Does your 200-watt solar panel actually produce 100 watts of power? Bring your calculator and notepad, we’re going to do some maths!
Solar panels have gone a long way from a novelty to a reliable source of clean electricity for homes and businesses.
And yet buyers keep asking: How much energy does a solar panel produce?
As an electrical engineer and a solar homeowner, I’ll answer this question from both a technical and real-life point of view.
Most solar panels you can find today are rated between 250 and 550 watts of power. The wattage (W) is what solar manufacturers and installers put first in the product description. To get the energy production of solar panels in a day, we need to multiply that number by the number of peak sun hours.
However, if you read the fine print, those power ratings only count in the most ideal condition — the sky is clear and the sun is more-less directly above.
The actual solar panel's output depends on different factors like the orientation of your roof, weather, shading, time of year, and sun hours.
So, let’s explain each of those factors and see how much you can really get out of your solar panels.
To calculate how much energy a solar panel produces, you need to multiply its power rating by the time period you’re interested in.
Hey, but aren’t energy and power the same thing?
No, and that’s important to understand.
energy = power × time
In other words, energy is the amount of power used in a certain time and it’s measured in kilowatts per hour (kWh).
So, for example, if you’re considering a residential solar panel with a power rating of 500W, you can calculate the energy one solar panel generates in a day by multiplying the power rating with the number of peak sun hours for your location, let’s say 4.5.
500W × 4.5h = 2.25 kWh of electrical energy per day
To see what your solar panels produce a month, you need to multiply that figure by 30.
2.25 kWh × 30 = 67.5 kWh of electricity per month
Keep in mind that in sunny states like California, Arizona, and Florida, you can get 5.25 peak sun hours a day or even more.
And if we know that the average US household uses about 900 kWh per month, we can calculate how many solar panels we need to offset that energy usage
900 kWh ÷ 30 days ÷ 4.5 h = 6.7 kW solar system
A 6.7 kW solar system produces 30.15 kWh of electricity per day.
And to build a 6.7 kW solar system, you need 14 500-watt solar panels.
If you have a smaller household, you could cover your energy use with a less expensive 4 kW solar system that produces 18 kWh of electrical energy per day, and you can build it with just 8 500W solar panels.
Again, this applies in the ideal, testing conditions. In the next sections, I’ll explain things you should look for when planning your solar system.
The efficiency of solar panels is determined by the quality of their solar cells. The quality of a solar cell is related to its conversion efficiency — the percentage of solar energy that the cell converts into usable electricity.
The more efficient the solar cells, the more energy output the solar has, which eventually means you can meet your energy requirements with less surface area or fewer solar panels.
Over time, the efficiency of solar panels has improved from 15% to 20% so nowadays you can find highly-efficient solar panels that reach 23% in solar conversion.
When it comes to solar panel efficiency, the math is clear:
The more efficient the panel, the more it will produce compared to a less efficient panel.
Ok, but what’s more important, the wattage or the efficiency?
Both are equally important, but since the efficiency is always rated at 1m2 or sq. ft. of the panel surface, the best way to put it is this:
A 500W solar panel with a higher efficiency rating is always smaller than a 500W panel with a lower efficiency rating.
This means two things:
For example, a solar panel with an efficiency of 22% is measuring 89.72” by 44.65” which is 2.6m2 or (28 sq. ft.)
We can calculate the solar output of a square meter:
2.6 × 0.22 = 0.57
Multiply that with the number of peak sun hours and you get:
0.57 × 4.5 = 2.57 kilowatt-hours per day
Divided by 10.7 we get the solar output of a square foot of 0.24 kilowatt-hours per day.
This is becoming increasingly important with the current trend of downsized housing.
Also, keep in mind that solar panels lose efficiency over time. This means they produce less electricity from the same amount of sunlight.
The materials that solar cells are made of degrade over time, so the average productive life of solar panels is 30 years.
According to the National Renewable Energy Laboratory (NREL), the output of solar panels degrades at a rate of 0.5% per year. This means a 20-year-old solar panel will produce approximately 90% of the electricity it produced when out of the box.
This means you don’t have to dispose of your solar panels right after the official end-of-life.
They can still run for many years.
Monocrystalline solar panels in general produce the most electricity per square footage.
