This is the fourth DIY solar post in a series on solar power systems, here covering the best solar panels for you and how to choose between different solar panel manufacturers.  While this application was done for an RV solar panel system, the concepts are intended to apply for any do it yourself solar power system.  We hope you enjoy this entry (The Solar Series: Choosing Solar Panels) and links to the rest of the DIY solar panel system series can be found at the bottom.

After determining how much electricity we’re going to use, where we’re geographically building our off-grid solar power system and what that translates to in terms of the size of the components (System Sizing), we’re ready to start selecting parts and solar panel manufacturers.  Because they’re the most important and satisfying part of this whole exercise, we’ll look at the solar panels themselves first.  This will include an explanation of the different crystal cell types, array voltages, temperature coefficients, solar panel efficiency, product comparisons and why to prefer one over the other.  Cost, size, power requirements and all kinds of factors come into play which is why there’s no specific answer, only you can select the best solar panels for you!

If all this seems a little daunting and you just want to honestly know which brand currently makes the best solar panels, I’d go with the Sanyo HIT line, which also happens to be what I own.  As you can see from all my research below, they’re the most efficient solar panels, least temperature sensitive, will work for home and mobile installations and are definitely worth the slightly higher incremental cost, but please read on to see how I drew that conclusion.

Solar Cells: Mono, Poly & Amorphous

There are three different types of solar crystal cell technologies available on the market today: monocrystalline, polycrystalline and amorphous.  These refer to the physical metal/silicone building blocks that make up the actual solar cells.  Each panel contains numerous individual cells that are wired together, and the fabrication of these cells has a large effect on how the panels perform.

Monocrystalline means ‘one crystal’.  In this case each individual cell is grown from a single silicon crystal.  This allows for the most uniform and consistent construction of a solar cell and makes or the best solar panels, but also the most expensive.  I describe it as “best” mainly due to being the most efficient solar panels.  Top of the line monocrystalline panels can have an efficiency of up to 20%, which means of all the sun’s rays that strike the panel, 20% of it is converted to electricity, the rest is converted to heat or reflected.  A higher efficiency number means the panel can be smaller and still produce the same output as a less efficient panel, important if you have space restrictions like on an RV.

Polycrystalline means ‘many crystals’.  While an oversimplification, this is sort of like the bologna or processed meat of the solar world, multiple different crystal bits are fused together to form one solar cell.  You can tell just by looking at them because they have a flaky, woodchip board look to them of small blue chunks, where a monocrystalline panel will just be a solid, uniform colour.  Polycrystalline panels are very widely used because they are cheaper than monocrystalline with efficiencies ranging from 10-15%.  This equates to them taking up roughly 30-50% more space, but they are more cost effective.

Amorphous means ‘lacking form or order’, or in this case no defined crystal structure.  Amorphous panels are created by spraying a very thin silicon layer to create the panels, so while it’s quick and uses less material, the silicon doesn’t form itself into the rigid, uniform crystal structure of the other cell types.  This results in an even greater loss of efficiency down to about 6%, but in turn a lower price.  Also, because of the thin silicon layer, it’s possible to create amorphous solar panels that are flexible, you can essentially buy it by the roll and slap it on anything.  This is the newest of the solar cell technologies listed so it’s not as prevalent (also attributable to the low efficiency) and there’s less variety, but it should become more prominent in the coming years.

Array Voltage – Considerations

As stated in prior posts, your off-grid solar power system is likely going to run off 12 Volts DC in the battery bank and appliances.  So that must mean you need 12 Volt panels, right?  Not necessarily…

The first thing to note is that currently 140 Watts is the largest 12 Volt solar panel commercially available, higher wattage is only available at higher voltages.  So if you’re building a system like mine that requires over 400 Watts, you’d already be looking at 3-4 panels.  This in itself might not be a problem if you have the space, it’s just going to be more work to mount and wire up and slightly more expensive (again, like I said, best solar panels is subjective).

Another point to keep in mind is that all solar panels are listed based on their ‘nominal’ voltage (usually 12, 24, 36, etc.), which means the voltage they run at the very lowest light levels to produce any current.  A panel rated at 12 Volts will output something more like 15-17 Volts in the middle of the day.  This actually turns out to be a good thing because batteries need to be charged by a higher voltage, but it’s something to be aware of, especially with this potential drawback: if your battery bank is large and not overly depleted, it could be sitting at 12.4 Volts early in the morning and the sun striking the panels only causes them to output 12.3 Volts, which means no charging will occur.  It’s only a matter of small percentages, but any sort of waste like this may as well be avoided if possible.

