If you’re considering adding solar panels to your home, one of the first questions that most people ask is: “How much electricity does a solar panel actually produce?”
This isn’t a one-size-fits-all answer as there are many different kinds of solar panels, and many different sizes. In short, solar panel production depends on a variety of factors — including panel wattage, efficiency, and total sunlight exposure. At the array level, production is simply a matter of panel output x number of panels. Understanding these variables is the first step in understanding how much offset you can expect when adding solar panels to your home.
With that said, let’s take a closer look at the questions of panel production and efficiency. In this guide, we’ll explore:
How panel production is calculated
The role of efficiency in performance
Differences between monocrystalline and polycrystalline panels
How panel output impacts overall array size
Every solar panel has a wattage rating — typically between 350 and 450 watts for modern residential models. This rating has grown over time, so older panels may produce less electricity, depending on age. The wattage rating tells you the maximum power the panel can produce under Standard Test Conditions (STC): full sunlight at 77°F (25°C) with no shading.
For example:
A 400-watt panel can generate up to 400 watts per hour under ideal conditions.
Over one peak sun hour, that’s 0.4 kilowatt-hours (kWh) of energy.
At this point it would also be beneficial to revisit the difference between a kilowatt, and a kilowatt-hour. In short, Kilowatts (kW) measure power — the rate at which electricity is being produced or consumed at a specific moment. For example, a 7 kW solar array can generate up to 7 kilowatts of power under peak sun conditions. Kilowatt-hours (kWh), on the other hand, measure energy — the total amount of electricity produced or used over time. If that same 7 kW system operates at full capacity for one hour, it will produce 7 kWh of electricity. In simpler terms, kW is like the speed of your car, while kWh is like the total distance you’ve traveled.
Efficiency measures how much of the sunlight hitting the panel is converted into usable electricity. If a panel is rated at 20% efficiency, that simply means that 20% of the sunlight’s energy becomes usable electrical energy — the rest is lost as heat or reflected away from the glass surface.
Higher efficiency = more power is available from less space.
This is critical for homes with limited roof area or complex rooflines that limit the total number of panels that can be installed.
Two panels of the same physical size can have very different outputs depending on efficiency. For example, a 65" x 39" panel at 17% efficiency might be rated at 340 W, but the same size panel at 21% efficiency could be rated at 420 W.
Efficiency isn’t random — it depends on a number of factors:
Type of solar cell – Monocrystalline cells are generally more efficient than polycrystalline (more on that soon).
Cell purity – Fewer defects mean better electron flow.
Temperature coefficient – High heat can reduce efficiency; panels with lower temperature coefficients maintain output better in hot climates.
Anti-reflective coatings – Help panels absorb more light instead of reflecting it.
Manufacturing quality – Premium brands often have tighter quality control, leading to more consistent performance.
When comparing solar panels, two of the most common types you’ll encounter are monocrystalline and polycrystalline. The main difference between them comes down to how the solar cells are made, which in turn affects their efficiency, appearance, and price.
Monocrystalline panels are manufactured from a single, continuous crystal structure, which allows electrons to flow more freely through the cells. This results in higher efficiencies, typically ranging from 19% to 23%, and better performance in low-light conditions. They’re easy to spot by their uniform black color and rounded cell edges, and because they generate more power in the same amount of space, they’re ideal for homes with limited roof area. The tradeoff is cost — monocrystalline panels are generally more expensive per watt than their counterparts.
Polycrystalline panels, on the other hand, are made from multiple fragments of silicon crystals melted together. This production method creates more boundaries within the cell structure, which slightly impedes electron movement and lowers efficiency to around 15% to 18%. Polycrystalline panels tend to have a blue, speckled appearance and square-shaped cells. While they require more panels (and more roof space) to produce the same amount of electricity as monocrystalline panels, they are typically less expensive, making them a cost-effective option when roof space is not a limiting factor.
In hot, sunny climates like Arizona, the higher efficiency of monocrystalline panels often gives them an edge, especially when aiming to maximize production from a limited footprint. However, for homeowners with ample roof space, polycrystalline panels can still be a smart choice for balancing upfront costs with long-term energy savings.
Read more: Polycrystalline Cells vs. Monocrystalline: Which Is Better?
A straightforward way to estimate a solar panel’s daily energy output is to multiply the panel’s wattage by the number of peak sun hours in your location, then adjust for typical system efficiency losses using a factor of about 0.8. For example, a 400-watt monocrystalline panel in Phoenix, which averages around six peak sun hours per day, would produce roughly 400 × 6 × 0.8 = 1.92 kilowatt-hours (kWh) of electricity each day. Over the course of a year, that adds up to approximately 701 kWh per panel.
