How to calculate the surface and the profitability of solar panels?

You are thinking of installing solar panels on your house and you are wondering about the surface you need on your roof, the savings you can make and especially the profitability of your project? You are on the right page. 

In this ultra-practical guide, we will help you estimate the surface of solar panels you will need and calculate the profitability of your investment. You'll see, it's simple and quite intuitive!

Estimate your future energy savings in a few questions

The project of installing solar panels at home is motivated by the desire to save money on the energy bill and by the wish to lower one's carbon footprint. To estimate your potential energy savings, ask yourself the following questions 

1. What is your annual electricity consumption ? Check your electricity bills to find out your consumption in kilowatt hours (kWh);
2. What is the electricity rate you are currently paying ? Check the cost per kWh on your electricity bill;
3. What is the available surface on your roof for the installation of solar panels ? Measure the surface of your roof in square meters and estimate the exploitable part for the installation of solar panels;
4. What is the level of sunshine in your area ? Find out the average annual solar irradiance for your area in kWh/m²/year. You can use solar atlases or local weather data to get this information;
5. What is the efficiency of the solar panels you are planning to install ? Solar panels generally have an efficiency between 15 % and 22 %, with a theoretical maximum of 31 %.

Once you have answered these questions, you will have everything you need to calculate the following indicators: 

• The number of solar panels required according to the required power;
• Solar panel production;
• The cost of the photovoltaic installation ;
• The profitability of the solar panels, taking into account the consumption options of the photovoltaic electricity (total self-consumption, total resale or hybrid use).

What is the available surface for your photovoltaic installation?

Your solar panels can be installed in a location that meets a few conditions: 

• Possibility of orienting them towards the south to capture a maximum of solar radiation;
• Possibility to adjust the inclination according to the latitude of the installation site;
• Unshaded location (no buildings, trees or other obstacles) ;
• Sufficient ventilation to avoid overheating;
• Reasonable accessibility for easy maintenance and troubleshooting;
• Location according to the law.

Contrary to what one might think, solar panels can be installed on many types of surfaces, not just roofs: building facades, parking lots, open land and even bodies of water. Rooftops, however, remain the most obvious choice for residential installations.

How much space is available on your roof?

You will simply measure the area of your roof with a tape measure, taking care to subtract the area occupied by fixed obstructions such as chimneys, roof windows, vents and antennas. Also subtract locations that require a north orientation, as well as those that do not allow for a slope between 30 and 45 degrees. Finally, you can eliminate shaded areas, unless you opt to install microinverters or a bi-tracker inverter.

After estimating the available area, you can determine the maximum number of solar panels you can install on your roof. Which brings us to the next point.

How to calculate the number of solar panels needed?

It is very simple. Just follow these steps: 

1. Calculate your annual energy consumption: the information is on your bill (in kWh);
2. Estimate the amount of sunshine in your area;
3. Determine the energy production needed by dividing your annual consumption (in kWh) by the average number of hours of sunshine per year in your area. This will give you the production power needed in kilowatts (kW) to cover your consumption.

Example If your annual consumption is 6,000 kWh and your region benefits from 4 hours of average sunshine per day, the necessary production would be 6,000 kWh / (4 hours x 365 days) = (4 hours x 365 days) 4.11 kW.

1. Choose the power rating of the solar panel you are considering purchasing. It is usually between 250 and 400 watts per panel;
2. Finally, calculate the number of solar panels needed by dividing the required power output (in kW) by the nominal power (or peak power) of the solar panel (in kW). We will develop this point in a moment. Round this number up to the next whole number to determine the number of solar panels you will need.

Example If the required production is 4.11 kW and you choose 300 watt solar panels (0.3 kW), you will need 4.11 kW / 0.3 kW = 13.7That's about 14 solar panels. Please note: make sure that the available surface on your roof is sufficient to accommodate the number of solar panels you need. It is always recommended to consult a professional to evaluate the feasibility and efficiency of your photovoltaic installation. Note: Solar panels used in residential installations range in size from 1.6 to 2 square meters, with an average of 1.8 square meters.

