# Understanding the Nominal Power of a Photovoltaic System

The nominal power of a photovoltaic system, also known as peak power, is the maximum electrical power that the system can produce. Discover how it is calculated and how it affects systems classification.

Knowing the nominal power of a photovoltaic system is essential to navigate between consumption and actual energy needs. But what does peak power really mean, how is it calculated, and how do you size the system?

Let’s take stock of the situation, but remember that in order to design a well performing photovoltaic systems, you need first of all to size them correctly and, therefore, it is advisable to use the right solar design software that combines both design and financial analysis in a single solution.

## What is the nominal power of a photovoltaic system?

The nominal power of a photovoltaic system, also called peak power, is the **maximum electrical power** that the system is capable of producing, calculated with reference to standard operating conditions.

Standard conditions refer to:

- temperature of 25°C;
- incident solar radiation of 1000 Watt/m
^{2}; - sun position at 1.5 AM, where the sun forms an angle of 48° with the zenith.

## How is the power of a photovoltaic system calculated?

The calculation of a photovoltaic system’s power is done by considering the different modules that make up the system, specifically by **summing the individual nominal powers of each module** belonging to the system, obviously **calculated under standard conditions** as seen above.

In recent years, the transition to a sustainable energy system based on renewable sources has become crucial and pressing.

The nominal power of a photovoltaic system is therefore a crucial aspect to consider. Understanding how to accurately calculate and evaluate the power of a photovoltaic system is essential **for designing and installing efficient and reliable systems**. Fortunately, there are specific software for designing photovoltaic systems to assist you.

## What unit of measurement is used for nominal power?

The unit of measurement used to indicate the nominal power of a photovoltaic system is the **kilowatt peak** abbreviated as **kWp**. To avoid confusing this unit of measurement with that of kilowatt-hour, which is instead the unit of measurement of electrical energy, let’s look at the meaning of the letters that make up its abbreviation:

- the
**letter k**stands for kilo, meaning a thousand times the value of the accompanying unit of measurement; - the
**letter W**signifies Watt, the unit of power measurement, so 1kW (kilowatt) = 1,000 Watts; - the
**letter p**stands for peak.

In the photovoltaic sector, therefore, the abbreviation kWp stands for kilowatt peak and is used to indicate the **value of the nominal power**, i.e., the theoretical maximum instantaneous power produced by a module or the entire system. It is worth noting that this is a theoretical power as the electricity production of photovoltaic modules is never constant but varies according to various criteria such as:

- the tilt and orientation of the panels;
- the solar irradiance of the specific site;
- weather conditions.

### What is the difference between kWp and kWh?

As mentioned earlier, it is important **not to confuse the two units of measurement, kWp and kWh**. The recommendation is essential as both refer to electrical power and thus both relate to a photovoltaic system. But how do they differ? The main distinction between the two units of measurement, which would lead to huge confusion if interchanged, lies in the **time factor**.

In general, the kilowatt, equal to 1,000 Watts, is the unit of measurement used to define the **electrical power of a photovoltaic system** and indicates how much energy is produced per second.

The **kWh**, kilowatt-hour, is the **power of electricity produced and supplied in an hour by 1 kW**.

The **kWp** indicates instead the **nominal power of the system**, which in turn represents the average power over a year. The calculation conditions regarding the time factor, between kWh and kWp, are evidently different.

## How to choose the power of a photovoltaic system?

To choose the power of a photovoltaic system and proceed with the sizing of the photovoltaic system, it is essential to refer to the **energy needs** to “dose” the system’s power based on actual and/or projected needs and consumption.

Obviously, the precise calculation must be carried out in relation to the specific project and with specific solar design software. In general, however, we will have smaller photovoltaic systems with a low nominal power – up to 50 kWp – for residential buildings and larger systems with a higher nominal power above 50 kWp for industrial plants.

Let’s see how to get an idea of sizing:

**photovoltaic power for residential buildings**: the first factor to consider in evaluating the power of the photovoltaic system is consumption, which can be inferred directly from the bill. As a first approximation, it can be estimated that the photovoltaic system should have a nominal power of about the average annual consumption divided by a factor of a thousand. Assuming a bill indicating an annual consumption of 3000 kWh, a 3 kWp system could be sufficient;**industrial or commercial photovoltaic power**: as with residential systems, the energy consumed, deducted from the bills issued by the local operator, must first be taken into account. However, in the case of industrial or commercial systems, the aim is to maximize business profitability. To do this, the calculation of the system’s power will be based on the consumer’s load profile, considering the instantaneous power of electricity absorbed.

As mentioned, calculating the nominal power of a photovoltaic system is a complex process that requires a precise evaluation of several key factors such as:

**available surface**: the first step is to assess the available surface for installing solar panels. This surface can be on the roof of a building, on a ground structure, or on other suitable structures. The size and arrangement of the surface will directly influence the system’s capacity;**efficiency of solar panels**: each solar panel has a specific efficiency, indicating the percentage of incident solar energy that can be converted into electricity. This value can vary based on the type and model of solar panels used;**solar irradiance conditions**: the solar irradiance conditions of the installation site of the photovoltaic system are a key factor in determining energy production. These conditions include the tilt angle of the solar panels, the orientation relative to the sun, and the amount of solar irradiance received in that specific area throughout the year;**correction factors**: some correction factors must be considered in calculating the nominal power of the photovoltaic system. These factors include shading due to trees, buildings, or other surrounding structures, as well as ambient temperature, which can affect the performance of solar panels.

To obtain an accurate assessment and optimal sizing of the photovoltaic system, we recommend using a software for designing and sizing photovoltaic systems. You will be able to accurately calculate the nominal power of a photovoltaic system, consider all the above-mentioned factors, and have an accurate estimate of the system’s capacity.