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How to calculate how many solar panels you need

It is important to know how many solar panels you need, as this will enable you to optimise the initial investment whilst taking advantage of the savings in electricity. Here's how to calculate how many solar panels you need.

How many solar panels your house needs

The number of solar panels that a home needs varies between 4 and 18 photovoltaic panel modules. To opt for more or fewer panels to make the investment of the installation profitable will depend on the annual electricity consumption for the house.

The following table will provide some guidelines for the number of modules depending on the type of consumption for the house.

 

Number of solar panels according to your annual consumption

Annual consumption Number of solar panels Consumption profile
< 2000 kWh 2 - 4 Low
2000 - 4000 kWh 4 - 6 Medium - low
4000 - 6000 kWh 7 - 9 Medium
6000 - 8000 kWh 10 - 15 Medium - high
8000 - 10000 kWh 16 - 20 High

What factors influence the number of solar panels a home needs?

Although the consumption you make is essential to be able to determine the size of the photovoltaic installation for our house, there are other factors that it is also important to take into account when calculating the number of solar panels.

  1. Annual consumption for the house.
  2. Quality and performance of the panels.
  3. Type of solar panel.
  4. Installed capacity.
  5. Orientation of the roof.
  6. Geographical location.

 

1. Annual electricity consumption

You can use your last bill to make a quick estimate of your annual consumption. For example, if the bill is for two months, you just multiply the total kWh consumed by 6. If you receive an invoice every month you should multiply by 12.

However, as electricity consumption is often not the same in all months of the year, you can go to the client area to find out exactly what your annual consumption is in kWh.

 

2. Quality and performance of photovoltaic panels

The quality of the panels is essential for their performance when it comes to producing energy. If the system is better quality it will cost a little more However, it will make better use of the sunlight and be able to produce more electricity.

If you choose the solar panels for your home well this will enable you to reduce the number of solar panels you need to install. Furthermore, solar panels with a higher performance will enable you to recover the investment in the installation faster.

It is always advisable to use panels from manufacturers with more experience and a good reputation in the production of photovoltaic panels. For example, Endesa X only uses solar panels manufactured by CanadianSolar, one of the most innovative companies in this type of system.

 

3. Type of solar panel according to cell type

Performance is the ability of the panel to produce electricity when sunlight strikes it. The technology used to manufacture the cells contained in the photovoltaic panel will affect its performance, and there are essentially three types of panels.

Type of solar panel Performance
Monocrystalline 16 to 23%
Polycrystalline 15 to 19%
Amorphous 8 to 12%

Crystalline panels (poly- - and mono-) give the best results in performance and are currently used for the installation of photovoltaic panels in homes. Amorphous panels are cheaper but are currently in disuse due to their low performance and shorter useful life.

 

4. Installed capacity

The capacity of the solar panels will indicate the amount of electricity that your domestic installation is capable of producing. Power is usually indicated in units of Wp (peak power in Watts). So the total power for the installation is the sum of the power for all the panels of which it is made up.

Number of panels * power for each panel (Wp) = Installation power (Wp)

For example, an optimal solar panel has a power of 300 Wp. If your installation consists of 6 solar panels you have a total power of 1800 Wp. This power value corresponds to 1800 kWh of electricity annually under theoretical conditions

It should not be forgotten that the WP value indicated by the manufacturers is measured under optimal conditions of solar radiation (Solar radiation = 1000 W/m2 and Temperature = 25°C). Therefore, to correctly calculate how many panels your house needs, you also need to make a study of both the orientation of the installed surface and the geographical location of your home.

 

5. Orientation and inclination of the roof

In Spain, the optimal orientation for solar panels is towards the south. This is the orientation that best takes advantage of solar radiation for the production of electricity in a domestic self-consumption installation.

There is no need to worry if you cannot have that exact orientation. In the case of orientations towards the southeast or southwest, the loss of efficiency is only between 1% and 4% which is perfectly acceptable.

In the case of east and west orientations, the fall in efficiency can reach up to 20 or 30%, so it may be necessary to install more solar panels to reach the necessary production power. In the case of north orientations it is important to make a very thorough preliminary study to evaluate whether it is worth installing the panels.

With regard to the inclination, to calculate how many solar panels you need, you will be limited to a certain extent by the structure of the building. Nevertheless, the panels are installed with their own supports so they will be able to have the most suitable inclination.