Available area on the roof (m²)	Average area of a solar panel (m²)	Maximum number of solar panels on the roof
4	1.8	2
8	1.8	4
12	1.8	6
16	1.8	8
20	1.8	11
30	1.8	16

How to calculate the power of your photovoltaic installation?

To determine the power of your future photovoltaic installation, you should simply refer to your normal electricity consumption. The power of your installation will have to be as close as possible to it to maximize savings. Before giving you the steps to follow, let's first explore the concept of peak power, which is essential for estimating the power of your photovoltaic installation.

Peak power: what are we talking about?

Peak power, also known as power rating, is a standardized measure used to describe the maximum capacity of a solar panel to produce energy under ideal laboratory conditions and at time "T". It is usually expressed in watt-peak (Wp) and is used to compare the efficiency and performance of different solar panels. 

We're talking about ideal conditions, and you guessed it: your solar panels won't reach that power. It is generally estimated that the production capacity real is around 85 %. In sum, 1 Wp will give you 0.85 kWh.

In concrete terms, how do I determine the power of my future photovoltaic installation?

If you have followed the steps we have developed, you can very simply estimate the power of your future photovoltaic installation. Small reminder : 

• Calculate your annual energy consumption in kWh, based on your bill or your Linky meter;
• Estimate the amount of sunshine in your area;
• Determine the energy production needed by dividing your annual consumption by the number of hours of sunlight;
• Divide the required power output (in kW) by the actual efficiency of the solar panels (85 %) to obtain the total peak power required for your installation;
• Select the power rating (or peak power) of the solar panels you plan to purchase, usually between 250 and 400 watts per panel;
• Calculate the number of solar panels needed by dividing the total peak power needed (in kW) by the nominal power (or peak power) of the solar panel (in kW). Round this number up to the nearest whole number.

Total self-consumption, total resale or hybrid configuration?

You don't know what we are talking about? Don't panic, we'll explain. When you install solar panels, you have several options for using the electricity produced. Here is an overview of the three main configurations:

1. Total self-consumption In this case, all the electricity produced by your solar panels is used to power your own home or building. You consume the solar energy produced and reduce your dependence on electricity from the grid.
2. Total resale In this configuration, you sell all the electricity produced by your solar panels to EDF. You will receive an income for the energy sold, often in the form of a feed-in tariff guaranteed by a contract. This can be an attractive option if you want to generate additional income, but you will continue to buy the electricity you need from your supplier, without directly benefiting from the savings from self-consumption.
3. Hybrid configuration It is the self-consumption with resale of the surplus. You consume the electricity produced by your solar panels to power your home or building and sell the excess energy not consumed to EDF. This option allows you to take advantage of the savings linked to self-consumption while generating additional income thanks to the resale of the surplus energy.

The choice between these configurations depends on several factors: your financial goals, your energy consumption, government incentives and local (and current) feed-in tariffs. A professional will help you make this decision based on your situation.

Please note: EDF is committed to buying back the electricity produced by your photovoltaic installation for 20 years, at the following prices: 

Power of the installation in kWp	Feed-in tariff per kWh in partial self-consumption	Feed-in tariff per kWh for total resale
≤ 3 kWp	0,10 €	0,2022 €
≤ 9 kWp	0,10 €	0,1718 €
≤ 36 kWp	0,06 €	0,1231 €
≤ 100 kWp	0,06 €	0,1070 €

How can I estimate the profitability of my photovoltaic installation?

