Selecting the Right Solar Panels

Switch to Solar considers a number of factors when designing and installing a solar energy system for your home or business, with one of the most important being the selection of your solar panels themselves. Different solar panels perform better in different climate conditions and operating circumstances, and we’re experts in helping you choose the right photovoltaic modules for the typical conditions in Maryland, DC and Virginia. Here’s a bit more on how the we choose solar panels for our clients.

Solar Panel Test Conditions

The electrical output of your solar panels (also known as solar or photovoltaic/PV modules) is rated by manufacturers under standard test conditions (STC). Solar panel model names typically correspond to their rated power at STC. However, this is only one specification to consider when choosing a solar panel.

Standard Test Conditions

Manufacturer solar panel testing is performed under factory conditions, allowing for consistent comparison between photovoltaic modules. However, these test conditions are not comparable to common outdoor operating conditions. Standard Test Conditions are:

    • Solar cell temperature = 25 degrees C (77 degrees F)
    • Solar irradiance (intensity) = 1000 W per square meter (peak sunlight intensity, comparable to clear summer noon time intensity)
    • Solar spectrum as filtered by passing through 1.5 thickness of atmosphere (ASTM Standard Spectrum)

If the manufacturer rates a solar panel’s output at 100 W of power under STC, that solar module will be called a 100-watt solar module. This solar panel will often have a production tolerance of plus or minus 5% of the rating, which means that it can produce 95 W and still be called a 100-watt photovoltaic module.

PVUSA Test Conditions

PVUSA Test Conditions (PTC) attempt to replicate real-world conditions. PVUSA Test Conditions are:

  • 45 degrees C (113 degrees F) cell temperature
  • 1000 W per square meter solar irradiance
  • 1 m/s wind speed

The PTC rating is typically between 70% and 85% of the STC rating. Solar panels have less output under these conditions because of the material properties of the solar modules themselves. Most solar panels become less efficient as their temperature increases, producing less power.

Other Solar Panel Specifications to Consider

Solar Panel Size

The larger the solar cells within your solar panel are, the more sunlight they can intercept and the more current your solar panel can produce. Typical crystalline solar modules have 6-inch, square cells that operate at about 0.5 voltage at maximum power (Vmp) and can produce eight amps or more.

Larger solar panels mean fewer are installed to generate the same amount of power. Installing fewer solar panels means that there will be fewer connections between the solar modules, which can improve system reliability. However, higher voltage solar panels can also limit system design options for homeowners, because residential solar arrays are limited to 600 volts of direct current.

Solar Module Power Tolerance

Power tolerance is the maximum variance between a solar module’s actual power output and its STC rating. For example, a 200 W solar panel with a 5% power tolerance may produce anywhere from 190 to 210 W at STC. To ensure your solar panel can produce the amount of power it is specified for, Switch to Solar looks for a narrow (or positive only) power tolerance.

Solar Panel Rated Power per Square Foot

This value, also known as “power density,” reflects power output at STC per square foot of a solar panel’s area. Higher power density requires less space to produce a certain amount of power. For customers with limited mounting space, Switch to Solar will install solar panels with a higher rated power density.

Solar Panel Efficiency

Solar panel efficiency is calculated based on the power rating of a photovoltaic module—its output under 1,000 W per square meter. Calculating the solar module size in square meters means a ratio can be made to calculate efficiency (1 square foot = 0.0929 square meters).

For limited space, efficient solar panel models might be more effective, but they also cost more. Cost often takes priority over efficiency when there is enough space for the solar panel array.

Maximum Power Temperature Coefficient for Photovoltaic Modules

The maximum power temperature coefficient is the change in your solar panel’s output power at temperatures other than STC. You can use this figure to calculate how much solar module power is lost or gained due to temperature variations. The closer the coefficient is to zero, the less your solar panels’ output will be reduced by increasing temperatures.

Photovoltaic Module Nominal Operating Cell Temperature (NOCT)

The 77-degree F cell temperature used for testing solar panels is not a typical operating condition.

In full sun, solar modules typically operate between 60 to 90 degrees F above the ambient temperature depending on how they are mounted. On a 77-degree day, solar cell temperature may be closer to 122 degrees F. On a hot summer day, solar cell temperature often reaches or exceeds 149 degrees F.

As your solar panels’ temperature rises, voltage decreases, which results in a proportional decrease in power at the same level of irradiance.

The nominal operating cell temperature (NOCT) is measured with your solar panels exposed to an ambient temperature of 68 degrees F and 800 W per square meter irradiance, which is close to the average irradiance an array is subject to. This gives a better idea of the effect of putting your solar panels in the sun. The lower the NOCT, the more power a photovoltaic module produces.

Solar Panel Warranty

Every solar panel carries two warranties:

  • A materials (or workmanship) warranty covers the solar panel’s parts and workmanship, and generally provides service or replacement of defective solar panels by the manufacturer.
  • A power warranty guarantees that the solar panel will produce a certain percent of its rated STC power for a given time period.

A typical solar panel power warranty is 90% of rated power for up to 10 years and 80% for up to 25 years. All solar panels that have a 10-year materials warranty have at least a 25-year, 80% rated-power warranty.

Parts that are used to assemble solar panels, such as quick connects and sealed junction boxes, have their own manufacturers’ warranties and are not covered under a materials warranty.

Solar Panel Reliability and Longevity

Solar panels are very reliable; they typically perform for 20 to 30 years. However, there is currently no industry-wide standard for reliability for solar panels and inverters. Therefore, Switch to Solar independently evaluates all the solar power components it installs based on:

  • Compliance with required and proposed standards
  • Extended reliability testing
  • Quality management systems
  • Warranty return rates
  • Manufacturer reported and independently verified performance data

We typically choose solar panels that meet IEEE 1262 or an equivalent standard for our home and business solar installations.

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