Generally, the optimal inclination when installing solar panels on a house depends on the latitude of the locality.

 

Table to calculate the angle of solar panels on a house

Season Angle of inclination for the solar panel
Winter 29 degrees+ (latitude * 0.9)
Spring and autumn Latitude - 2.5 degrees
Summer (Latitude * 0.9) – 23.5 degrees
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For example, the city of Toledo is at a latitude of 39.8568. If you multiply this by 0.9 you get a value of 35.87112. So the optimal inclination angles for the solar panel system would be: 64.9 degrees in winter, 37.4 degrees in spring/autumn and 12.4 degrees in summer.

Modifying the inclination of the installation is an annoying and expensive process, usually in the studies prior to installation an angle of inclination for the panels is selected that works optimally during all seasons of the year.

 

6. Geographical location of the house.

When calculating how many solar panels a house needs, it is essential to take into account the geographical location of your house. In Spain each province receives a different amount of solar radiation so, depending on the area in which you live, you will need to install more or fewer solar modules.

For example, in the province of Málaga the average value for the energy produced throughout the year is 1700 kWh per m2, while in the province of Lugo it is 1270 kWh per m2. So in Lugo you would need between 1 and 3 additional modules to produce the same energy as in Málaga.

It is generally recommended to request a customised study to ensure that the installation of solar panels in your home fits your energy needs taking all these factors into account.

Restrictions on the number of solar panels on a house

Square meters available

With the new more efficient solar panels, the surface area of the roof or where you intend to install the solar panels is not usually a limiting factor.

However, it is important to bear in mind that if you want to produce a large amount of energy you may be limited by the available surface area. In those cases it is better to have panels with higher power that generate more energy in the same number of square metres.

 

Hours of sunshine versus hours of consumption

The hours around the middle of the day, when there is greater light intensity, are the hours when more energy is produced by your solar panels. If those hours coincide with the hours where you make the highest consumption of electricity, you will get the most out of your installation.

However, if you need to consume more energy at another time of day, a traditional panel system may not be as cost-effective. In this case, there are two solutions:

  1. To install an electric accumulator: these devices store the excess electricity generated by your solar panels so you will be able to use it at another time. These are currently expensive devices with a short life cycle, so they are not recommended unless you are prepared to make a large investment. 
  2. Compensation mechanism: there are electricity tariffs that compensate for a surplus of energy generated by solar panels and the corresponding amount is discounted from the bill. All the electricity you use in the hours when your solar panels are not producing is offset by the energy you do not use when they are producing electricity, so you make a saving on your bill

In single-family homes, it is generally advisable to take advantage of this compensation mechanism to make the most of energy production from solar panels.

 

How much does a photovoltaic installation cost depending on the number of panels?

Currently the price of a photovoltaic electrical installation for a house is between €0.8 per kWh and €1.2 per kWh of annual electricity consumption. This is approximately between €500 and €600 per solar panel or module for the complete installation.

Annual consumption Number of panels Price for the installation
3300 kWh 6 4000€ - 5000€
5000 kWh 9 5000€ - 6000€
6600 kWh 12 6000€ - 7000€
9900 kWh 18 7000€ - 10000€

As always, these prices are just a guideline and it is best to request a customised study in accordance with your needs. Furthermore, the prices do not include any possible discounts or subsidies which may be applicable in the different plans that exist to promote the use of clean energies.

 

Guarantee and extras included in the price

When calculating how many panels you need for your home and how much they will cost, you also need to take into account other expenses that may or may not be included in the final cost for the installation. The number of years of guarantee is also a very important factor to take into account, as it could mean the difference between whether or not future cost overruns may appear.

For example, at Endesa X our installations enjoy the following:

  • Solar modules with a 12-year guarantee.
  • Power inverter with a 10-year guarantee.
  • Structure of the installation with a 10-year guarantee.
  • Installation and procedures included.
  • Management of subsidies.
  • Fully comprehensive insurance free for 10 years.

Comparison of Electricity and Gas Tariffs

You don't need to adapt to Endesa's tariffs because they adapt to you. If you go to our catalogue you can compare the different tariffs for yourself. Or if you prefer, you can answer a few questions about your consumption habits and we will take care of comparing all the different electricity and gas tariffs and then make a customised recommendation.

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