304 North Cardinal St.
Dorchester Center, MA 02124

# How to size and calculate solar panels needed for an isolated system

In this article  for solar panels we are going to exemplify the calculation of the solar panels. necessary for an isolated/Grid connected solar panel system

If what you are looking for is the calculation of panels for a self-consumption installation in Africa , then better read our article on how many solar panels you need for a home with a self-consumption installation.

The structure of the article will present

• how solar panels are connected in series or parallel?,
• how much energy a solar panel produces? and based on that, how to calculate
• how many photovoltaic solar panels are necessary to generate electrical energy?,
• from solar energy, to those who need to put solar panels at home and who do not have a connection to the electricity grid, for example.

It serves as an introduction that installing high quality solar panels is of vital importance for a power generation installation of this type. It pays to do a simple internet search looking for reviews and references before making a purchase.

### Power generated by a solar panel

We assume that you already know how solar panels work, and the fundamentals of photovoltaic solar energy. Otherwise, you can find plenty of information on our blog. Thus, to calculate the energy generated by a solar panel during one day (Epanel), we must use the following equation:

Ep = Ip × Vp × PSH× 0.9 [Wh/d]

• Ip and Vp are the maximum current and maximum voltage of the panel,
• PSH are the peak sun hours, and
• 0.9 would be the coefficient of performance of the panel (typically 85-90% when discounting losses).
• The resulting energy would be expressed in Wh/d.

That would be the energy generated by a single solar module, but if, for example, we want to know how much energy a photovoltaic self-consumption installation with several solar panels is going to generate, we would simply have to apply the following formula:

Power Generated = Iphotovoltaic × Vphotovoltaic × PSH × 0.9

The current, in this case, would be the maximum resulting from the association of the photovoltaic modules connected in parallel of each branch (string), and the voltage would be the result of the sum of the voltages of each branch (string) connected in series.

The electrical symbol that is usually used to graphically represent a photovoltaic panel is as follows: ### Series and parallel solar panels connection:

In most photovoltaic projects, especially isolated solar installations and depending on the power of the installation, it will be necessary to associate several plates in series or parallel to obtain the desired voltage and current levels.

For the connection of photovoltaic solar modules, there are three possible options:

#### 1-Parallel connection of solar panels:

• all the modules are connected by their positive poles and, separately, by all the negative poles. With this, what we achieve is to increase the current generated in the branch (sum of the currents of each panel) but maintaining the same voltage as that of one of the panels that make up the branch.
• In other words, if we connect the panels in parallel, at the output of the branch we will have the sum of the currents of each “sub-branch” and the output voltage of each “sub-branch”. We see it better with an example?
• Let us consider that we have an isolated photovoltaic installation made up of 3 branches in parallel with, for example, a 12V solar panel, with a maximum nominal voltage of 18.4V and a maximum current of 8.37A.

If there were no losses of any kind (hypothetical case), the diagram for connecting solar panels in parallel could be represented as follows: #### 2-Connection of solar panels modules in series :

• for this type of configuration, the positive pole of one module is connected to the negative pole of the next, and so on with as many panels as necessary.
• With this, it is possible to increase the voltage and maintain the same value of current generated.\
• The voltage generated will be equal to the sum of each of the voltages of each panel that makes up the branch (string), or in other words,
• we multiply the unit voltage by the number of panels in the branch, since we must always connect panels of the same characteristics with each other. Let’s see then with an example?

Let’s consider that we want to have solar panels for a motorhome, made up of a branch with 3 panels in series with 37.45V voltage and 8.98A maximum current. If there were no losses of any kind (hypothetical case), the connection scheme of the plates in series could be represented as follows: ##### example:

As we can see indicated in orange, at the output of branch (c), we will have the voltage resulting from the sum of each of the voltages of each panel that make up the branch in series (112.35V) and the current will be the same as that of one of the panels (8.98A).

Surely you are thinking, what happens if a panel breaks down and I have to change it for a different one? Well, let’s imagine that, since we can’t find the same panel on the market, you want to buy photovoltaic panels with the following specifications: 31.40V maximum voltage and 9.33A current.

What will happen when connecting this module in series with the other panels already installed, is that the entire branch (string) will start working at the current of lesser magnitude, in our case as the Solar World SW 290 module has a current ( 9.33A) greater than the modules already installed (8.98A), the installation will not be modified.

In the event that our module has a lower current than those already installed, it will affect the entire string and a drop in production will occur, therefore it is not recommended to use replacement modules with currents lower than those of the installed modules.

#### 3- Mixed connection of solar panels :

it would be the last configuration option that we can find, in this case it would be a configuration where we find branches with panels connected in series and, in turn, these branches, connected in parallel. This configuration is used when we must achieve very specific output currents and voltages, and then we “play” with the options that the different types of connection give us. Let’s see an example about it  As we can see in the electrical diagram, at the point (node) (c) of the first branch (string).we have the sum of the panel voltages and the unit current, at the point (node) (d).

which is the output of the system, we will have the same output voltage of each of the branches, but as output current it is the sum of the output current of each of the branches, since the two branches connected in parallel.

As a practical summary, let’s say that in series connections the total current (output) is equal to that of one of the panels that make up the branch (string) and the total voltage (output) is the sum of the voltage of each panel connected in series. In parallel connections, the total (output) voltage is equal to the output of each branch and the total (output) current is the sum of the currents in each branch.