After installing a solar panel array with a total rated power of 4.8 kW solar (for example, 12 x 400W PV panels), you might reasonably expect the PV panels to produce 4.8 kW per hour of electricity (4.8 kWh) during peak sunlight.
But, after monitoring your solar system, you’ll most likely find that the panels are not consistently producing their maximum rated power wattage.
Reduced electricity generation can be incredibly frustrating if you purchase your solar panel array without understanding how rated power for solar panels is calculated.
However, the reduced output makes perfect sense (and can be accounted for) once you know how power ratings for solar panels are determined.
You’ll find that unless conditions are exactly perfect, solar panels rarely produce their maximum rated power output in the real world.
Learn about the many factors that impact solar panel electricity output, including temperature, degradation, location, shading, and more.
What are Standard Test Conditions for Solar Panels?
The rated power for solar panels is determined by a laboratory test under Standard Test Conditions. These conditions are:
- An optimal operating temperature of 77°F (25°C).
- A sea level air mass (AM) of 1.5.
- Solar irradiance of 1000W/m2 (1kW/m2).
Optimal operating temperature refers to the solar cells within the panel — not the panel itself. So the corresponding ambient temperature can vary depending on the panel’s frame, coatings, colour, and other factors. Operating temperature is an important metric, as solar panels work at their highest efficiencies in temperatures between 59°F (15°C) and 95°F (35°C).
Air mass measures the amount of atmosphere that sunlight has to pass through to reach your solar panels. An air mass of 1.0 denotes that the sun is directly overhead, so laboratory tests use 1.5 as a more realistic estimate of real-world conditions.
Solar irradiance refers to the level of light energy shining on a given area. The 1000W/m2 rating means that these tests simulate direct sunlight in the brightest part of the day without any shading or clouds.
Laboratory conditions are ideal and almost never match real-world situations.
The best way to think of rated power wattage for any solar panel (e.g., 110W or 400W) is as a spec that measures the maximum amount of electricity its PV cells can produce per peak sunlight hour. NOT how much they WILL produce hourly.
6 Reasons Why Your Solar Panels May Produce Less Than the Rated Power
1. Heat
Since solar panels convert sunlight into electricity, most people assume a hotter day will generate more energy. This is not the case. While more sunlight generally allows solar panels to produce more power, it can also bring more heat, which actually has the opposite effect.
Solar panels come with a temperature coefficient rating that denotes how excessive heat will decrease the output. The temperature coefficient is expressed in a percentage of efficiency lost for every degree above the standard test conditions. Most panels have a temperature coefficient of around -0.3%/°C, meaning they lose approximately 0.3% of their output for every degree over 25°C.
Expect your panels to perform lower than their rated output on hot summer days. You can’t control the weather, but there are a few steps you can take to reduce the temperature of your solar array.
For instance, if you’re in a warm area, install your panels with a gap between them and the roof. Spacing allows air to flow around the panels and cool them down.
2. Degradation
Solar panel output inevitably decreases with age. Most solar panels are designed to produce at least 80% of their output capacity after 25 years. So, you can expect your panels to produce about 1% less of their maximum rated power output every year. If environmental conditions are harsh, this degradation could occur at a faster rate.
A variety of factors influence the degradation of your solar panels. Physical damage from hail, wind, and heavy snow can play a significant role in panel deterioration. UV radiation from sunlight will cause certain components of the panels to break down slowly. Thermal cycling — the expansion and contraction of components with the day-to-day and seasonal changes in temperature — can also degrade PV panels over time.
Other components in any solar power system also deteriorate over time. The wiring and connections will eventually show signs of age, including increased resistance and efficiency loss. Inspect your array regularly to identify faulty connections and components before they have too big of an impact.
Your solar battery’s storage capacity will also diminish depending on how much you use it. Battery lifespan is determined by cycle life — the number of times you can discharge/recharge the battery fully.
Depending on your solar battery chemistry, the cycle life can be anywhere between 500 (lead acid) to 3,000 cycles (LFP) before a noticeable decline in storage capacity. LiFePO4 (LFP) batteries — like those found in EcoFlow’s current line of portable power stations — tend to offer the most extended cycle life commercially available today.
3. Location
Geographical location is one of the most significant factors in solar panel performance, as this dictates the weather, peak sun hours, and other climate conditions your PV panels will experience.
For example, a solar array in the South East of Britain receives more peak sun hours than a city in the North. More direct sunlight helps your array produce power closer to its rated wattage.
Where you live also determines the orientation and tilt angle your PV panels need to be installed at to produce their maximum wattage. Generally, in the Northern Hemisphere, your panels should face south at a ~40° tilt angle (depending on your latitude).
In addition, the air quality and altitude of your exact location can affect panel output. Smog can reduce the amount of sunlight that reaches solar panels, while a higher altitude may result in a clearer atmosphere and more intense sunlight.
4. Shading
Shade is a significant factor in whether or not a solar panel can produce its rated wattage. The PV cells contained in a panel are connected in series, which causes the output of one cell to affect the rest. A similar effect is observed in a strand of holiday lights — when you remove one bulb, the rest of the lights also go out. So when one cell becomes shaded, the rest of the panel’s output will suffer.
Always install your solar panels where they will not receive shade from buildings, trees, or other structures. Some types of shading, like cloud cover, are unavoidable, but you can take preventive measures to avoid most shade.
If you have a mobile off-grid solar system — perhaps on a van or RV — you can always move your array to sunnier locations throughout the day.
5. Inverter Power Loss
Solar panels produce direct current (DC) power, but your home runs on alternating current (AC) AC electricity. Inverters are responsible for making this conversion from DC to AC power. In the process, some amount of energy is lost — the exact amount depends on the efficiency rating of your inverter. Most modern inverters are rated for 95% or higher efficiency.
Choosing the correct inverter capacity will help reduce power loss. If the inverter is too small, it won’t be able to handle and convert the full power output of your panels. And if the inverter is too big, it may not perform at full efficiency under light electrical loads.
6. Dirt and Debris
Your solar panels are constantly exposed to the elements. Dirt, bird droppings, dust, leaves, snow, and other debris will accumulate on the surface of your solar array. This will obstruct sunlight and reduce the output of your PV panels. Perform regular cleaning and maintenance to combat this effect and keep your panels operating at maximum efficiency.
Always practice extreme caution when performing any maintenance on your solar power system. This is especially important when accessing the roof to clean your panels. Don’t be afraid to hire a professional for potentially-dangerous jobs.
Frequently Asked Questions
Do Solar Panels Produce Their Rated Wattage?
Solar panels can only produce their rated wattage in ideal conditions, including an optimal temperature of 25°C, peak sunlight, freedom from obstructions like debris and shade, and an optimal tilt angle. Such conditions rarely occur in the real world, so solar panels usually produce less than their rated maximum wattage. 75% of rated wattage per peak sun hour is a relatively safe estimate in most climates.
Final Thoughts
Solar panels will rarely achieve an electricity output that matches their rated wattage. Real-world settings never match standard test conditions. At least a slight loss in power is usually inevitable. You can perform preventative care to maximise your panels’ output, including routine cleaning and maintenance, keeping your panels cool, and installing your panels at the optimum angle and orientation. EcoFlow is a leading manufacturer of high-efficiency solar panels, boasting an innovative lineup of rigid, portable, bifacial, and flexible options. Its monocrystalline PV panels meet or exceed the performance of comparable brands, ensuring that you maximise your electricity production.
