What Is kWp in Relation to Solar PV Systems?


Jack Ayre

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If you’re interested in solar panel systems or considering installing one in your property, you’ve probably heard several terms typically associated with them. These include measuring units like kW, kWh, and kWp.

While they might be a little confusing, understanding these units is the key to generating enough energy to power your home or business properly, which is where this guide comes in handy!

Here you’ll find out all the information you need about kWp meaning and its relationship to solar PV systems. Let’s dive in!

What Does kWp Mean in Solar PV Systems?

In order to understand what kWp means, you should first familiarise yourself with other related terms.

As you may already know, the energy consumption of electrical appliances is usually measured in watts (W). For that reason, the capability of a solar panel is also measured in watts (W) or kilowatts (kW).

However, several factors can affect the performance of a solar PV system, so the same panel can end up generating different amounts of energy, depending on these factors. In fact, the same panel can generate two different amounts of energy if you use them in different cities or under different conditions.

That’s why manufacturers use a more accurate unit to describe the amount of energy a solar panel can produce at peak conditions and performance, and that’s where the kWp comes into play.

kWp is short for kilowatt peak, and as the name suggests, it describes the maximum power output a panel can generate under industry-wide standardised test conditions (STC), which are defined as:

  • Measuring the solar irradiance of exactly 1,000 watts per square meter (W/m²)
  • Keeping an air mass of AM 1.5 spectrum
  • Maintaining a constant module temperature of 25 °C throughout the test
  • Testing in clear, sunny weather

What is the Importance of measuring kWp for Solar Panels?

As you can see, the kWp is the wattage a panel can produce under standardised optimal conditions. 

In other words, the kWp rating is not going to be the same as the amount of electricity a solar panel produces in the real world, as you can’t maintain the previous test conditions at all times and across different locations.

For example, a 1 kWp solar PV system will produce up to 1 kW of electricity under STC. This doesn’t account for any shade, debris, or other limitations found in the real world, so the system will end up producing less than 1 kW of electricity.

The key purpose of measuring each panel’s kWp is to have a constant reference that you can use to compare various models, brands, and types of solar panels available on the market while having an idea of the highest rate of generation it can reach.

Factors That Affect the Electricity a Solar Panel Produces

As previously established, there are various factors and key aspects that can impact the amount of electric energy a solar panel produces. Let’s have a brief overview of each one of them.


The location where you install your solar panels has a remarkable impact on its total yield. This is because different regions across the UK receive different amounts of daylight per day.

For instance, East Sussex is one spot that receives plenty of sunlight, especially during the summer. On the other hand, if you live in Glasgow or Greater Manchester, the same panel will have a noticeable reduction in power output.

This also includes the micro-location of the panels and where you install them around the house. 

For example, if you install a panel on a south-facing roof, it’ll typically generate more energy than a north-facing one, as the latter receives much less sunlight per day.

Weather and Clouds

The conditions of weather where you set up a PV system can also affect its kWh and yield. For example, on a sunny summer day with clear skies, the wattage of a solar panel will be at its highest and closest to its kWp rating.

As more clouds block sunlight, the energy a solar panel produces can be as low as 10 to 25% of its rated maximum capacity. 

Similarly, heavy rain and snow can inhibit solar panels from working properly, which ends up reducing their efficiency dramatically.


The ambient temperature is an often overlooked factor, although it has an impact on a solar panel’s wattage. 

Contrary to what some people might think, solar panels’ performance is negatively affected as the temperature increases. This means that on an exceptionally hot day, the energy production of a solar panel can be reduced by up to 25%.

That’s why proper spacing between the panels is necessary to ensure consistent airflow and prevent the panels from overheating.

The Orientation and Angle of the Panels

The photoelectric cells in solar panels can generate electricity when they come in contact with any type of light. However, the more direct and intense the light is, the more solar-to-electric energy it will convert.

For that reason, you need to adjust the orientation of your solar panels so that they get the maximum amount of sunlight during the day.

