Nominal power of solar panels: definition, calculation, and impact on system performance

Solar energy has become one of the cornerstones of the global energy transition. Yet anyone who has ever considered installing a photovoltaic system has likely come across a technical but crucial term: nominal power. This value, expressed in Watts-peak (Wp), lies at the very heart of every solar panel’s performance. Understanding what it means, how it’s measured, and how it affects real-world electricity production is essential to making an informed decision when choosing and evaluating your solar setup.

The meaning of nominal power in solar panels and energy production

Nominal power indicates the amount of electrical energy a photovoltaic panel can generate under optimal laboratory conditions. In other words, it’s the panel’s reference power, a standardized value that helps compare products from different manufacturers and estimate the total capacity of a solar system.

A key concept for assessing solar performance

Nominal power provides a theoretical foundation for estimating how much energy a solar panel can produce. However, it shouldn’t be confused with actual power, which varies based on environmental, climatic, and technical factors.

Nominal power vs. real power vs. peak power

• Nominal power is the value declared by the manufacturer.
  
  

• Real power is the actual output under the site’s real operating conditions.
  
  

• Peak power is the maximum output achieved at any given moment under ideal sunlight.
  
  
  

Why understanding nominal power matters

Knowing how to interpret nominal power helps properly size your system, avoid miscalculations in annual production, and plan investment costs more accurately.

What nominal power is and how it’s determined

Each photovoltaic panel has a nominal power rating that represents its ability to generate electricity under Standard Test Conditions (STC). These standardized conditions make it possible to compare different panels fairly and consistently.

Standard Test Conditions (STC)

Laboratory testing is performed with a solar irradiance of 1000 watts per square meter, a cell temperature of 25°C, and a solar spectrum of AM 1.5. Of course, these are ideal conditions rarely replicated outdoors. In real-world environments, panels typically deliver slightly less than their nominal rating.

How cell technology affects nominal power

The nominal power of a panel also depends on the cell technology used. Monocrystalline modules offer higher efficiency and stable performance even in low-light conditions. Polycrystalline panels are more affordable but slightly less efficient. Advanced technologies like PERC, TOPCon, and bifacial modules deliver higher nominal power within the same surface area, enhancing energy yield and long-term return.

How to calculate the total nominal power of a photovoltaic system

The total nominal power of a solar system is obtained by multiplying the power of each module by the total number of panels installed. For example, ten 400 Wp panels provide a system with a nominal capacity of 4 kilowatts-peak (kWp).

Practical examples

• A residential system with ten 400 Wp panels reaches 4 kWp, producing roughly 4,800 to 5,200 kWh per year.
  
  

• A commercial setup with twenty 350 Wp panels delivers 7 kWp, generating around 8,500 kWh annually.
  
  

• An industrial installation with fifty 400 Wp panels totals 20 kWp, producing over 24,000 kWh per year.
  
  
  

Nominal power vs. installed power

Installed power is the sum of the nominal power of all panels connected to the inverter. However, the inverter has a power limit: if the total nominal power exceeds the inverter’s capacity, energy clipping can occur, resulting in lost production.

Nominal, peak, and real power: practical differences

These three terms are often used interchangeably, but they represent distinct and complementary values. Nominal power is the manufacturer’s rated value, peak power is the maximum instantaneous output under ideal sunlight, and real power is the actual power generated in varying conditions throughout the day and the year.

Why real power is lower

Real output is influenced by several factors:

• Ambient temperature, which lowers cell efficiency.
  
  

• Panel orientation and tilt angle, affecting sunlight exposure.
  
  

• Shading and surface dirt, which can significantly reduce generation.
  
  

• Natural degradation of panels over time.
  
  
  

Factors affecting the effective nominal power of a solar system

Temperature effects

Every solar panel has a temperature coefficient, indicating how much power it loses as heat increases. The higher the temperature, the lower the efficiency. In summer, cell temperatures can exceed 45°C, leading to power losses of up to 10%.

Orientation and tilt

In Italy, panels facing south with a tilt angle between 25° and 35° are generally ideal. Even small deviations can affect yearly production. East- or west-facing arrays may generate slightly less energy overall but provide a more balanced output throughout the day.

System losses

Losses caused by inverters, cabling, and electrical connections usually range between 10% and 15%. A high-efficiency inverter and well-designed wiring minimize these losses. Additionally, panel degradation — typically 0.5% to 1% per year — gradually reduces the overall power output.

Cleaning and maintenance

Dust, pollution, and debris can block sunlight and lower efficiency. Regular cleaning can restore up to 5% of lost power. In dusty or coastal areas, routine maintenance is essential to keep performance at its best.

How to choose solar panels based on nominal power

Assess nominal power vs. available space

If roof space is limited, it’s best to opt for high-efficiency panels with greater nominal power. Larger surfaces, instead, allow for more flexibility, prioritizing the best cost-per-watt ratio.

The relationship between nominal power, efficiency, and cost

On average, a photovoltaic system costs between €1,200 and €1,500 per kWp installed. The higher the panel efficiency, the less space required to achieve the same power level — improving both spatial use and return on investment.

System types

Residential systems typically range from 3 to 6 kWp, commercial systems from 6 to 20 kWp, while industrial plants often exceed 20 kWp. In every case, panel selection must align with power needs, available space, and inverter capacity.

Comparing panels

Comparing panels by nominal power alone isn’t enough. You should also evaluate voltage, maximum current, positive tolerance, and performance warranty. A more efficient panel will deliver higher energy output for the same nominal capacity.

Nominal power and efficiency: two complementary indicators

Nominal power measures a panel’s capacity, while efficiency shows how effectively it converts sunlight into electricity. Two panels may have the same nominal rating but differ in actual performance due to surface area, design, or degradation rate.

A 400 W panel with an area of about 1.9 square meters typically has an efficiency of around 21%. Advanced technologies such as HJT and TOPCon now exceed 22%, while bifacial panels can harness reflected light to boost total energy yield.

FAQ: Nominal power in photovoltaic panels

What does nominal power represent?
It’s the power a panel can deliver under ideal laboratory conditions.

Is nominal power the same as real power?
No. Real power depends on local environmental conditions and system efficiency.

How can I estimate the annual production of a 3 kWp system?
On average, between 3,500 and 4,500 kilowatt-hours per year, depending on location.

Do two 400 Wp panels always produce the same energy?
Not necessarily: efficiency, cell quality, and temperature play a key role.

What’s the difference between kWp and kWh?
kWp measures installed power, while kWh indicates the amount of energy produced over time.

Why nominal power matters for an efficient solar system

Understanding the nominal power of solar panels is essential for estimating production, planning your investment, and ensuring maximum efficiency. A well-sized, high-quality system, properly installed, guarantees long-term performance and a stable return on investment. In a world where sustainability is no longer optional, nominal power is the foundation of every smart solar project.