How to calculate the power and size of a solar panel?
Important features
 Electrical energy produced and consumed
 Power
 sun incidence
 Performance
 Loss of performance
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 kilowattpeak, 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 3035° 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 : kilowattpeak
 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 .
West 
Southwest 
South 
southeast 
This 

0° 
1,075 
1,075 
1,075 
1,075 
1,075 
33° 
1,111 
1,042 
1 
1,042 
1,111 
45° 
1,190 
1,087 
1,042 
1,087 
1,190 
60° 
1,282 
1,136 
1,099 
1,136 
1,282 
90° 
1,818 
1,515 
1,471 
1,515 
1,818 
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
Abbreviation

Definition

W

watt

whah

watt hour

kWh

kilowatt hour

kWp

peak kilowatts

P

Power

p_p

peak power

AND

Energy

E_p

total energy produced

r

Performance

F_CO

Correction factor

E_ns

solar irradiation
