There are many technologies on which solar panels are made.
The most common panels are made on the basis of a silicon crystal. Further, they are divided into two types, they are polycrystalline, and monocrystalline. There are other variants, but these are the ones that are most often on sale. Despite the fact that polycrystalline panel is considered more efficient, in fact, it all depends on many factors, such as weather, conditions of use, etc.
By design, solar panels are divided into rigid and flexible.
Rigid solar panels are enclosed by glass on top and encased in a metal frame, while flexible ones are made on thick plastic that take the shape of the surface on which the panel is located. It is good to use flexible panels where you want them not to be visible, it can be attached directly to the roof of the car. Of the disadvantages of flexible solar panels, it is a limited lifetime, and they cost one and a half to twice as much as conventional. In addition, when installing the solar panel, it is recommended to leave a small gap for ventilation, because when heated, the panel efficiency drops. So it is better to choose a classic rigid solar panel, and the flexible should be considered only if it is not possible to install a conventional solar panel.
A modern solar panel gives approximately 150 watts of power from each square meter.
Although measurements in real life show somewhat smaller results, about 110-130 watts in sunny weather, and 70-80 watts in cloudy weather. So if the panel says the maximum power is 150 watts, that doesn’t mean you’ll get that value, but rather focus on 90-110 watts on average. Efficiency is also affected by the dirtiness of the solar panel, so you need to monitor its cleanliness. A very important factor is the angle of incidence of sunlight, so it is worth to provide a design that allows you to turn the panel towards the sun. The greatest efficiency is achieved when the rays of the sun hit the panel strictly perpendicular. It is best to provide a removable design, this will allow you to put the van in the shade, and the solar panel to the sun. When buying a solar panel, you should consider only two important parameters, this power output, and voltage.
If everything is more or less clear with the power, the voltage issued by the solar panel may differ from the voltage on-board network of your van.
There are panels for 12, 24, 48 volts, and we need to determine which of them is preferable. It is clear that you can not connect the solar panel directly to your van. You will need a special charge controller to make it work. Charge controllers come in two types, PWM and MPPT. PWM controllers are simpler and much cheaper than the MPPT, but they are also much less efficient. The principle of the PWM controller is simple, to put it simply, it “cuts off” the excess voltage produced by the solar panel, and leaves only the voltage necessary to charge the battery. When using such a controller, you need to ensure that the voltage issued by the solar panel, coincided with the voltage on-board network of the van. If the on-board network of the van has a supply voltage of 12 volts, then the panel must be at 12 volts. MPPT type solar panel controllers work on a different principle, and they are much more efficient. So if you do not have the task to save a lot of money, consider buying them.
MPPT controllers can be used with a solar panel that has a different voltage from the van’s on-board network.
Even if your solar panel is 48 volts, you can use it with the MPPT controller with a 12 volt onboard network. The thing is, if you use a PWM controller, and a 12 volt solar panel, in low light, the voltage may not be enough to charge the batteries. If you have a 48 volt solar panel and you use the MPPT controller, the voltage will be sufficient in most cases, we just get a lower charging current, and therefore a longer time to charge the batteries. Another advantage of the MPPT controller is that the solar panels can be connected in parallel or in series.
Keep in mind that you CANNOT connect a solar panel to the controller if it is not connected to the battery. And if you want to disconnect the battery from the MPPT controller, disconnect it from the solar panels first, otherwise the controller will fail. Modern controllers have an output, to connect the load directly. But I would not advise you to do so, as they are not designed for high currents, and long-term use. Connect the load directly to the batteries.