyssey PC1200's

[eamarquardt]

The specs for the battery say you can get 44 amp hours out of each battery with a reasonable load. If you draw 25 amps (100 amps for the four) you'll get 78 minutes or somewhat less than the 44 amp hours. So for your four batteries you'll have 176 amp hours available which is pretty good. Unless you go hog wild I don't think you'd need any charging system for a weekend. I'm guessing a reefer would need 3 amps for a 33% duty cycle or 24 or so amp hours a day for a medium Engle type box. With some lighting, a reefer, a fan on low I think you'd still make it through a couple of days without recharging.

If you want sustainable solar power make an estimate of your needs in amp hours. I don't have any experience with solar cells but the ratings are probably best case senerios (the sellers want to look as good as possible!). Maybe others can share their actual experience but I think a reasonable SWAG of what you can expect to get in a full day is maybe the equilivant of 4 hours rated capacity. For the 85 watt panel that would be maybe 28 amp hours and for the 240 watt panel that would be 80 amp hours. Remember you can return home with "empty" batteries so if your weekend useage is less than the 176 amp hours you get from your batteries plus what you can get from your solar cells you'll have enough.

In short, figure out how much you're gonna need to get you through a weekend and act accordingly.

Cheers,

Gus

[Engineer Guy]

Eamarquardt offers sage advice and solid math, as usual... Some other wizards on this Forum have nice installs.

See some Websites here to dive in and tweak your System to your needs. Solar House/Off Grid folks already have figured out what you need to know:

http://www.google.com/search?sourceid=c ... lar+panels

http://www.irv2.com/forums/

http://www.backwoodssolar.com/

In the 'Tech' section of the 'irv2' Site, some RV folks share their real world Solar insights. A fellow linked from there is VERY keen on what many would consider over-sized Panel -> Battery wiring [#4 vs. #10 AWG], but he makes some sound points. He also advocates putting charging and monitoring right at the Batteries so there's 'no' losses. He advocates charging up a Battery, with the Charger/Tender right there, to whatever the Battery Manf. suggests; not some voltage that is 'conventional' wisdom. These tricks net you more stored energy. This fellow further suggests using a House-quality, not RV, Panel -> Battery Tender/Manager. They have some nice performance subtleties that net you more power.

The newer >200 Watts Panels offer rooftop space economy. Another consideration might be to run the numbers of price/Watt. However, 'big enough' wiring and connectors cause multiple Panels to cost more installed. In the Trade, what you're looking for is 'Balance Of System' [BOS] re: Panel -> Batteries -> Control Electronics sizing WITH some System growth potential, hopefully. One fellow dumps 'excess' wattage into his Electric Water Heater in which he replaced the Anode plug with a resistance heating element.

Tilting Panels toward the Sun, based on Latitude, is one good trick. Following the Sun via [House] tracking Arrays, or Panel relocation throughout the day, can net you >20% more power. Staying out of shadows is another. Panels DO put out power on cloudy days; just not as much, obviously. Water and Snow 'bounce' cause higher Panel output.

Lots of Panels output more wattage in Sun than specified. Big Home Systems have Panels wired in series, not parallel. So the Array outputs high voltage at modest Amps, allowing the wiring to be smaller gauge. This would be unnecessary on a TD. But, this is why Home Systems have up to 600 VDC input specs on some Control electronics.

Knowledge is power, no pun intended, so knowing Battery condition via Instrumentation is key. You can put what are called 'pin jacks' in a convenient spot and plug Multimeter probes into those. This lets you 'recycle' a Multimeter into a Battery readout device w/o having access to the Batteries. Just use 'good size' wire so that the Multimeter reads voltage w/o significant, wire-based, voltage drop losses.

[astrotrailer]

I went with 160 watts to give me enough solar gain in the winter to keep my battery charged when the solar gain is less. My main load in the winter is running the propane furnace and my computer. I have way more than I need in the middle of the summer, but I use the extra power to run my Engel fridge in the hotter weather and to run personal swamp cooler on hot sunny afternoons. I have a Trojan 130 amp-hour deep cycle battery.

It doesn't make sense to have a 200 watt panel and only a small amp-hour battery to store the solar power. Batteries have an optimal recharge rate based on their capacity and I wouldn't put in more solar capacity than you need to meet that recharge rate.

There are a lot of good deals on 80-200 watt panels. With $500 you can buy a very nice panel and charger. You may need to buy a better deep cycle battery to make the best use of the solar system.

[bdosborn]

What are the advantages of having say a 240w panel vs a 85w panel if charging say 2 batteries.

Quick summary of bigger versus smaller.

Advantages:
-Slightly better price/watt.
- You'll probably will have enough capacity that you can leave the panel flat (I do at 220 watts).

Disadvantage:
-Shipping on panels over 135W can get murderously high because of the size of the panel.
-Most panels larger than 135W or so are at a voltage higher than 18V, (which requires an expensive MPPT controller) so check the panel voltage before you buy.
- The bigger the panel the more your first cost is.


One thing people often overlook is that you can only realistically expect to see 80% of a panels output, best case. The highest output I've seen from mine is 172W with the panels flat and in full sun. Extra capacity helps with the real world output of PV panels and cloudy days. That being said, an 85W panel on our teardrop worked so well I could leave it flat unless it was a cloudy day.

The rule of thumb for solar capacity: 1W/amp-hr of battery capacity. I have 220 amp-hrs of battery and 220 Watts of solar works nicely to replace 100% of my nightly usage. I have multiple panels but not because I planned it that way. I originally went with a 135w panel and added an 85W panel later because the price on panels has fallen so much I couldn't resist .
Bruce

[eamarquardt]

I have multiple panels but not because I planned it that way. I originally went with a 135w panel and added an 85W panel later because the price on panels has fallen so much I couldn't resist .
Bruce

I think having a couple of smaller panels might offer more flexibility and might be physically easier to handle and store than one big panel. Also, if you have an accident (Murphy was an optomist) and break a panel, you still have some capacity and the replacement cost would be less for a smaller panel than a larger panel. If your load is light and there is lots of light you might put out only one panel versus two. My story (and I'd stick to it) would be that I deliberately went with two smaller panels.

Just a thought. If I'm off the mark for one reason or another let me know.

Cheers,

Gus

[bdosborn]

I have multiple panels but not because I planned it that way. I originally went with a 135w panel and added an 85W panel later because the price on panels has fallen so much I couldn't resist .
Bruce

I think having a couple of smaller panels might offer more flexibility and might be physically easier to handle and store than one big panel. Also, if you have an accident (Murphy was an optomist) and break a panel, you still have some capacity and the replacement cost would be less for a smaller panel than a larger panel. If your load is light and there is lots of light you might put out only one panel versus two. My story (and I'd stick to it) would be that I deliberately went with two smaller panels.

Just a thought. If I'm off the mark for one reason or another let me know.

Cheers,

Gus

I agree. Multiple panels also lets you grow your system over time after you get a better handle of how much capacity you really need. That way you don't have to cough up as much money up front and you might find you don't need as much PV as you first thought.

Bruce