Solar panels on the Van

[SWbySWesty]

I found a place offering an 85Watt solar kit for $320. That's a sweet price.

UL Solar

For those of you with solar panels on your van, could you tell us about your system and what you are powering? I'm trying to gauge what size panel to use for the van so that I can base camp without idling or needing to drive around to run my radio, fridge, LED lighting and occasional laptop use. (yes I posted in the 12v/power subforum, but it isn't very active).

Let's hear your solutions!

[pods8]

Whats in the "kit"? Off hand the doesn't necessarily sound like a sweet price to me if its just the panel and a cheap PWM solar controller for instance.

Edit: I think its this one: http://www.ul-solar.com/product_p/stp085p-kit-pr.htm

Okay price for the components but I'm guessing you don't need the bracket or 30' worth of cable so you're wasting money there. You can get an 80W panel for $175 and that specific charge controller for $90 at solar blvd, then add the wire length you truly need. However since you're piece mealing it together you can pick any controller you want, you can get a cheaper BZ MPPT controller for $106 (I don't know the reputation of BZ but I've been eying it up myself to try). Just my $.02

[SWbySWesty]

Nice advice. Do you think generally that 85W is enough or maybe 100+ watts would be better?
As I read more, I'm becoming more comfortable piecing it together from local deals to save $$.

[pods8]

Nice advice. Do you think generally that 85W is enough or maybe 100+ watts would be better?
As I read more, I'm becoming more comfortable piecing it together from local deals to save $$.

Your call based on how much sun you're getting and draws, etc. People have posted up their thoughts in the past and 80-100watt range seem fairly common in areas with good sun from what I've read. You mentioned fridge, I assume we're talking a 12V compressor fridge (ie low amp draw) right?

I don't have solar yet so I can't weigh in more heavily, I'm just pointing out sources I've been eying up for myself. Since I'm in more overcast country (PNW) if I finally trigger on a solar setup I was leaning more towards 175watt 24V panel on a MPPT controller. My thought process there is that would be over-sized for my needs in full sun but I'm usually not going to get full sun. In overcast conditions I believe solar cells start putting out less voltage if I understand correctly (same amperage but since the volts are down you end up with less wattage). A 12V panel is ~17V in full sun so there isn't much room to drop in voltage before it falls to an unusable level for battery charging (which needs to be more like 14V). However a 24V panel is ~35V in full sun so in overcast conditions there is a much larger span of usable voltage before it drops too low for battery charging. Again if I'm understanding correctly a MPPT controller does a DC to DC voltage conversion to optimize the usage of the solar panel output. So normally a 175 watt 24V panel is putting out 4.95amps at 35.2V. If you used a PWM controller to drop that to 14V it just chops off the extra charge and you'd only get 4.95amps at 14V, where as a MPPT controller should convert that over to 12.45amps at 14V (in full sun). In periods of overcast conditions as long as the panel is still kicking out around 14V I'd be able to get at least 4.95amps from it, any additional voltage would be converted over to more amps. Again that is my understanding of how it works, someone correct me if I'm off base.

[dwh]

Solar Guppy is an electronic engineer who does R&D, design and testing of solar equipment under contract to various manufacturers, including Xantrex and Morningstar (i.e., he does this for a living). One of the things they hire him to do, is to evaluate competitor's products. He has a full test lab and has tested many, if not most, of the solar charge controllers and inverters on the market.

This guy doesn't pull punches and he doesn't play favorites. He tested a couple of BZ controllers in his lab. Here's his opinion:

http://www.wind-sun.com/ForumVB/show...2&postcount=45


Also, here's his opinion of lowbuck eBay ""MPPT" charge controllers, some of which he has personally tested in his lab - in a nutshell, they were PWM units that were sold as MPPT units...buyer beware (post #6) :

http://www.wind-sun.com/ForumVB/showthread.php?t=7850

[SWbySWesty]

OK, so I understand getting a bigger panel, but as a solar n00b, does the charge controller convert the 24V from the panel to the 12V required by the house battery system or are you using a 24V house system?

In reality, most of my usage comes at night. I'm generally out and about during the day (hiking, moto-ing, whatever). I basically want to get a full charge on my two batteries during the day. I'm in Southern Cali and most trips are to the desert or beach. Open sun. I'm looking to have the panel removable for mtn trips but that's neither here nor there.

[dwh]

My thought process there is that would be over-sized for my needs in full sun but I'm usually not going to get full sun. In overcast conditions I believe solar cells start putting out less voltage if I understand correctly...

You've got it right, but I think you may be over estimating how much power you'll get in overcast conditions. Most reports I've seen for non-thin film PV modules would seem to indicate that you can expect around 10% of the PV module's rated output in overcast.