The calculation of the profitability of your photovoltaic installation will be based on five essential variables: 

1. Initial cost of installation Consider the total cost of the installation, including solar panels, inverter, mounting hardware, batteries (if used), wiring and installation cost;
2. Estimate the savings on your electricity bills by multiplying the amount of electricity produced by your solar panels (in kWh) by the current cost of electricity (in euros per kWh). The savings will depend on your level of self-consumption and the configuration of your installation;
3. Income from the resale of surplus electricityif applicable;
4. Financial aid and grants If you opt for self-consumption, you are entitled to a premium per kWp installed and a reduced VAT rate. Ask your local authority about other subsidies you may be eligible for.
5. Include maintenance and replacement costs of the components of your photovoltaic system in your profitability estimate.

This will allow you to calculate the return on investment (ROI), or the time it will take for the savings and revenue generated by your installation to cover its initial cost. Here's the formula: 

ROI (years) = (Initial cost - Financial assistance) / (Annual savings + Annual income - Maintenance and replacement costs)

Once you have calculated the return on investment, you will have an idea of the profitability of your PV installation and the time it will take to recoup your initial investment. Keep in mind that these estimates are based on assumptions and may vary depending on actual power generation conditions and future electricity costs.

Calculation of solar panels: we summarize the essential

To estimate the energy savings from the installation of solar panels, it is essential to take into account the following elements

1. Your annual electricity consumption and the electricity rate you currently pay;
2. The area available on your roof for the installation of solar panels and the level of sunlight in your area;
3. The efficiency of the solar panels you are considering installing.

With this information, you can calculate :

• The number of solar panels required according to the required power;
• Solar panel production;
• The cost of the photovoltaic installation ;
• The profitability of the solar panels, taking into account the consumption options of the photovoltaic electricity (total self-consumption, total resale or hybrid use).

To estimate the profitability of your photovoltaic installationit is important to consider :

1. Initial cost of installation;
2. Savings on your electricity bills;
3. Income from the resale of excess electricity, if any ;
4. Financial aid and grants available;
5. Maintenance and replacement costs of the components of your photovoltaic system.

Solar panel calculation : the FAQ

Still have questions? The answer may be in this concluding FAQ.

How much do 100 m² of solar panels yield?

To estimate the potential income of 100 m² of solar panels in France, several factors must be taken into account, such as the power of the panels, the sunshine in the region and the feed-in tariff. 

Here is a sample calculation to make it clear: 

1. Let's assume you are using solar panels with a power output of 300 watts (W) each. A typical solar panel has an area of about 1.8 m². With 100 m², you can install about 56 panels (100 m² / 1.8 m² per panel).
2. The total power of the installation would be 16.8 kW (56 panels × 300 W).
3. Let's assume that your region has an average of 1,500 hours of sunshine per year. The annual energy production would be about 25,200 kWh (16.8 kW × 1,500 hours).
   • The feed-in tariff depends on the configuration of your installation (partial self-consumption, total resale). According to the EDF tariffs mentioned above for an installation of ≤ 36 kWp: feed-in tariff for partial self-consumption = 0.10 € per kWh, and feed-in tariff for total resale = 0.1231 € per kWh.

The potential annual revenues would therefore be as follows:

• In partial self-consumption: 25 200 kWh × 0,10 € = 2 520 €
• In total resale: 25 200 kWh × 0,1231 € = 3 102 €

This is a rough estimate, and actual figures may vary depending on the efficiency of the solar panels, the orientation and tilt of the installation, and potential shadows.

What is the surface area for 10 photovoltaic panels?

Assuming that each photovoltaic panel has an area of about 1.8 m², the area required for 10 panels would be about 18 m² (10 panels × 1.8 m² per panel).

What surface of solar panels for 9 kWp?

The area required for a 9 kWp photovoltaic system depends on the power of the solar panels used. Assuming that you use solar panels with a power of about 300 W per m², here is how to calculate the required surface: 

• 9 kWp = 9000 Wp (since 1 kW = 1000 W) ;
• 9,000 Wp / 300 W/m² ≈ 30 m².

In this case, a surface of about 30 m² would be necessary for a 9 kWp installation. Please note that the exact surface can vary depending on the specific power of the panels you choose.