When discussing solar panel size, it’s important to distinguish between physical dimensions and wattage rating. Most residential panels measure roughly 65 inches by 39 inches, with 60-66 individual solar cells. Commercial solar panels are somewhat larger at roughly 78 inches x 39 inches with 72–78 cells.
A solar panel's wattage rating, however, is determined by the panel’s efficiency and the type of solar cells it uses. This means that two panels with identical dimensions can have very different power ratings — higher-wattage panels often produce more electricity from the same amount of space simply because they are more efficient.
While panel output matters, the total array size ultimately determines the system’s total output, and how much of your home’s electricity is offset by the Sun’s ample energy. Here’s an easy formula to determine total system output of a solar array.
Panel Wattage × Number of Panels ÷ 1,000 = System Size (kW)
Example:
20 panels × 400 W = 8,000 W = 8 kW system.
1.92 kWh/day per panel × 20 = 38.4 kWh/day.
Now let’s look at how you match system production to your home’s usage to arrive at an offset. It all starts by looking up your annual energy usage. Most of the time, this data is available through your utility’s user portal. Here are some easy guides for downloading your usage from SRP or APS.
Once you have the data, simply add up the total kilowatt hours consumed for the entire year. It’s important to have a full year because your energy consumption differs by season - especially in Arizona where summer temperatures drive a dramatic increase in AC use that generally drops just as dramatically in the cooler winter months. Follow this formula to determine the total number of panels and offset.
Formula:
Annual Usage ÷ Annual kWh per Panel = Number of Panels
Example:
Annual usage: 12,000 kWh
Each panel produces 700 kWh/year → 12,000 ÷ 700 ≈ 17.1 panels → round up to 18.
Panel efficiency plays a big role in determining how many panels you need to meet your energy savings goals. For example, upgrading from a 17% efficient polycrystalline panel rated at 350 watts to a 21% efficient monocrystalline panel rated at 425 watts can significantly reduce the total number of panels you need to accommodate on your rooftop. In Phoenix, for example, a home targeting 10,000 kWh of annual production might need around 17 of the 350-watt panels, but only about 14 of the 425-watt panels to achieve the same output. Remember, increased efficiency doesn’t necessarily translate to increased panel size, so a higher efficiency panel will produce the same or more from the same footprint. This is a meaningful difference if roof space is limited or shading blocks a portion of the roof from full sun exposure.
It’s also important to remember that efficiency doesn’t remain constant forever. All solar panels experience gradual performance loss over time, known as degradation. Premium panels often have a slower degradation rate, typically between 0.25% and 0.5% per year, meaning they can maintain a higher percentage of their original output for decades. Monocrystalline panels not only start more efficient but often keep that edge well into their 25- to 30-year lifespan, delivering more electricity — and more savings — over the long term.
In real-world conditions, solar panel output may fluctuate throughout the year or in response to a variety of outside factors. Seasonal variation plays a big role, with longer summer days providing more hours of sunlight for production, while shorter winter days result in less production.
Shading from trees, chimneys, or nearby structures can also reduce output of your solar panels, even if the shaded area is small. Dust, pollen, and other debris that accumulate over time can can also sometimes cut production. Finally, while Arizona’s abundant sunshine generally keeps annual totals high, extreme heat can slightly lower a panel’s instantaneous efficiency. Despite these variables, a well-designed and well-maintained Arizona solar system should deliver strong year-round performance.
Read: Maximize Your Solar Panel Production: Tips and Insights
Solar panel efficiency determines how much of the sunlight hitting the panel is converted into usable electricity. In short, higher efficiency panels offer more watts from the same amount of space. Of the two types of panels - monocrystalline, or “Mono” and polycrystalline or “Poly” - monocrystalline panels generally offer better efficiency and space savings, while polycrystalline panels are typically more affordable but require more roof area to achieve the same output.
Understanding how individual panel production scales into the total array size and output is essential for planning a successful solar installation that meets your savings goals. A professional solar assessment can help guide this process while offering clarity on actual savings numbers based on your local utility rates and your personal consumption patterns throughout the year.
Ready to see how solar could work for your home? Contact us today to schedule your free, no-obligation solar assessment and find out exactly how many panels you need — and which type will give you the best return on your investment.