How to calculate the power and size of a solar panel?

How to calculate the amount of electrical energy generated or consumed

The amount of electrical energy is expressed in watt hours or Wh. It is simply a matter of multiplying the power in watts, by the time during which it is consumed or charged. For example, a 1000 watt radiator running for one hour will have consumed 1000 Wh or 1 kWh. The kWh is the standard unit of measurement that can be found on your energy provider’s bills .

To estimate the energy needs of each of your appliances, simply multiply the rated power in watts by the usage time in hours . You will obtain in Wh the minimum amount of energy to produce to power your devices .

It should be noted that the power produced by a solar panel is greater at noon than at the beginning of the morning or late in the afternoon, you will surely need to use a battery to store the electricity and a converter to power your devices.

What factors influence the production of electricity from a photovoltaic panel

The energy production of a photovoltaic panel depends on the quality of the equipment , as well as its environment and the circuit it feeds.

solar panel power

Expressed in kWp or kilowatt-peak, it is the electrical power produced under ideal conditions . Depending on the technology, one square meter of solar panel can produce 60 to 150 Wp . Once the peak power is known, you can calculate the total area of ​​solar panels that you will need.

The solar radiation of the area

Fewer kilowatt hours (kWh) per peak kW (kWp) are produced in Galicia than in Malaga. This magnitude, the sun, is interesting because it makes it possible to evaluate the annual production taking into account the place where the panel is installed based on the peak power of the panels .

Installation performance

If the panel is not oriented to the south and with an angle of 30-35° with respect to the horizontal, it produces less electricity than it is capable of, since the sunny surface is smaller . Taking this performance into account makes it possible to improve the installation.

How to calculate the energy produced

The total energy produced E_p represents the number of kilowatt hours (kWh) produced by the panel in a year . It is calculated using the following formula:

Ep=r*Ens*PC

But what you are interested in is calculating the peak power of the panel that you want to buy to cover your electricity needs or sell a certain amount of energy to an electricity provider.

By transforming the previous formula, we obtain:

P_C≥(E_p*F_CO)/E_ns

with F_CO=1/r with a correction coefficient to compensate for performance losses .

single panel performance

The performance of a panel depends on its components:

• 6% for amorphous silicon;
• 15% for polycrystalline silicon;
• 16 to 24% for monocrystalline silicim.

Abbreviations used

• kWh : kilowatt hour
• kWp : kilowatt-peak
• E_p : total energy produced
• r : yield
• ens : sun
• P_c : peak power
• F_co : corrective factor

How to determine solar radiation E_ns

Check the following map. Locate the place where you are going to install the solar panels, look at the color of the area where you are and consult the legend. In case of doubt, take the smallest solar radiation value for greater safety .

Atlas of Solar Radiation in Spain. Source AEMET(c) State Meterology Agency

How to compensate for performance losses

In the following table you can find by what factor to multiply the annual production of the panel to compensate for the losses related to orientation problems .

Correction factor F_CO=1/r

The losses in the conversion, the storage in the battery or the passage through the cables , suppose an order of ten percent. They depend on your installation and are difficult to accurately assess without knowing the nature of each installation. Choose a sufficient margin if you are not sure.

Summary of the different abbreviations and their meaning