The No.1 criterion of choosing solar panels is output power, which should be sufficient to power your system. So first you need to have a good estimation of the power consumption of your system in one day. For example, if you want to provide continuous power to a 12Watt PC, your panels should be able to generate 12Watt*24Hour = 288Wh. However, the wattage specification of solar panels is rated under a ideal environment, where the Sun is directly overhead at local noon and the atmosphere is clear and dry. So the real output of your panels depends on the season, the weather and the latitude, where your system is going to be deployed.
In a panel's specification, it usually says that the panel is P Watt measured under the Sun irradiance of R Watt/m2. Suppose the panel is perpendicular to the sunlight, the real output wattage is
P * RealRad / R,
where RealRad is the real solar irradiance at the panel surfance, depending the distance that the sunlight travels in the atmosphere and the atmosphere condition. Given the month, day and local time, we can calculate the angle and height of the Sun in the sky, and thus obtain the distance d that the sunlight travels in the atmosphere. The the real Sun irradiance is
ReadRad = SolarCon * Bras(Tbf, d),
where SolarCon is the solar constant irradiance at top of the atmosphere, Bras() is Bras Solar Total (Diffuse + Direct) Irradiance Model, and Tbf is the turbidity factor, which is 2 for clear sky and 5 when smoggy.
The following figures shows the output power a panel (P=120Watt and R=1000Watt/m2)generates in a clear summer day (August 1st) and a clear winter day (December 1st) at Urbana-Champaign, IL (N40:06.59, W88:14.35).
Typically in your system, there is a battery to store the energy output from the solar panels, and a charge controller to optimize the battery charging process. So the energy eventually stored in the battery further depends on your battery type, and the charge controller. Click here to download the detailed solar panel calculator. For an introduction of batteries and charge controls, see battery and charge controller.
A solar panel has two output wires: positive(+) and negative(-). And the ends are usually MultiContact-3 male/female connectors. You can directly connect the solar panels to the battery. But a charge controller can improve the charging efficiency and battery lifetime by regulating the charge voltage and current. We will talk about the charge controllers in detail later.
Most charge controllers or batteries use standard screw or terminal connectors. So you need adapters from MutiContact-3 to standard connectors, unless you want to just cut the MultiContact-3 connectors and put other connectors onto the wires.
To make such adapters, you actually only need one MultiContact-3 cable for each panel, and cut it in half. Buy a longer one if you need adapters as well as extend cables.
Be caution when you want to use longer extension cables, since more energy will be wasted on the cables. Using heavy duty wires (preferable at least 10AWG) to reduce the energy waste on wires. Here is a chart about how to choose wire sizes.
If your system needs multiple solar panels, you can wire them in parallel or in sequence. The output voltage of a solar panel is relatively constant through the daytime (typically 15.2Volt). So the variance of the output wattage throughout a day is reflected by the output current. Therefore you can wire multiple panels in sequential to obtain higher current, or in parallel to obtain higher voltage. Even though higher voltage can reduce the energy loss on wires, you may need voltage inverter to match the input voltage for devices. And the voltage inverter also incurs energy loss.
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