There are several types of solar panels, but the most popular types are
At this point, you should know that the efficiency of an individual solar cell is not the same as the efficiency of the solar panel.
Solar cell conversion efficiency can reach 42% in some cases, but you’d be hard-pressed to find commercial solar panels with efficiency greater than 27%.
The performance of solar cells is measured in a laboratory, while the solar panel is the whole system that consists of the cell panel, front glass, wiring, and even the way the solar cells are arranged.
Also known as single-crystalline panels, these panels are produced from the purest silicon. This type of crystal is grown using an expensive process as a single piece which is then cut into wafers that will become solar cells. Monocrystalline solar panels are known to deliver the highest efficiency in standard tests when compared to the other two types — from 22% to 27%.
Monocrystalline panels can be rigid or flexible, so nowadays they are the first choice whenever the most power is needed or the space is limited.
You can recognize this type of solar panel by the rounded cell edges and dark color without impurities.
Also known as multi-crystalline panels, polycrystalline panels are less efficient than those made of monocrystalline solar cells. Why? Because of the way they’re made. The silicon for polycrystalline panels is not grown as a single cell but as a block of crystals. These blocks are cut into wafers to make individual solar cells. Most polycrystalline solar panels sold today have an efficiency between 15% and 22%.
You can recognize a polycrystalline panel by square-cut cells and blue speckled color.
Thin-film solar panels are made by covering a glass, plastic, or metal surface with one or more layers of photovoltaic material. Due to this, thin film solar panels are flexible and lightweight, so they used to be the main type for portable solar panels and solar panels for RV use. Thin film solar has pretty much the same output as polycrystalline panels, about 15-22%. However they degrade faster than both mono and poly panels, so their price is also lower.
Thin films are dark in color and have no visible solar cells.
As I mentioned, the rated solar panel wattage and the conversion efficiency refer to the testing conditions, which means ideal sunlight shooting straight down.
However, in real-life scenarios, there are factors that reduce the actual output.
We covered that in the previous section in depth, but let me sum up:
If you want the top output, go for monocrystalline panels. If you have extra roof space, you can go with less efficient polycrystalline panels.
Solar panels with a higher power rating (W) will put out more energy per day/month/year than those with a lower power rating.
The conversion efficiency of a solar panel tells you what percentage of solar energy it can convert into usable electricity. Higher efficiency means a higher energy output, but also that the solar panel can put out more power per square foot.
Latitude is the point on Earth in relation to the equator and it’s measured in degrees. For example, Nebraska has a latitude of 41 degrees, which means the sun is always south of your house and never directly above it.
In this case, you can maximize the output of your solar panels by placing them on the south-facing slope of the roof.
A roof that is tilted at an angle of between 30-45 degrees gives the best overall performance for most latitudes, as it’s able to catch most of the sun hours.
Make sure your roof is free from shadows from nearby trees or buildings. Anything that blocks the sunlight for a part of the day will make your panels less efficient. Minimal shading in the morning is not a problem, but when the sun’s up you want your roof free from shade.
Not all locations get the same amount of sunlight. And this doesn’t depend on the latitude only. Some areas benefit from high atmospheric pressure which results in a lot of sunny days with little cloud cover.
And not only dust. Leaves, twigs, tree sap, and even snow reduce the efficiency of your solar panels. When your panels are covered with dirt, it’s like they're in a shade all day. You can maximize their output by cleaning your solar panels every once in a while.
As crazy as it sounds, hot temperatures actually reduce the efficiency of solar panels. Heat always increases electrical losses, so if you live in a cold part of the country with a lot of sunny days, your panel's efficiency is maxed out.
If you live in a part of the world that gets all four seasons, the output of your panels will change with the seasonal cycle. Again there’s no rule to it. The summer will give you the most sunlight as days are longer, but it’s also the hottest season which takes away a bit of the panels’ efficiency. In fall you can expect more overcast days as well as dealing with extra leaves and debris.
A solar panel produces between 1.1 and 2.5 kilowatt-hours of power in one day, which amounts to 33 to 75 kWh per month.
As an average home in the US uses about 900 kWh, you will need between 27 and 12 solar panels to cover that usage, depending on the panel efficiency and how many watts each solar panel produce.