The sun is an inexhaustible source of energy and more and more private individuals are now investing in a solar and photovoltaic system. But it is often difficult to assess the number of panels needed to supply a house with electricity.
The number of panels to be installed depends on several factors. In addition to the house’s size, the panels’ performance and production capacity play a critical role in the decision-making process. In this guide, find out how many photovoltaic solar panels you need to install to supply your home with electricity.
Nominal power, real power, loss of efficiency: the concepts to know in this calculation
To determine how many solar panels to power a house, you need to master some basic notions on solar energy. Indeed, the number of photovoltaic panels needed for a house depends on several factors, such as:
- The nominal power of the solar panel expressed in peak watts (Wp) and corresponding to the maximum power that the panel can produce under optimal conditions,
- Your annual electricity consumption expressed in kilowatt hours (kWh). To find out about it, you can refer to average ranges corresponding to your situation. For a more personalized estimate, refer to your most recent electricity bills.
- The actual power of the panel expressed in Watts, which takes into account the loss of efficiency of the panel of approximately 15% between the nominal (maximum) power indicated by the manufacturer, and its production power in real conditions, where the sunlight remains variable and the panels cannot always be installed in an optimal way.
Here’s how to differentiate between these different concepts.
The nominal power of a solar and photovoltaic panel
The rated power is the maximum power that the panel is capable of producing under ideal conditions. This value, generally expressed in peak watts (Wp), corresponds to the maximum electrical power that the panel can produce under optimal conditions of temperature (25 °C), sunshine (1,000 W/m²) and angle of incidence of solar rays (due south or southwest orientation, 30-degree inclination and open environment).
The rated power of a solar and photovoltaic panel depends on its technology, surface area and design. For example, a monocrystalline photovoltaic panel (the most efficient on the market) will generally have a higher nominal power than a polycrystalline or amorphous panel of the same size.
Note that the power rating does not represent the amount of energy the panel can produce over its lifetime, but rather its maximum capacity under ideal conditions. The amount of energy actually produced will depend on several factors such as local weather conditions, the orientation and inclination of the panel, shading and its proper maintenance.
The real power of a solar and photovoltaic panel
Expressed in Watts (W), the actual power of a solar panel should not be confused with its rated power (expressed in watt-peak). As explained earlier, determined by the manufacturer of the photovoltaic panel and indicated on the panel’s technical sheet, the rated power is the maximum power that the panel is capable of producing in ideal conditions that do not really reflect the reality of the field.
Because, in reality, the actual power of the photovoltaic panel depends on many factors, such as:
- The outside temperature,
- The sunshine in your area,
- The orientation and inclination of the panel,
- The quality of the inverter and the electrical components used.
In practice, the actual power of the solar panel is therefore often lower than its nominal power. A solar and photovoltaic panel produces around 75% of its peak power under good conditions. This leads to a loss of yield of about 15%, which must be taken into account in your calculations.
For example, a panel with a nominal power of 400 Wp produces 340 W in real conditions.
The loss of efficiency of a photovoltaic panel
Loss of efficiency refers to the ratio between the nominal power of the panel and its actual power. The notion of yield loss is essential since it allows you to take into account, in your calculation, the inevitable energy losses during production in real conditions.
The loss of yield is expressed as a percentage. As mentioned earlier, it is estimated on average at around 15%. Therefore, to calculate the number of solar panels needed, the so-called conversion factor 0.85 (1- (15/100) is used.
How to calculate the number of solar panels to install on your roof?
Once you understand these concepts, you will be able to calculate the number of panels you need for your installation.
To do this, you need to divide your annual electricity consumption (converted into Wp) by the power of your solar and photovoltaic panels (in Wp). The formula is, therefore as follows:
Number of solar panels = annual electricity consumption / nominal power of the panel
Let’s say you consume 4,000 kWh of electricity per year (average consumption of a family of 4). This is equivalent to a Watt-peak installation with a power of: 4,000/0.85 = 4,705 Wp (remember our conversion coefficient 0.85 taking into account the efficiency losses between the peak power and the actual power of your installation).
The nominal power of a conventional solar and photovoltaic panel (polycrystalline type panels) is about 300 Wp. You will divide 4,705 by 300, giving you 15 solar panels to install on your roof.
Then, you can calculate the surface area of your installation by multiplying the number of panels by the surface area of a single panel, generally around 1.6 m². Here, the result will be 15 panels x 1.6 m² = 24 m² of panel area to be installed.
The rated power of solar and photovoltaic panels varies depending on their technology and efficiency. The more powerful the panel, the fewer
panels you will need to cover your electricity needs.
Types of photovoltaic panels and their performance
Three types of photovoltaic panels are currently available: monocrystalline, polycrystalline and amorphous panels. Let’s take a look at their performance.
The monocrystalline photovoltaic panel
Made from a single silicon crystal, the monocrystalline panel is the most efficient solar and photovoltaic panel model currently on the market. Monocrystalline panels are easily recognizable: they are very dark, or even completely black for some models. They are also very popular with customers for their aesthetic, discreet and elegant side.
Their nominal power, around 300 to 400 Wp, is also the best in the sector. This means that fewer solar panels will be needed to cover your electricity needs than with other technology.
