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  • Writer's pictureSarah Lozanova

Solar Panel Wattage & Output, Explained

Updated: May 18, 2022

Before installing panels, solar shoppers often get several quotes for a solar energy system. Unfortunately, it can be tough for a novice to compare bids for an apples-to-apples comparison. One of the obvious likely differences is in the solar panels. Often, the wattage of solar panels and the manufacturer varies by the solar proposal.

In general, the higher the wattage, the greater the solar power production. However, the topic is a bit more complicated than that. So, let’s explore how solar panel efficiency, wattage, and the number of solar panels in an array impact total solar electricity output so that you can educate your customers on this important topic.

Solar Panel Ratings: The Basics

Solar panel ratings can be a critical consideration for solar shoppers. If they see that the solar modules in a competing bid have a higher rating, they will likely take their business elsewhere. However, there is more to a solar panel than just the efficiency and wattage of the panel. Often, solar installers pick a specific product based on the warranties, durability test results, prices, appearance, and other factors.

It can be helpful to inform customers on why you like certain solar panels and why they are an attractive option. Helping them understand the wattage and efficiency is an excellent place to start. Many solar installation companies have this information listed in their proposals and mention it when conducting a sales visit or site survey.


Solar panel efficiency ratings have increased significantly in the last decade or two, enabling more homes to produce 100% of their electricity needs from a rooftop solar system. Solar panel efficiency relates to the amount of sunlight that is converted into solar electricity. A higher percentage means a more efficient solar module.

High-efficiency solar panels generate more solar energy in a limited amount of space. Solar panel efficiency is especially critical when space is tight, and a homeowner wants to maximize production from every inch of space possible.

Most solar panels are 14% – 23% efficient, and some residential solar panel manufacturers are known for producing more efficient panels, including LG, Panasonic, and SunPower. Also, efficiency can vary within the solar panels from a given manufacturer depending on the specific model. Typically, black solar cells (monocrystalline) are more efficient than blue solar cells (polycrystalline).

Also, the efficiency of a solar panel decreases over time due to degradation, but not all panels degrade at the same rate. The power performance guarantee 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.

From an efficiency standpoint, there are two primary considerations: how efficient the solar panel is on day one, and how efficient it will be 5, 10, 20, or more years into its lifespan. The output at the end of the warranty period can provide long-term generation information.

Unfortunately, more efficient solar panels often have a higher cost. When space for solar panels is not limited, high-efficiency solar panels may not be one of the top deciding factors when selecting a photovoltaic module. Other times, efficiency may be less critical than warranties, aesthetics, durability, country of origin, price, and other considerations.

If you have customers who seem overly concerned about higher efficiency panels, help them identify if this is a top priority compared to other features. Sometimes, solar shoppers think it is critical, even if they have plenty of roof space with direct sunlight for their array. Perhaps they would rather have a lower efficiency solar panel but add solar batteries for energy storage capabilities or extend the warranty on their inverter.


The wattage of a solar panel reflects its performance under optimal conditions. To calculate the wattage of a solar panel, just multiply the amps by the volts. The wattage of solar panels has increased significantly in the last decade or two, with most solar panels on the market for residential and commercial use with wattages in the 300s or low 400s.

However, wattage and efficiency are not synonymous because the solar panel size varies by model and manufacturer. For example, a small solar panel may be highly efficient but not have a high wattage rating because of its small size. Likewise, a relatively large solar panel with a low efficiency will not necessarily have a high wattage.

Size And Number Of Solar Panels

To calculate the capacity of a solar system, multiply the number of solar panels by their wattage. When all else is equal, solar power systems with more and higher wattage panels will generate a greater amount of energy. The same applies if a couple of solar panels aren’t functioning properly. Just having a few panels malfunctioning can make or break a home being net zero with net energy billing.


Although solar panels have a nameplate reading for optimum conditions, the actual power output can be significantly lower when there are not ideal conditions. For example, if the solar panels are covered with ice or snow, actual panel power output decreases considerably. Likewise, cloudy conditions or even high temperature conditions can decrease PV panel output.

Typically, solar generation is higher in the summer months than in the winter. One reason for this is that the days are the longest in June and July and the shortest in December and January. Also, there is less chance that panels will be covered in snow, and many climates are sunnier in the summer than during the winter.

For this reason, solar installers don’t calculate annual power generation just on the hours of sunlight and the wattage of the solar system. Unfortunately, these are do not cover all real-world conditions.

Online estimation tools like the PVWatts calculator from the National Renewable Energy Laboratory take weather data into account and the available sun hours. Although the weather can vary a bit seasonally, PVWatts estimates tend to be very accurate.

Why Does Solar Panel Output Matter?

When designing a solar panel system, you want the output to create the most value for the customer. Often, if a solar system is oversized, the customer will not get additional benefit from it because of how net metering rules work.

Likewise, if a solar system is undersized, the customer might still need to pay more than necessary on their electricity bill. Therefore, solar output is critical for optimum sizing, which helps boost the return on investment of the solar system.

How Many Solar Panels Does The Average American Household Need?

The average home needs about a 6-kW solar system to produce all of the kilowatt hours of electricity it consumes throughout the year. However, the required amount of power can vary considerably based on the size of the home, types of loads, and lifestyle habits.

Typically, a home that’s heated with electricity will need a larger solar system than a home that uses natural gas, especially if it is located in a colder climate. Often, newer homes use less electricity because they have newer appliances, are well insulated and air sealed, and do not use inefficient incandescent lightbulbs. Regularly charging an electric vehicle or running air conditioning also consumes a lot of power.

If you install 370-watt Panasonic Evervolt panels, you would need about 16 for a 6-kW system. However, if you use a higher wattage module, you need fewer panels. For example, with the 400-watt REC Alpha or the 400-watt Q CELLS Q.Peak Duo, you would only need 15 panels for a 6-kW array. If you compare these to the 250-watt solar panels that were widespread about a decade ago, you would have needed about 24 250-watt solar panels for the same output.

Average Wattage Of Solar Panels

The average wattage of solar modules keeps increasing as solar technology advances and matures. Many models are in the mid- to upper 300-kW range, but some top 400, including the Tesla T420s, which have an output rating of 420 watts.

Conclusion: Take Care To Explain Solar Panel Ratings To Clients

When solar customers shop around, they often examine their solar panel options with different installers. The power rating and efficiency of solar panels are easy metrics to compare and something that many potential customers are considering. Being able to explain why power ratings are important and how they impact total system output is essential for providing excellent customer service.

For more articles to help explain challenging solar concepts to prospective clients, read the GreenLancer blog.

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