How to understand and compare solar panel specifications
The specifications outlined in a solar panel's datasheet provide insights into its expected performance under specific conditions. When shopping for solar panels, it can be hard to identify the most crucial metrics to pick the best solar panel.
We recommend focusing on key specifications such as power output, efficiency, and the temperature coefficient of the panel. Depending on your location, other ratings may also prove valuable. Considering these factors, you can make a more informed decision when selecting a solar panel and comparing solar quotes.
Power output
Listed as: Pmax, PMPP
The power output of solar panels is a fundamental rating measured under Standard Test Conditions (STC), a standardized set of laboratory conditions for testing all solar panels. Sometimes referred to as the panel’s wattage or size, the power output describes the amount of power a solar panel can produce.
Most home solar panels today typically boast power ratings of around 400 watts. However, panels with at least 370 watts can effectively meet the needs of most homeowners.
Understanding a panel's power output is crucial as it directly influences the number of panels required to cover your electricity bill. Higher power output means fewer panels are needed.
It’s important to note that although panels are tested under STC for comparison purposes, how they operate in real-world conditions will differ. To better understand performance in practical scenarios, some panels undergo additional testing under PVUSA test conditions or PTC (PVUSA Test Conditions). Some manufacturers, like Silfab, provide PTC ratings, which can also be labeled NOTC (Nominal Operating Cell Temperature).
Module efficiency
The efficiency of a solar panel represents the percentage of sunlight that the panel can convert into usable electricity. The average solar panel has efficiency ratings ranging from 19% to 21%, while the highest efficiency home solar panels currently available boast maximum efficiency ratings of up to 22.8%.
Some brands prominently feature the efficiency rating in their marketing to attract customers. However, it's crucial to note that the power output is a more reliable indicator of a solar panel's energy production. The panel's efficiency is already factored in when its output is rated.
Efficiency becomes particularly significant for homeowners with limited roof space. High-efficiency panels can generate more power in less space, making them an excellent choice for smaller roofs. However, for those with ample space, panels within the average efficiency range are more than capable of meeting energy needs.
Temperature coefficient
Listed as: Temperature coefficient (PMPP), Temperature coefficient (Pmax)
Solar cells, constructed from semiconductor materials, see a decrease in voltage as their temperature rises. Simply put, as solar panels get hotter, their power output decreases slightly.
The effect temperature has on solar performance is represented by the temperature coefficient, which measures how much power output drops for every degree above 25°C the surface of a solar panel reaches.
Most solar panels today have a temperature coefficient between -0.3% and -0.5% per degree Celcius. The closer the temperature coefficient is to zero, the better. For example, Panasonic’s EverVolt panels stand out with some of the market’s best temperature coefficients at -0.26%.
Keep in mind that the performance decreases due to temperature are generally minimal. Nevertheless, the temperature coefficient shows how the panel can handle real-world conditions.
Cell type
Each solar panel is made up of a number of individual solar cells that have been wired together. The type and construction of the solar cells will impact how the panel performs.
Monocrystalline solar cells are widely preferred for home installations as they are the most efficient and look great! Some websites may mention polycrystalline and thin-film solar panels, but these aren’t typically used in home installations.
Monocrystalline solar cells can be manufactured in several ways to enhance performance. Modern solar panels use PERC (passivated emitted rear contact) or TOPCon cells, which feature additional layers to boost efficiency and output ratings.
Many manufacturers use half-cut solar cells, where the traditional cell is cut in half to decrease resistive losses, and the entire panel is wired to minimize shade's impact.
Dimensions
The physical size and weight of a solar panel are crucial factors, particularly for homes with complex or older roofs.
Typically, solar panels weigh around 40 pounds, with dimensions exceeding five feet in length and 3.25 feet in width. Specific dimensions can vary among different solar panel models.
In most cases, residential roofs can easily bear the additional weight of solar panels and accommodate an average-sized solar system. However, for older homes, you should seek a professional inspection to ensure the roof can safely support solar panels. Additionally, if your roof features a complex shape, the dimensions of the panels become more important.
Generally, your solar installer can find the right-sized panels for your home based on your roof’s characteristics.
Maximum surface loads
Listed as: Maximum static load, snow load, wind load
Solar panels are installed on your roof, exposing them to all the elements! Manufacturers design panels with this in mind, so they’re built to weather high winds and snow.
Most solar panels are rated to hold 5,000 or more pascals (Pa) of pressure, which equates to two to four feet of snow, depending on the snow’s density. Some homeowners don’t have to worry about this - but those who live in snowy regions may want to know how much snow their solar panels can handle!
Additionally, panels are built to endure wind speeds of about 141 miles per hour, typically only experienced in hurricane-like conditions. If you experience these winds, you’ll have more than damaged solar panels to worry about!
Finding the right solar installer can help you get the best solar panels
We believe it’s important for consumers to understand some of the technical aspects of solar panels, like their power output and efficiency, so that they are better prepared to compare solar quotes and not get tricked by unsavory solar salespeople.
But, when you find a reliable solar company, you can trust that they will use good quality solar panels. Finding a trustworthy solar company is the key to getting the best solar installation possible. Check consumer reviews, see how long the company has been in business, and shop around to better understand what is offered in your area.
Teddy Theil
Solar Industry Expert
Andy is arguably the most qualified rooftop solar expert in America. He is in a unique position, having founded a large solar company but now being independent of any particular company. He has appeared on CNBC, has been referenced in public hearings concerning the rooftop solar industry ( such as the recent CPUC NEM 3 decision) and has been referenced by many major publications such as the Wall Street Journal, Business Week and many others.