One last variable to consider when selecting array voltage is how far away from the panels you’re going to put the charge controller and batteries and what thickness of wire you’re using.  The wire will act as a resistor and cause power loss.  Checking out this Wire Resistance Calculator and using an example of 30 feet of 10 AWG wire, that equates to 0.03 ohms.  Power loss equals current squared times resistance (P = I^2*R) so for 400 Watts of panels at peak output at 12 Volts is 33 Amps (remember P = V*I) and using the first equation that’s 33 Watts of lost power, over 8%!  Naturally you could use thicker or shorter wire runs, but the exact same setup using 36 Volt panels only has a current of 11 Amps and a power loss of 4 Watts, now down to a much more comforting 1% loss.

Array Voltage – Implementation

I’ve definitely glossed over it to this point, but one of the most important factors of the solar system is that there needs to be a charge controller in between the solar panels and the batteries and the controller choice will affect whether you can use nominal voltages other than 12.  Without poaching too much from the upcoming Choosing a Charge Controller post, if you buy a standard and cheaper PWM controller you will be forced to use panels and batteries of the same voltage, likely 12.  However, if you opt for the more advanced MPPT controller technology (which I use and recommend heavily) then you have opened up the possibility of using a higher array voltage than battery voltage.  This really broadens your options on panel choice.

Lastly, the wiring configuration of the panels can also be used to modify array voltage.  If you decided you needed 260 Watts, got a great deal on two 12 Volt panels but are convinced you’d get better efficiency with a 24 Volt array, you could just wire the two panels in series on the same circuit.  If you wanted to double the output, you could wire two 12 Volt panels in series, then parallel them with another two 12 Volt panels in series, still yielding the desired 24 Volts.  Or alternatively if you’re only looking for 12 Volts, then you could wire all 4 panels in parallel.  I don’t have a definitive answer on the array voltage subject, it’s up to you to weigh the pros and cons based on your individual setup.

Efficiency, Temperature Coefficients and Other Factors

Now that you’ve got a rough idea of the amount of panel wattage you need, and what voltage you want to run them at, it’s time to start comparing actual products.  I just wanted to outline a couple parameters you’ll see on solar panel data sheets from manufacturers and what they mean.

Solar panel efficiency we’ve already touched on, it’s how much energy a panel is able to harvest from the sun, the higher the better.  In some cases you’ll see this broken down even further to cell and module efficiency.  Cell efficiency relates to the raw sun-absorbing material in the panel and how good it is, whereas module efficiency takes into account the construction of the panel as a whole including non-absorbing bits such as the frame.  Obviously module efficiency will always be slightly lower and is the “true” solar panel efficiency.

While the importance of this next number will be applicable in varying degrees depending on where you’re installing your solar off-grid system, the temperature coefficient of a panel can have a drastic effect and is a concept worth understanding regardless.  In short, the power generated by a solar panel is temperature dependant, the colder it is the more it will output due to intricacies of the photoelectric effect.  The temperature coefficient is just a multiplier on how much panel output power will change per degree Celsius (an alternate voltage temperature coefficient is also often available).  A typical power temperature coefficient would be around 0.4%/^oC, centered on 25 ^oC.  So if you have a 200 Watt panel, in peak direct sunlight at 25 ^oC you should get 200 Watts of output like you paid for.  However, if it’s really hot out and the panel gets to 50 ^oC, 0.4% * 25 degrees = 10%, so you’ll only be getting 180 Watts.  Likewise if it’s cold and 0 ^oC you’ll get 220 Watts.

There are a couple important side effects of solar panel temperature dependency.  The first and most relevant for an RV solar power installation like mine is it gets a lot hotter on the aluminum roof than what the weatherman says it is.  On a 25 ^oC day it will easily be 35 ^oC up there, so you need to be acutely aware of this and how it will lower the amount of energy harvested.  Installing the panels with a standoff or some separation for air flow can help this to some extent.

