The heart of every PV system is the array of photovoltaic modules. Today, the overwhelming majority of PV modules (more than 95%) are crystalline silicon.

But despite the fact that most PV modules utilize similar technology, there can be considerable variations in performance. As such, we consider understanding four key differences critical to making a smart decision between different proposals. So here we go...

1. Nameplate Ratings versus Real-World Performance (or STC v. PTC... WTF?)

It would seem obvious that a "208" module is better than a "195", right? Not so fast grasshopper. What if the so-called 208 only really put out 183.3 watts while the "195" put out 183.5 watts? Confused yet?

The explanation lies in the manufacturers' use of the "Nameplate rating" in the model number and how they derive those figures. For example, Sharp's ND2081U module has a nameplate rating of 208 watts, and this comes from the testing protocol used by Sharp and every other PV manufacturer. In the factory, PV manufacturers use Standard Test Conditions (STC) which keep the PV test temperature at a nice constant 78 degrees F. In the real world, even a chilly day can see hotter PV module temperatures than that since dark surfaces absorb light and heat. On a hot summer day, PV modules can heat up to over 150 degrees F. And there's the rub.

So the industry came up with a better testing protocol -- called "Photovoltaics for Utility Scale Applications" (PV-USA) or PVUSA Test Conditions (PTC). PTC were developed at the PV USA test site in Davis, California and represent a more real life condition of 20 degrees C. ambient temperature (68 F.), 10 meters above ground level, and wind speed of 1 meter per second. As PV cells average about 20 degrees C. above ambient temperature in the real world (104 F.) and cell voltage drops as temperature increases, a PV module's power output in real life conditions will always be lower than the power measured at the factory where cell temperature is maintained at a controlled 77 degrees F. (25 C). But since there are differences in technology and manufacturing processes, the ratio of PTC to STC can vary dramatically. So the first lesson in evaluating PV modules is to ignore the nameplate rating and go right for the PTC ratings. But some manufacturers don't publish these data on their spec sheet. (!) Don't worry, you can look up any module sold in California here (The CA Energy Commission's web site), and here are some of the best selling ones.

2. Watts Are Important, but So Are Inches and Feet

Now that you know about the difference between STC and PTC watts, you can simply just make sure you buy the highest PTC (used by the CA Energy Commission, or CEC) rating available, right? Alas, it's not that simple. What probably matters more than the wattage of a PV module is the watts generated per square foot. The wide range of PV technologies available today range from 5 watts per square foot to 15 watts per square foot ("WPSF"), and 200% improvement over the bottom end of the range!

As the above chart shows, many common modules are in the 10-11 WPSF range but even the difference between 10 and 11 is 10%, so it makes sense to only get as space-efficient product as you need. If products in the 10-11 WPSF range won't generate enough power, then paying a small premium for higher efficiency products like Sanyo could be a smart investment.

While there is a lot of discussion lately about "thin film" solar products (~5% of the market) such as amorphous (non-crystalline) Silicon, CdTe (Cadmium Telluride) and CIGS (Copper, Indium, Gallium and a form of Selenium), these materials usually yield from 5-9 WPSF, typically ruling them out for most applications due to lack of space.

3. Some Like it Hot, Others... Well, Not So Much

The next thing that anyone considering an investment in solar power needs to know is the degree to which hot temperatures will degrade the performance of their system. Photovoltaic modules perform best at cooler temperatures, and some lose less production as temperature rises than others, due to the materials and technologies used. In the field, due to their light-absorbing properties, solar cells will average about 20 degrees centigrade above ambient temperature, or (all in Fahrenheit) 106 degrees on a 70 degree day, or 136 degrees on a 100 degree day.

Temperature Coefficient of Power ("TCoP")is expressed at the percentage loss with each one degree increase in temperature (Celsius). The closer to zero (the less negative the number is), the better it's hot weather performance. If a PV module has a TCoP of -0.50%, then that module will lose 10% of it's output with a 20 degree centigrade increase in cell temperature -- a truly amazing stat. (Aren't you glad you know this now?) You may note that some manufacturers don't publish this data ("n/a"). We think that it's not good business to withhold important information from a customer and we now have a policy that we won't sell any products that don't have published data on all key performance metrics.

4. In This Case, Being Intolerant is a Good Thing

As the manufacturing process for making PV modules includes a lot of variables, each and every PV module is tested at the end of the manufacturing line to determine its performance and nearly every manufacturer take some latitude in allowing modules that fall slightly short of their intended rating. For example, if Company X was looking to make 200-watt modules (STC), and one tests at 191 watts, then they might label it as a "200" and note a "tolerance" of +/- 5%. Because these products are priced on a per-watt basis, then Company X selling this module as a "200-watt" yields 4.7% more revenue for the manufacturer while giving the customer 4.5% less output. Pretty shocking, isn't it?

As a result of this practice, you can see there's little incentive to ever sell a module in the "plus" range (a module intended as a "200" that tests at 209 watts), because they will simply sell it as a "220", assuming the same +/-5% tolerance.

For the US market, the most common tolerance is +/-5%, yet there are some manufacturers that sell at +/-9% and there are some that actually sell at +/-0%.

As you can see, every PV module varies in important ways. At Sunlight Electric, we believe there can be a place in the market for each and every one of these products, as getting more of one thing usually involves trading off another, and it's our job to educate our customers as to the trade-offs and make recommendations on the optimum choices based on our experience.