Ideally, you need to keep the angle of the solar panels close to the latitude of your house, which is around 30 to 45 degrees from the horizon. This ensures that your panels are south-facing, and therefore, receive maximum sunlight per day.

Type and Efficiency of the Solar Panels

Another aspect that can greatly impact the solar panel’s ability to generate electricity is its type.

This is because different solar panel technologies have different efficiency ratings. Here’s a quick look at the most popular types and their average efficiency rate.

Type of Solar Panel CellsAverage Efficiency Range
Amorphous (Thin Film) Silicon Cells Around 6% to 7%
Polycrystalline Silicon CellsAround 13% to 16% 
Monocrystalline Silicon CellsAround 15% to 23%
High Concentration Photovoltaic CellsAround 30% to 40%

Maintenance and Lifespan

In addition to the previous factors, the condition of the solar panels themselves can affect their output. 

Similar to clouds and snow, other elements of weather can also widen the gap between the panel’s kWp and its realised performance. 

These include dust, debris like fallen leaves, animal droppings, bird nests, etc. For that reason, you need to clean the panels more often to maintain their efficiency. 

Additionally, you should also know that as solar panels’ years in service increase, their performance might start to decline by up to 0.5% per year.


What is the difference between kWp and kWh?

Many people confuse kWp and kWh, thinking they’re the same thing. While they’re related, they’re two different terms.

A kWh is short for kilowatt hour, and as the term suggests, it describes the amount of energy that an electrical appliance consumes over an hour. 

This means that kWp is the maximum theoretical output of a solar panel while kWh is the realistic measure of electric power generation. 

How many kWh does an average house consume per day?

The average UK household consumes anywhere between 8.5 to 10 kWh of electricity per day, which reflects around 255 kWh to 300 kWh per month. 

Of course, this is just an average figure that can vary depending on several aspects, such as the size of your house, the number of family members, your energy consumption habits, etc. 

How much power is enough to take a property off the grid?

Getting off the grid is perfectly legal in the UK. However, it usually requires a lot of solar panels and high-capacity batteries to go off the grid, especially if you want to use power-intensive devices like fridges, electric heaters, air conditioning, etc.

As a general rule, you’ll need around 4.4 kW solar output to get a three-bedroom house off the grid, although this also depends on your energy-saving habits and the efficiency of the panels you use.

Final Thoughts on kWp Meaning

Understanding the concept of kWp in solar panels is essential to compare the efficiency and performance of various panel models and pick the one that suits your needs.

However, kWp won’t necessarily reflect the actual power you’re going to get from the solar panel, so you have to keep the previously mentioned factors in mind to keep your panels’ performance close to their theoretical peak.

One way to make sure that you get the most value for your money is hiring a professional solar panel installer, so don’t hesitate to call ESE Solar to get a reliable and high-efficiency system with an excellent warranty system for a worry-free purchase!

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Born in North West England, Jack kicked off his career in the insurance arena, dealing with claims for three years. Always up for a challenge, Jack leapt out of his comfort zone and embarked upon 5 years of excitement with the military.

Working for the Ministry of Defence, Jack specialised in counter-terrorism which took him to places such as Dubai, Oman, Iraq, Norway and Bahrain. During this period he worked with various Governments implementing strategic operations to prevent terrorism.

After travelling around various countries Jack increasingly came across various forms of solar from large solar farms to domestic solar panels. From here Jack took a keen interest in all things solar and started reading about the technological capabilities with a view to entering the industry after his career in the Ministry of Defence.

Once home after 5 fruitful years, Jack had 3 months leave in which he immersed himself in a solar crash course and got applying for jobs within the solar sector. He came across ESE Solar, a long-established forward-thinking company whose head office was only a stone’s throw away from him. He picked up the phone to see if there were any vacancies and within a week had a successful interview and obtained a full-time job, starting in the technical team.

Within the first few months, Jack onboarded in various online courses during work and social hours to speed up his knowledge of all things solar. Due to his strong work ethic and thirst for knowledge Jack was promoted to Head of Technical at ESE Solar. He currently leads the team and oversees the product development of new technologies within the company alongside assisting where necessary on their maintenance department.

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