How to read solar module specs

Start by taking a look at a typical 12V solar module spec sheet. Here is one for a 100 watt Grape Solar product, a popular module sold through THD and elsewhere.

At the top you'll see an overview touting the product attributes. Pretty much everything listed there is normal for any standard glass front silicon solar module, as is the 10 year/90% 25 year/80% warranty. The 50 lbs per square foot wind and snow loading is roughly enough to hold up to a wind speed of 125 mph, so if you roof mount one of these no towing at speeds higher than that..... The built in bypass diodes are there for higher voltage systems when you string multiple modules in series, they won't do anything for you in 12V systems. 

Moving down to the mechanical specs you see that the module has 36 156mm x 104mm solar cells (or 4x6 inches, the 4.4 inch dimension is an error). The cells are all connected in series in the module. The output cables are 900mm (about 3 ft) and use MC4 "comparable" connectors. "Comparable" is an interesting choice of words, I'll bet a lawyer was involved in that one....MC4 is the standard for connecting solar modules. You'll need mating connectors to connect to these. IP65 is the IEC rating system, similar to the US NEMA ratings  it means the junction box on the module is dust tight and protected against water jets, but is not water tight (you can't immerse it in water). 

Next the electrical specs. 

The first one says that you can use this module in circuits up to 600V in the US (UL rating) and 1000V in Europe (IEC rating). This is important for larger residential and commercial systems with lots of modules in series but not for our trailers. 

The second is the 100 watt power rating. The 0%, +6% means that none of the modules should test lower than 100watts but could test as high as 106 watts. This is rated at standard test conditions which are 25C (77F) module temperature, 1000 watts/square meter irradiance, and something called AM1.5. 1000 watts/square meter is the typical full solar irradiance you get on a clear day with the module pointed at the sun, at sea level, no clouds or high humidity. This is also know as "1 sun". 

AM stands for air mass, which defines the light spectrum that the module sees in the test. The more atmosphere the sun shines through tne redder the light gets, because the atmosphere scatters the blue light, which is why sunsets are red and the sky is blue. AM1.5 is defined by a sun angle from the zenith of about 48 degrees which give 1.5 times the shortest path length of the sun's rays (AM1, when the sun is directly overhead). 

The third line specifies the type of silicon crystal that the solar cells are made from, in this case polycrystalline (as opposed to monocrystalline for some other products). Mono cell modules are a bit higher in efficiency but tend to cost more. You'll see lots of reasons from various manufacturers about the pros and cons of mono vs poly, but other than efficiency vs price, it really doesn't matter in practice. 

The 4th line specifies the voltage where the 100 watts occurs, (Vmp) in this case 18V. Solar modules don't set the voltage when charging a battery, the battery does (unless you're using a MPPT controller), so if your battery voltage does not match this Vmp you can't get 100 watts from the module. Of course, you never charge a 12V battery at 18V, so you will never get 100 watts from the module (unless you use a MPPT controller and its a really cold day). More on why the 18V later. 

The 5th line specifies the current at max power (Imp), where the 100 watts occurs, in this case 5.56A. While you will never get 18V from the module, you will get close to the 5.56A into your battery whenever you are getting 1 sun irradiance, so  this is the key number needed directly to determine how many amp hours you will supply to your battery for a given number of daily full sun hours. 

You can find the daily sun hours for your location, month, and solar module orientation here: 

The 6th line is the open circuit voltage, the voltage you get if you just connect the module to a voltmeter with no current flowing.  The 7th line is the short circuit current, the current you get when you short out the module leads and the voltage is zero. These numbers are used in the design of larger solar systems but aren't really important for our purposes. 

The next line is the module efficiency, watts electric out divided by watts solar irradiance in. 

The next three lines describe how voltage, current, and power change with module temperature. As you can see, voltage and power go down with increasing temperature, current goes up very slightly. Overall you lose 0.45% power per degree C, practically all in voltage which is why the module starts out at 18V at standard test conditions. As the module heats up, that 18V drops down to something close to the charge voltage of a 12V battery. 

Finally, down at the bottom, titled "other performance data", you see the power tolerance again (0% below, up to 6% above), the operating temp range, the fuse rating (10A) and something called NOCT. The 10A is the fuse size you should use to protect one of these modules in case of a short circuit. If you have more than one module in parallel, you'll need multiple fuses. 

NOCT stands for normal operating cell temperature, which is the temp the cells will get to when the ambient temperature is 20C (68F) and you have 800 watts per square meter (80% sun) irradiance. In this case that is 45 degrees C (113F). This is actually a pretty low temperature, in reality in summer conditions the cells can get to well over 70C (160F), higher in the desert. At 70C your module max power voltage will be down around 14V. 

That's about it but if you've taken the time and aren't bored, feel free to ask any questions.