The maximum power of solar panel Pmax = open circuit voltage × Short circuit current, which is their ideal power, and usually, we measure the solar panel as the rated power PM. In practice, the rated power is less than the maximum power, mainly because the output efficiency U of solar panels is only about 70%. In use, due to the different light intensity, the power at different times is also different. According to the experimental data, its actual average power P = 0.7pm.

If the solar panel is to drive the load directly and make the load work stably for a long time, the rated power of the load is PR = 0.7pm. If the power of the solar panel is selected according to the power of the load, the power of the solar panel is PM = 1.43pr. That is to say, the power of the solar panel should be 1.43 times the load power.

When choosing the power of the solar panel, the power consumption of the load should be selected reasonably, so that the power generation and power consumption can be in a state of balance. Of course, the power generation of solar panels will also be affected by the season, climate, geographical environment and lighting time, and other factors.

The use of battery (here only take summer as an example to introduce the use of solar panels and batteries in general)

Battery is a kind of container for storing electric energy, which is often used as the energy base of other circuits. As the power generated by solar panels is limited, the storage capacity of the base should be expanded as much as possible, but not unlimited, because solar panels can only generate electricity in the daytime, and its daily power output M = generating power (maximum output power) × Effective illumination time × Therefore, its daily energy is equal to the product of output current and effective illumination time, i.e. C = IH (ah).

The capacity of the battery is the product of the discharge time and the discharge current, so the calculation formula is C = IH (unit ah, is the rated 1A current discharge for one hour). How to calculate the capacity and power of solar panels and batteries We can change the formula of electric power from P = IU to P = IUH / h = Cu / h.

According to the above formula, the capacity of the battery can be calculated. To be more accurate, the charging efficiency of the battery should be considered. The charging efficiency of the battery is generally between 65% and 80%. The charging efficiency depends on the charging mode, that is, the charging rate and the utilization rate of active substances inside the solar panel. The general experience is that the charging efficiency is selected according to the charging time rate and current rate.

The longer the charging time is, the smaller the current is, the higher the conversion efficiency of power safety is, and the higher the compensation value is; The shorter the charging time, the higher the current, and the lower the conversion of safe electric energy, as shown in the attached table：

For example, for a module with a single crystal silicon solar panel, the maximum output power PM (rated power) is 25W, the peak voltage (rated voltage) UMP is 17.2v, the peak current (rated current) is 1.45a, the open-circuit voltage is 21V, the short circuit current is ISC is 1.5A, and the effective lighting time in a certain area is 12 hours. The daily power generation of the solar panel and the required battery capacity can be calculated.

It is known that PM = 25W, H = 12h, u = 17.2v, the power generation efficiency of solar panel is u = 0.7, and the compensation value of battery is n = 1.4

Power generation of solar panels M = PM × h × u=25 × twelve × 0.7=210W

According to the above formula C = pH / u = 25 × 12 /17.2=17.44Ah

Then the actual effective capacity of the battery needs to be above C = 17.44 / 1.40 = 12.46ah

So in practice, we can choose about 14Ah capacity battery.

In the South and the North, due to the difference of geographical latitude, the sunshine time in summer and winter can be calculated according to the following formula H = 12 ± 0.09 ɡ, H is the sunshine time, 12 is the average sunshine time, 0.09 is the difference of longitude and latitude ɡ It indicates the geographical latitude of the location, ± in summer, + winter. Generally, the lighting efficiency is about 0.8 in summer, 0.7 in spring and autumn, and 0.5-0.6 in winter, so the effective lighting time is H = H × Light efficiency.

Matching of battery and solar panel

At present, there are many kinds and specifications of solar panel products. For batteries, there are generally 6V, 12V, and 24V. So how to connect the solar panel with the battery Generally speaking, the rated output voltage of the solar panel is 1.3-1.5 times higher than that of the storage battery, which is determined by the charging efficiency of the storage battery. The charging of the solar panel does not have the same choice as the charging of the storage battery with the commercial power. Moreover, its power fluctuation is relatively large when charging the storage battery, First, consider the cost of solar panels.

If the charging time rate of the battery is C10 and the charging compensation value is 1.4 times, then the voltage of the solar panel for a battery with rated 12V voltage should be 12V × 1.4 = 16.8v, which is close to the limit charging voltage of the battery. It’s the same with charging the mobile phone with solar panels. The higher the charging voltage is, the greater the charging power will be. Then other charging time and charging compensation values will be calculated separately.

When the solar panel is charged in parallel with the battery, it is also necessary to prevent the trees and buildings from blocking the light of the solar panel, or the solar panel can not generate electricity on cloudy days and nights. Therefore, a rectifier diode must be connected in series in the circuit to prevent the battery from discharging the solar panel in reverse when the voltage drops or does not generate electricity, When using diodes in parallel, pay attention to choose the same internal resistance.

For high-power solar panel modules (low-power is not required), to prevent the solar panel from being blocked under strong light, some of them will be seriously heated and damaged due to the lack of light, it is better to parallel a bypass diode at the two poles of the output end of the solar panel, and the current value of the bypass diode should not be lower than that of the solar panel.