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How do I calculate how many solar panels I need for my house?

Thanks to the new regulation regarding the sun tax, individuals take the opportunity to start using solar energy in our homes. In addition to contributing to the protection of the environment, it is profitable in the long term and allows you to save on the electricity bill. And with this some questions arise such as;

In the following post we solve all your doubts so that you know what aspects to assess before carrying out a residential self-consumption installation and therefore specify how many panels you would need for a photovoltaic installation.

Discover those factors to take into account to know how many panels you need in your houfsolar panelsse:


The most suitable system depends on the energy consumption of your home, its monthly use throughout the year, the surface you have for the installation of the PV modules, the distribution of the consumption profile 24 hours a day and the rates contracted with the electricity company.

 1) The energy consumption of your home. Determine how much energy we need.

This is one of the most important factors to know how many solar panels we will need in our photovoltaic installation.

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The annual consumption and the monthly use allow to obtain the consumption distributed by months. A day without consumption does not allow to take advantage of the energy of a direct self-consumption without batteries.

2) The surface that you have for the installation of the PV modules

The space in the building and its orientation determine the maximum number of PV modules that can be installed. It is usually the factor that determines the power to be installed.

How should my roof be to install solar panels?

The PV power that can be installed on a well-oriented roof (with a coplanar structure) is around 150Wp/m2. On a flat terrace with a structure to tilt the panels, we can consider about 80Wp/m2.

The two standard panel formats are 72 cells: 2x1m/320Wp, and 60 cells: 1.6x1m/280Wp. It is often easier to take advantage of a roof space with the smallest module.

3) Consumption profile

The best performance of the installation is obtained when the consumption occurs during the hours of PV generation. Thus, a pool treatment plant programmed to work at night could not take advantage of the energy of a self-consumption without batteries. On the other hand, working in the central hours of the day, it would take almost 100% advantage of it.

*The specifications of this point may change depending on the needs of the user.

 4) Rates contracted with the electricity company

It is recommended to contract a rate with hourly discrimination, formerly it was the so-called night rate. This rate consists of dividing the day into two periods.

A schedule with a higher price (day) and a schedule with a cheaper price (night). In this way, during the day, the solar kWh would be used to a great extent, while at night the reduced price of the rate would be used.

The hours of these periods also vary depending on the time of year (summer and winter).

Correctly dimension our photovoltaic installation:


Once we are clear about the previous premises, we will have to make certain decisions in order to correctly size our photovoltaic installation. We must bear in mind that with the current regulation of electricity self-consumption there is the possibility of injecting surpluses into the electrical network or disposing of these surpluses in what is commonly called “zero injection”. Since September 2019, it is advisable to carry out the installations with the injection of surpluses into the electricity network, so that the electricity company makes an assessment of that energy and compensates it (at the corresponding price) with the consumption made from the electricity network in hours without solar production (at the price of the current rate).

One of them is to choose between connection to the network with battery or without battery. In general, it is always more efficient to consume at the same time that the solar generator is producing energy, that is, during the day.

  1. Without batteries , when the energy generated is greater than the demand for energy, the excess is injected into the network (in the case of having surplus injection) or lost (in the case of having zero injection). The direct connection to the network (without batteries) allows to reduce the consumption of the network when the generator is producing energy and simultaneously it is consumed in the house. Available solar energy that is not consumed instantly is either injected into the grid or lost. The following image represents the daily operation graphically:



As we can see, most of the consumption occurs after sunset, during the night, so the amount of energy saved is limited by daytime consumption. The recommended powers for this configuration are as follows:


The approximate power generation in Spain is between 1400 and 1700 kWh/kWp per year (depending on your location). Assuming an average use of 50% taking into account the consumption profile, you can obtain the recommended size for your installation:




2. With batteries , the excess power generation during the day allows the recharging of the battery that is consumed during the night. The connection to the grid with batteries allows to take advantage of the surplus energy of the PV generator during the day to be able to consume it at night. In this case, part of the solar energy that would have been wasted without batteries can be used. The following image represents the daily operation graphically:



As we can see, the excess generation during the day is used after sunset. The recommended powers for this configuration are as follows:


To calculate the recommended PV generator for an installation with batteries, it is interesting to oversize it by 50% compared to that recommended for an installation without batteries.

To calculate the necessary capacity in the battery, it can be considered that the objective is to supply the typical nocturnal consumption of the house. In a typical home, it is common for 35% of consumption to be carried out when there is no sun. This way:

Battery capacity = Daily consumption (kWh) x Night consumption percentage (%)


Whether we include batteries or not

Up to 25-30% of the total energy consumed can be covered by a PV generator without batteries. In order to increase the use of the PV generator to higher values, it is necessary to include batteries in the system.


  • See the working voltage with which our installation will work . Depending on whether the consumption is lower (light bulbs, television, etc.) or higher (fridge, washing machine, dishwasher, sewage pump) we will use a 12-24 V or 48 V installation.
  •  Choose the electronic equipment that makes up our installation (Inverter, Regulator, Charger). You can enter our article ” What is an isolated solar kit “ for more information.


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