A Guide to Buying Solar Panels

Which solar panels should you buy?  When you decide to install a solar PV system, you’re making a commitment to purchase a product that you’ll be living with for the next 25 years. Aside from your house, few investments have that long a lifespan. Given that, and the size of the investment – the typical home solar PV system can cost £6,500 to £10,000 installed – you would think people would put as much thought and care into buying solar panels as they do into buying their next automobile.

Unfortunately, the factors that influence quality in solar panels aren’t as commonly and easily understood as engine sizes and fuel economy. The ratings are littered with jargon and industry-speak and numbers that make little sense to most laypersons. Here’s a brief overview of the factors that you should consider when deciding which solar panels to buy.

Wattage Ratings

The wattage rating should be a good guide to determining the output and power of solar panels, but it’s not as simple as all that. The nameplate rating on solar panels is arrived at under what they call “Standard Test Conditions”, which often bear little resemblance to the typical British seasons. Those conditions are considerably cooler than most real world situations – and solar PV efficiency drops considerably as the panels heat up. Instead of going by the STC rating, look for the PTC rating – PVUSA Testing Conditions. Those figures will give you an output rating that’s far closer to real world performance than the STC rating.

Power Tolerance

On the spec sheet for your solar panels, you see a listing called “Power Tolerance”. That figure represents the range of performance you’ll see based on the wattage rating. For example, a power tolerance figure of +/-5 on solar panels that are rated for 200 watts means that the panels will be able to output between 195 and 205 watts at peak power. That doesn’t sound like it makes much difference until you consider the effects across an entire solar PV array, which may consist of as many as 16 panels. If all of them are operating at 10 percent below the actual wattage rating, it’s a significant difference.

Keep in mind that solar panels with a lower wattage and tight power tolerance rating can actually outperform those with higher wattage and a larger range.

Performance Guarantees

Look for solar panels with a performance guarantee. Most reputable manufacturers will offer one that guarantees a minimum level of output over a fixed period of time. The most common guarantee is for 90 percent of the maximum output for 10 years and 80 percent of the maximum for 25 years.

Temperature Coefficient

Some manufacturers provide a number called a Temperature Coefficient of Power or TCoP. The figure tells you the percentage of power loss suffered by the solar panels for each degree above 0 C the solar panels reach. As noted above, solar panels lose efficiency at higher temperatures. The TCoP measures that power loss. Look for manufacturers who provide a TCoP rating, and choose solar panels with TCoP ratings of -0.5 percent or less. If there is no TCoP rating, expect that the panels perform more poorly than that in hot conditions.

When you talk with a solar PV consultant, ask about the solar panels used by his company for installation. Many companies buy their solar panels from only one source, while others offer solar panels from a variety of manufacturers. The more you know about the system to be installed the better able you will be to make the best choice for your needs.