How many monocrystalline panels should I install on my roof?
If the nominal power of your monocrystalline panel is 400 Wp and the annual production capacity required for your home is 4,705 Wp (as in a previous example), you need to install about 11 monocrystalline panels of this power on your roof, on an area of 17 m² (11 panels x 1.6 m²).
The polycrystalline photovoltaic panel
Polycrystalline photovoltaic panels are the most widely used solar panel models by individuals. They have a satisfactory quality/price ratio.
They work the same way as monocrystalline panels, but they are made from several silicon fragments. Visually, they are blue and their coating looks a bit like a mosaic. Their nominal power, which is lower than that of monocrystalline photovoltaic panels, is between 250 and 350 Wp.
How many polycrystalline panels should I install on my roof?
If the nominal power of your polycrystalline panel is 350 Wp and your home’s required annual production capacity is 4,705 Wp, you need to install about 13 polycrystalline panels of this power on your roof, covering an area of 20 m² (13 panels x 1.6 m²).
The amorphous silicon photovoltaic panel
Amorphous photovoltaic panels are the least expensive but also the least efficient solar panel models. Their nominal power, which is much lower than that of other types, is between 40 and 100 Wp.
How many amorphous panels should I install on my roof?
If your amorphous panel’s nominal power is 100 Wp and your home’s annual production capacity is 4,705 Wp, you need to install about 47 amorphous panels of this power on your roof, covering an area of 75 m² (47 panels x 1.6 m²). This type of panel is, therefore, not very profitable for individuals and is rarely installed on homes. Easily transportable, they are more suitable for mobile use.
To help you navigate, here is a table summarizing the average power of each type of photovoltaic panel and the approximate number of panels to be installed for a 3 kWp and 6 kWp installation.
What are your solar energy production goals?
Now that you know how to calculate the number of solar and photovoltaic panels installed on your roof, according to their technology and power, you must determine the amount of energy to be produced according to your objectives.
What needs do you want to cover with your solar panels? Do you want to sell all or part of your production? These questions are essential to size your photovoltaic installation in the best possible way.
What are your annual electricity needs?
In practice, total self-consumption is difficult to achieve, but it is possible to cover between 50 and 70% of your energy needs quite easily and thus lower your electricity bill.
To provide about 70% of the electricity consumption of a family of 4, installing photovoltaic panels with an average total power of about 3 kW is necessary. This corresponds to about 8 monocrystalline panels or 12 polycrystalline panels.
However, this is an average. If you live alone and consume little electricity, there is no need to plan for an installation that is too expensive. Depending on your needs, installing solar thermal panels connected to a solar water heater may be more appropriate.
Conversely, if you have a heated swimming pool or if your house has an air conditioning system, a photovoltaic installation with a power of 6 kWp will be more suitable for your electricity consumption.
Do you want to resell all or part of your production?
While self-consumption is often the main objective of individuals who invest in a photovoltaic project, you can sell part of your electricity to an energy supplier. In this case, it is best to adjust the size of your photovoltaic system in order to produce more energy.
The total resale of the surplus to EDF or Engie is also possible. You will then be able to set the number of panels to be installed in your home more freely, taking into account the average tariff set by the State for purchasing your electricity.
What are the external factors to take into account when sizing your solar installation?
Do you know the average power of a photovoltaic panel, and have you estimated your electricity needs? You’re ready to take the next step! There are several factors to consider when determining exactly how many photovoltaic panels you need to install on your home.
The solidity of your roof
The weight of a solar and photovoltaic panel is not negligible. Therefore, it is necessary to take this into account before the installation of the panels. If your home is old, it might not support too many panels. The weight of a solar module depends on how it is attached and its components.
A panel weighs between 11 and 12 kilos per square metre, excluding the fixing device. For an installation of 3 kWp of power, you will need 8 monocrystalline panels of 1.6 m2, i.e. about 12.8 m2 of surface area. Your frame must, therefore, be strong enough to accommodate this additional weight of more than 160 kilos, with the cables and fixing brackets.
If in doubt, have your roof checked by a professional installer before the work begins. Depending on the condition of your framework, you may need to reduce the number of panels in your photovoltaic installation.
The environment and geographic region of your home
The amount of energy produced by your solar and photovoltaic panels depends on their maximum efficiency, production capacity, and environmental factors.
The southernmost regions are more suitable for installing solar modules. However, be careful, as your panels are sensitive to high temperatures. Indeed, during heat waves, their yield decreases.
Your panels must be installed in an environment without shades to produce maximum electricity. If there are large trees near your house, for example, you will need more photovoltaic solar panels to obtain the same amount of energy as with a perfectly unobstructed installation.
The orientation and inclination of your photovoltaic panels
The ideal orientation for a solar and photovoltaic panel is to the south. In this way, the sensors will be exposed to sunlight for longer. The inclination of your panels should be about 30 degrees to capture as much light as possible.
Therefore, it is essential to choose the roof slope intended to accommodate your photovoltaic solar panels. A preliminary technical study is necessary before choosing the location of your solar collectors. Your qualified craftsman will help you identify the best possible location for your investment.