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  • Writer's pictureSarah Lozanova

Understanding Solar Panel Wattage And Output

Updated: Jul 14

Solar panel ratings can be a critical consideration for solar shoppers because they provide information on how much electricity the system will generate. However, many solar shoppers also find solar panel output and wattage information confusing.

If a customer is comparing bids between two companies, one might have higher energy estimates for an almost identical system because they used different assumptions for shading or weather conditions. Being able to explain key terms and concepts to solar shoppers, such as power output, solar panel wattage, amps, efficiency rating, and volts, is critical for helping them make informed decisions. Let’s explore some of the key concepts you need to be able to clarify with your potential solar customers.

What Are Solar Panel Ratings And How Are They Determined?

A solar panel rating measures the peak output of a solar panel in watts, typically under ideal conditions known as peak sun hours. Solar panel wattage ratings usually indicate the maximum energy produced when exposed to direct sunlight at 1000W/square meters. However, factors such as temperature, shade, dust, dirt, weather, orientation, and tilt angle can affect the power output of photovoltaic panels, so solar panel ratings are usually assigned following standard test conditions to achieve consistent results.

This wattage information is used to calculate the capacity of the solar power system by multiplying the solar panel wattage by the number of solar panels in the system. In addition to watt peak, other solar panel ratings include a temperature coefficient, which considers the effect of temperature on the panel's power output, and conversion efficiency, which measures the amount of sunlight converted into electrical energy.

In addition, there are ratings for other types of equipment, including solar inverters and batteries. For example, the efficiency rating of inverters is commonly more than 90%, but never 100%.

Factors That Go Into Solar Panel Ratings

Numerous factors associated with solar panel ratings can help customers compare solar panels.


Solar panel efficiency is a measure of total energy converted into electrical energy and is usually expressed as a percentage. Residential and commercial solar panels have an average efficiency rating of 15 to almost 23%, but researchers have developed more efficient PV panels in laboratories. The most efficient solar panels are commonly a dark, non-reflective color, and usually black solar cells (monocrystalline) are more efficient than blue solar cells (polycrystalline).

Solar panel efficiency ratings can vary between manufacturers and models, and SunPower Maxeon produces some of the most efficient solar panels currently on the market. High-efficiency solar panels are often more expensive per watt, so they are not ideal for all solar customers. Solar panel efficiency is most important when unshaded roof space is limited, and less critical when no space limitations exist.

Keep in mind that models with a high output aren’t necessarily more efficient because the size of solar panels varies. For example, a 450-watt solar panel may be less efficient than a smaller 400-watt panel.


Solar panel wattage is the total amount of power the solar panel can produce in a given amount of time. This is usually measured in watts and is calculated by multiplying the solar panel’s voltage, amperage, and the number of cells in the solar panel. The power rating of a typical solar panel varies between 40-480.

Lower-watt solar panels are commonly smaller and more portable. Although higher-wattage solar panels exist, such as Trina Solar’s 600+ watt module, they are often too large for widespread use.


Solar power output ratings assume ideal conditions for generating solar electricity, and a solar system's total power generation depends on the solar panels' wattage. However, actual power production will vary depending on the weather and sun conditions, such as shading. Modules produce less energy when temperatures exceed 25° C (77° F). Accurately estimating actual solar power output is critical for generating electricity bill cost savings estimates.

Solar panels work best in direct sunlight, so clouds, dust, or smog can reduce power output. On cloudy days, solar panels may produce less than 50 percent of the possible electricity.

Type Of Solar Panel

Monocrystalline solar panels are made from a single crystal or cylindrical ingot of silicon. They usually have a higher efficiency rate and are some of the most popular residential solar panels.

Polycrystalline solar panels are made from raw silicon that has been melted and then cooled to form square-shaped crystals. These solar panels typically have slightly lower efficiency, but are less expensive to produce than monocrystalline panels due to the manufacturing process.

Thin-film solar, also called amorphous silicon solar cells, are made from thin layers of silicon deposited on a substrate, such as glass or metal. These solar panels are commonly less efficient, but they are the least expensive to produce of all panel types and can be easily integrated into existing structures. Thin-film solar panels have great potential for many applications as researchers improve efficiency and output.

Number Of Solar Panels

When all other factors are equal, the greater the number of PV panels, the more renewable energy the array will produce. Where there is significant roof shading but sufficient space, some installers help overcome output issues by installing more solar panels. In addition, off-grid solar systems need more panels plus a solar battery to have home power around the clock.

Orientation And Environmental Factors

Although solar energy system ratings usually assume ideal conditions, real-world conditions vary. Anything that decreases the intensity of the sun hitting the solar PV module can decrease output. For example, if solar panels are oriented facing east instead of on a south-facing roof, they will have more energy production in the morning, but a lower total daily output, when all other factors are equal.

Likewise, dirty solar panels, smog, and even wildfire smoke can all reduce solar electricity generation. Therefore, some solar homeowners may need to clean their panels periodically to prevent dirty, pollen, bird droppings, debris, etc. from accumulating on the face of the solar module.

Solar Panel Degradation

The efficiency of a solar panel decreases by a fraction of a percent over time due to degradation, but not all panels degrade at the same rate. The power performance guarantee from the panel manufacturer offers some indication of what someone can expect from a module over time by looking at the guaranteed output at the end of the warranty period.

Solar panel degradation can be particularly concerning if home energy use increases over time, perhaps from installing an electric vehicle charging station or switching to an electric heating system. Even if the solar system was sized to produce 100% of the home’s electricity, solar panel degradation can prohibit this as the system ages and production decreases somewhat.

How Many Solar Panels Does The Average American Household Need?

The average U.S. home uses 886 kilowatt-hours of electricity per month, and most households need between 17 and 21 solar panels to meet their electricity needs, according to EnergySage.

The number of solar panels needed for the average American household depends on:

  • Home energy usage: If the home consumes more power than the average, they may need more solar panels. Viewing a year or more of electricity bills is a good way to determine total household consumption.

  • Available roof space: The average residential solar panel dimensions are around 65 by 40 inches, but this does vary by the model. Commercial solar panels tend to be larger and have more solar cells.

  • Shading: Shading decreases solar electricity production, so you may need to install more solar panels to compensate.

  • Local climate: The sunnier the climate, the more clean energy the PV modules will produce.

  • Budget: If your client has a limited budget, they may not be able to afford sufficient panels at the outset. But, installers could set the system up for easy expansion in the future if budget changes.

  • Solar panel wattage: You will need more solar panels if using lower-watt panels.

Conclusion: Take Care To Explain Solar Panel Ratings To Clients

An important aspect of providing excellent customer service to potential solar homeowners is providing them with relevant information to make an informed purchasing decision. Being knowledgeable about the solar rating standards and how they relate to the real-world solar panel output helps ensures that customers can confidently decide to go solar. As a solar energy professional, it’s useful to understand solar panel ratings, to show you are knowledgeable about renewable energy.

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