In today’s market, the vast majority of solar panels produce between 250 and 400 watts of clean energy. On your solar installation quote, you might see a number like 245W, 300W or 345W next to the name of each panel. They all refer to a solar panel’s capacity, power output and wattage.

How to Calculate How Much Energy a Solar Panel Produces

All solar panels are rated according to how much DC (direct current) power they produce when tested under standard conditions.

The output of a solar panel is expressed in units of watts (W) and represents the theoretical power production of the panel under ideal sunlight and temperature conditions. The wattage of a panel is calculated by multiplying volts x amps. Volts refer to the force of electricity and amperes (amps) determine how much energy is being used over time.

Most home solar panels have power output ratings ranging from 250 to 400 watts, with higher power ratings generally considered better than lower ones. Pricing in solar is usually expressed as dollars per watt ($/W) and the overall cost of your solar system depends on the total watts generated by those panels.

For instance, if you’re a California homeowner who gets 5 hours of direct sunlight per day, you could calculate your solar panel output like this:

• 5 hours x 290 watts (an example wattage of a premium solar panel) = 1,450 watts-hours, or roughly 1.5 kilowatt-hours (kWh)

So, the output for each solar panel in your array will be about 500-550 kWh of energy per year.

What Factors Determine How Much Power a Solar Panel Generates?

The amount of energy a solar panel can produce depends on two key factors: cell efficiency and solar panel size.

Let’s take a closer look at each one of these factors.

Solar Panel Efficiency

Today, most solar cells are made of silicon and can convert 20 percent or more of the sunlight that hits them into usable energy. This has led to solar panels that are able to generate more than 400 watts of power.

Higher efficiency = more energy produced. As a result, high-efficiency solar panels convert more of the sun’s energy into electricity to produce more power for your home.

Number of Solar Cells and Solar Panel Size

To simplify, we can divide solar panels into two groups based on their size: 60-cell and 72-cell. Most 60-cell solar panels are roughly 5.4 feet tall by 3.25 feet wide and can generate 270 to 300 watts of electricity per panel.

On the other hand, 72-cell panels are larger than 60-cell panels because they have an extra row of cells. They’re able to produce an average of 350–400 watts. Because of their large size and weight, 72-cell panels are typically used on commercial solar projects, not on residential homes.

Environmental Factors: Shading, Orientation, and Hours of Sunlight

The efficiency of a solar panel and the number/size of its individual solar cells directly impact its rated power output.

In the real world however, other factors affect how much power a solar panel will actually produce:

Shading will produce less electricity from your solar panels. Though a solar panel’s wattage rating indicates the amount of energy it can produce when in full sunlight, other factors—such as shade from trees or buildings—can lower that output.

Your panel’s orientation can impact your solar panel’s output production, but a panel’s output rating doesn’t take this into account. Ideally, your panels should be positioned so that they face directly towards the sun. In reality, roof planes are rarely made at maximum efficiency angles.

Hours of sunlight refers to the amount of time during a day or year that your panels are exposed to the sun’s rays. More hours of sunlight for your solar panel means higher energy output.

What Can You Power With a Single Solar Panel?

In the above example, the solar panel produces 1.5 kilowatt-hours of electricity per day, or about 45 kilowatt-hours per month. That’s enough energy to power a handful of small appliances. In order to produce enough energy to offset usage of your AC unit, refrigerator, cooking appliances, etc.—you’ll need more panels.

How Much Energy Will an Entire Solar Panel System Produce?

The amount of solar energy produced by a single panel is important, but it’s also necessary to know how much power you can generate on your roof. Let’s do the math:

Using the example above, let’s say you get an average of five hours of sunlight daily (this is an average amount for most California homeowners) and your solar panels are rated at 290 W. If you install 30 of those premium solar panels on your roof—which would be an 8.7 kW system—you could net an 8,700 watt, or 8.7 kW solar panel system.

When you multiply the five hours of direct sunlight estimated above by 8.7 kW, you get approximately 43.5 kWh of electricity produced daily. A final conversion will tell us how many kWh the solar panels produce in a year: multiply 43.5 by 365 days, and you get 15,800 kWh of electricity produced annually by 30 premium, 290 W panels. Since the average American family uses about 10,600 kilowatt hours of electricity per year, that would be more than enough energy to run your home on solar power. Again, that’s if your usage is within the 10,600 kilowatt hour average. 

Why Does Solar Panel Output Matter?

Power output is one of the most important measurements for your home or commercial solar energy system. Solar photovoltaic energy systems are typically priced by the amount of electricity they can produce (expressed in watts or kilowatts).

Solar panel wattage refers to a panels’ ideal power production under perfect sunlight and temperature conditions. The wattage is calculated by multiplying volts x amps, where volts represent the force of electricity and amperes (amps) refer to how much energy that current draws.

For grid-tied systems, open-circuit voltage and the voltage of your system are less significant factors. For homeowners who want to go off-grid or install a home battery storage solution, it’s an important part of a PV system. The savings from your solar energy system come from the electricity it generates over time (expressed in kilowatt-hours).

How Do Size and Quantity Impact Output? 

It is important for consumers to consider the overall quality and performance of a solar panel, not just its power output.

Some panels have a higher power output rating because they are physically larger, rather than more efficient or technologically superior.

If two solar panels both have 15 percent efficiency ratings, but one has a power output rating of 250 watts and the other is rated at 300 watts, it means that the 300-watt panel is about 20 percent physically larger. This is why measuring a panel’s efficiency—as opposed to simply its capacity—better reflects how well it performs. 

Practically speaking, a 5kW (kilowatt) solar panel system could consist of either 20 250-watt panels or 16 300-watt panels. Both systems will generate the same amount of power in the same location. While a 5kW system may produce 6,000 kilowatt-hours (kWh) of electricity each year in Boston, that same system is expected to produce 8,000 kWh yearly in Los Angeles due to the amount of sun.

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