How CNET's Solar Table Helps Us Test Rooftop and Portable Solar Panels Objectively

Every solar panel company makes bold claims about its panel efficiency, maximum wattage availability, temperature coefficient and other specs in optimal conditions. Anyone who does product testing knows that bold claims on spec sheets don't always measure up when it comes to hands-on tests, making it hard to pick the right solar panel for you. 

That's where CNET's solar table comes in, helping us test rooftop and portable solar panels in an objective, transparent and repeatable way. 

The solar table was built by the team at our Louisville testing lab, where CNET experts test rooftop and portable solar panels, solar batteries, portable power stations, solar generators and more. You'll only find it at CNET labs. The solar panel testing table allows us to test the solar panels under ideal testing conditions, control external factors like air mass and ambient temperature and measure the wattage and panel temperature over time. 

This proprietary technology will give us objective numbers in our hands-on testing to help us compare panels across manufacturers and types. 

How does CNET currently test solar panels?

CNET has been comparing and evaluating rooftop and portable solar panels for years, with a holistic approach to evaluating the specs and comparing them across manufacturers. Where specs haven't been available, we'd reach out to the manufacturer to gather information and use our own expertise and experience with solar panels to identify ones that are poorly made or have any other issues. 

Type of solar panel 

We looked at factors like the different types of solar panels. Most residential solar panels these days are monocrystalline models. They typically produce more electricity than polycrystalline or thin-film solar panels, giving them a better return on investment. 

Solar panel efficiency 

We also look at solar panel efficiency, which is the amount of sunlight energy a solar panel can turn into electricity. For example, a solar panel with 20% efficiency is not converting 80% of the sunlight that hits it into electricity. Efficiency tends to vary across panel types and manufacturers, but to give you a frame of reference, the most efficient residential solar panel currently available is a Maxeon panel with a 24.1% efficiency rating. 

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Temperature coefficient

CNET also considers the temperature coefficient since solar panels become less efficient as temperature rises. That's why they can become less effective during extreme heat waves. The temperature coefficient rating measures the amount solar panel efficiency changes for every degree Celsius over 25 (77 degrees Fahrenheit). If a solar panel has a temperature coefficient of -0.5%, it means the amount of solar energy production it loses for each degree Celsius of temperature rise.  The way it works is you look at the panel's output at two different temperatures. Say your panel goes from 25 degrees Celsius (77 degrees Fahrenheit) to 35 degrees Celsius (95 degrees Fahrenheit), and output drops from 100 to 95 watts.

You would take that 5-watt decrease in production and divide it by the 10-degree increase (always Celsius here): 5/10 = 0.5. That's your temperature coefficient. Essentially, this is the percent of power output you lose for each increase in temperature of 1 degree Celsius. 

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Wattage rating 

Solar panel size plays a role in wattage rating. On paper, the higher the wattage rating, the more power the solar panel will produce. A 400-watt solar panel, in perfect conditions, should produce 400 watt-hours of electricity. Conditions can often be impacted by clouds, particulate matter (pollution, dust, wildfire smoke) and other things that come between the sun and the panel. 

Other factors

Finally, CNET experts examine other factors surrounding the solar panel, including the cost and production warranty level and length. Depending on the manufacturer, the standard warranty length for solar panels can range from 10 to 30 years. Longer is better. On a more minor level, we also consider the appearance and color of the solar panel, with some preferring a sleek, all-black. 

The scoring breakdown chart also gives you an idea of how CNET weights each factor when calculating a score. Up until now, CNET hasn't been able to give each of these factors an objective, comparable measurement, but that's set to change with the solar testing table. 

What is CNET's solar testing table and how does it work? 

The solar table is a proprietary tool built by CNET to help us measure the capabilities of solar panels and compare them across manufacturers. It also helps us evaluate power output and temperature response under controlled conditions designed to mimic one sun, or one kW/m2 of light. This is the output at standard test conditions, with a solar panel set at a 90-degree angle to the sun at the equator with an air mass of 1.5.

It's built with custom parts, but essentially, it consists of a 6- by 10-foot extruded aluminum, adjustable-height table with up to five 1,000-watt LED solar simulating lights mounted above it to replicate sunlight. Yes, these are cannabis grow lights. While they don't perfectly replicate the sun's full spectrum, they do come awfully close.

The entire array is covered under a Mylar heat-retentive enclosure, while the testing room itself contains a ventilation system that includes a ducted portable AC unit, heater, and in-line ventilation fans. There are a pair of K-type thermocouples, one measuring the air temperature under the Mylar sheet and another measuring the temperature of the solar panel itself, allowing for constant, precise temperature monitoring.

Also in the room, you'll see an iTech 8513B+ DC resistive load bank to draw up to 600 watts of power, along with current and voltage transducers for output measurement. Finally, CNET uses proprietary software for temperature and power output monitoring and logging.

How we'll use the solar table to recommend solar panels for you 

Our goal is to control as many external factors that impact solar panels as possible while collecting useful, repeatable data.

Here are the steps CNET experts follow while testing:

1. Our tester centers the solar panel on the table underneath the lights, being sure to orient the cable connections to the side nearest the DC load and connected cabling.
2. We run the thermocouple cable between lights to end in contact with the surface of the panel while ensuring it blocks none of the light from reaching the panel surface.
3. We engage the room AC system and, if needed, an additional box fan and portable AC.
4. We calculate the operating resistance for the model we're testing by dividing the operating voltage by the operating current.
5. We configure the DC load bank in constant resistance mode and set the resistance value to the operating resistance calculated previously.
6. Monitor software until ambient enclosure temperature, as well as panel surface temperature, is below 65 degrees Fahrenheit.

This testing produces raw data that show the room temperature, room humidity, Current (A), Voltage (V), Wattage, Corrected Wattage, Ambient Temperature and Panel Temperature.  

This allows us to measure and chart various factors. In the above example of a Duracell 100-watt portable solar panel, we can chart the wattage versus panel temperature over time, confirming that as panel temperature increases, wattage decreases. This lets us compare the solar panel's tested efficiency to the efficiency the company claims in its specifications.

Going forward, CNET will be able to gather this data for every single rooftop and portable solar panel we recommend so that you'll know the solar panels you're getting installed are the best ones for you. 

We'll also continue to evaluate more subjective factors, based on our experience with particular solar panel brands, the appearance of the solar panels themselves and more. 

We also need to give you the important caveat that the average person getting a rooftop solar panel installed at their residence will have little to no choice when it comes to hardware. 

Most solar installers will have a preferred set of hardware they will install, and you may only have one or two choices if you're able to weigh in at all. Not to mention that panels need to be sized right for your home and use case. You may want a Maxeon panel because you've heard good things about it, and the installer may have a good reason for pushing you toward a different option based on your specific needs and use case. 

That's why it's equally important, if not more important, to pick the right solar company to install your solar system. The ease of installation, financing options and warranty and customer service support you get are all directly impacted by your solar company. Balancing the objective measurements of solar panel hardware, with more subjective factors is how CNET experts will make the best solar panel recommendations for you.