On the cold end of the spectrum, while the extra energy output is nice, you need to make sure your system is designed to handle the corresponding increase in voltage.  If I was building a larger commercial system with two 36 Volt panels in series for a 72 volt array and I bought a charge controller that’s maximum input rating is 80 Volts, if my voltage coefficient is 0.15 V/^oC and it’s -25 ^oC in the middle of winter, all of a sudden I have an 88 Volt array and my charge controller has either shut itself off or is on fire.  Of course if it’s -25 ^oC and you’re where I’m from in Alberta it’s the middle of winter and you aren’t getting much sun anyways, but you still need to be aware of it.

Best Solar Panels Comparison

To tie all the above pieces together, I’m going to use the real life example of how I selected the best solar panels for Bessie the RV.  The first step is obviously to find a few different panels that will suit your parameters of array voltage, wattage output and physical dimensions.  There are plenty of good sites online with Solar Sphere having the best selection, along with Go Green Solar or Real Goods Solar, several of which offer free shipping on larger orders (or if you’re in Alberta or BC check out Larry at Solar Wyse and tell him Blimpy sent you).

Ultimately I arrived at three different panels that could do the trick, the Canadian Solar CS6P 220 Watt (data sheet), the Sanyo HIT 215 Watt (data sheet) or the BP SX 3140 140 Watt (data sheet).  I’ll table out all the other important parameters of each setup from the above datasheets and briefly explain why I went with Sanyo as the best solar panels for me.

	Canadian Solar CS6P	Sanyo HIT 215	BP SX 3140
Cell Type	Polycrystalline	Monocrystalline	Polycrystalline
Power (Watts)	220	215	140
Number of Panels Req’d	2	2	3
Dimensions	64.5” X 38.7”	62.2” X 31.4”	59.4” X 26.5”
Nominal Voltage	24	36	12
Temperature Coeff (%W/ oC)	-0.43	-0.336	-0.5
Price	More	Most	Least

I distilled all the important info here for you, but you should be able to do the same for any brand of solar panel.  One thing worth noting when reading the datasheets, the Sanyo one contains a lot more detailed information and the CS and BP sheets don’t even list their module efficiency.  Sure, you could easily calculate it, but I immediately felt this indicated Sanyo knew they had a superior product worth showcasing, whereas the other two intentionally withheld information they didn’t want you thinking about.

As you can see the three panels miraculously highlight all the parameters I outlined earlier.  They all have different nominal voltages.  The BP panel is much larger per Watt and less efficient because it is polycrystalline, but it’s also the cheapest.  You can also see that all monocrystalline panels are not created equal as each manufacturer has their own unique design process and materials.  The Sanyo is slightly more efficient and much less temperature sensitive, and correspondingly more expensive.

In an ideal world the easiest comparison you’ll be making is price per Watt, the lower the number the better.  These days in Canada that ranges from $3-5/Watt, but it varies greatly and is lower in the US.  I just provide that number as a guide to make sure you’re not getting supremely ripped off.  Once you start factoring in the array voltage and temperature sensitivity stuff it becomes a lot harder to just base it on price tag.

So after all that, how did I ultimately decide on the Sanyo panels?  Temperature sensitivity played a role with me anticipating being in the southern US for extended periods of time.  Nominal voltage didn’t matter because I knew I wanted an MPPT controller.  No, the real reason was simply size.  Take a look at the attached picture of the doodles of the roof of my RV and you’ll see the limiting factor is a width of 3’.  Try as I did to pretend the Canadian Solar would fit, or that I could squeeze on the third BP panel, I knew I was only deluding myself so I dropped the hammer on two brand spanking new Sanyo HIT 215 Watt panels and haven’t looked back since.

Best of all, they work like a dream…because they were designed to!

——————————————————————————————-

So that’s how to select the best solar panels.  Over the coming weeks I will be creating posts for each aspect of the creation of off-grid solar power systems with the hopes that you can follow along and use the the info for your own DIY solar panel system.  Of course it gets a little technically involved in spots, so feel free to ask for clarification in the comments or drop me an e-mail.

Topics

RV Solar Panels – Goals, Rationale

How Do Solar Panels Work – Power, AC & DC

Solar Power System Sizing – How much power do you need?

Best Solar Panels – Selecting the best solar panels for you

Solar Batteries – Selecting the best solar batteries for you

Solar Charge Controller – Choosing between MPPT and PWM

Installation (coming soon…)

Maintenance (coming soon…)

The Solar Lifestyle (coming soon…)

Popularity: 100% [?]