I'd look for people in your area who have some solar installed and see if I could find out what they are getting in the real world in your area.

[dwh]

OK, so I understand getting a bigger panel, but as a solar n00b, does the charge controller convert the 24V from the panel to the 12V required by the house battery system or are you using a 24V house system?

"12v", "24v", etc. are what are called "nominal voltage". Those numbers basically indicate a range, not an exact voltage.


In order for power to flow from the solar to the battery, the solar has to have a have a voltage (you can think of voltage like pressure) higher than the battery. So most "12v" solar panels actually put out around 17v - 21v.

To charge a "24v" battery, you have to get the battery up to 28.8v, and so a "24v" solar panel will actually put out over 32v.

Thus, any PV module (solar panel) that puts out under 32v or so would be considered a "12v" panel since it's voltage is too low to be used on a "24v" system.


Only one type of solar charge controller does any voltage conversion - MPPT.

Normal charge controllers, and also the newer technology (but not as new as MPPT) "PWM" controllers just connect the solar panel straight through to the battery and then cut off when the battery gets up to a certain voltage. Unlike a "regular old charge controller", which just cuts off when the battery is full and then cuts in again when the battery voltage drops to a certain point, PWM will "pulse" many times a second, checking the battery voltage and giving it a little shot whenever needed to keep it topped off.

The solar panel voltage has to be somewhat higher than the battery voltage for the power to flow toward the battery, but the bigger the difference, the less the efficiency - so a say, 85w panel that puts out 17v will charge a "12v" battery more efficiently than an 85w panel that puts out 30v.

MPPT functions like a transmission - it can down convert the higher voltage from a solar panel to a lower voltage which can more efficiently charge a battery. The neat trick about that, is if you have the same watts, but lower the volts, the amps goes up. It's a bit like downshifting a tranny to get more torque.

However, MPPT controllers (at least the good ones) aren't cheap, and they only get you maybe a 10% (or less) bump in amps on average, so for a small system - usually not worth the expense.

In reality, most of my usage comes at night. I'm generally out and about during the day (hiking, moto-ing, whatever). I basically want to get a full charge on my two batteries during the day. I'm in Southern Cali and most trips are to the desert or beach. Open sun. I'm looking to have the panel removable for mtn trips but that's neither here nor there.

For a flat mount panel in SoCal, I'd figure on 5 hours (on average) of "good sun" per day. So, 85w * 5 hours = 425 watt*hours per day of harvest.

Divide that by the voltage, 425wh / 13v = 32.6 amp*hours per day.

So that's a decent guess as to your average harvest - 32ah/day. Now as long as your loads don't suck more than 32ah out of the battery in 24 hours, an 85w panel should be able to keep up with the demand.

You could get a couple more hours of harvest per day if you put the panel on a tilt mount and parked so it pointed at the sun, or detached it and set it facing the sun.

Also keep in mind when figuring out the mounting that -any- shade on a solar panel, even a small shadow, will cut the panel's output down a LOT...often to pretty near nothing.

[SWbySWesty]

Thanks for the info. My battery system is 110AH total capacity. I run an Engel40 (2.2 amps her hour, maybe 16 hours out of 24 it's on). I don't know the power of the laptop, but it wouldn't be for long (a movie's length). The only thing is that I'd plug my laptop into my stereo which is 17watts RMS (52peak) and a 200watt sub amp. Not sure the power on that.

[pods8]

Solar Guppy is an electronic engineer who does R&D, design and testing of solar equipment under contract to various manufacturers, including Xantrex and Morningstar (i.e., he does this for a living). One of the things they hire him to do, is to evaluate competitor's products. He has a full test lab and has tested many, if not most, of the solar charge controllers and inverters on the market.

This guy doesn't pull punches and he doesn't play favorites. He tested a couple of BZ controllers in his lab. Here's his opinion:

http://www.wind-sun.com/ForumVB/show...2&postcount=45

That one was the BZ 500, I was looking at the 250 but I understand the same issues likely apply. I'll definitely do some more reading before triggering (I'm not that antsy to buy yet).

Edit: Thanks for the good link (and subsequent searches/reads it lead to) I burned a lunch hour on it. Lots to think about, but a general conclusion is forget the BZ controller.

You've got it right, but I think you may be over estimating how much power you'll get in overcast conditions. Most reports I've seen for non-thin film PV modules would seem to indicate that you can expect around 10% of the PV module's rated output in overcast.

I'd look for people in your area who have some solar installed and see if I could find out what they are getting in the real world in your area.

I wasn't aware the drop off would be so sharp but I had similar thoughts about trying to get some real world reports first. Thanks for the info.