Solar energy: power, power and thick cables

In the first part about #solarenergy (a first lesson in technology)  we have seen how we get from sunlight to stored electricity. That is not very difficult in concept, but if we want to connect that in a good way, then we have to calculate some. What you don't want is for things to catch fire, because your cables are too thin and burn out. Then you've lost all the energy you put into it anyway. And that is, of course, a shame, besides being a waste of all the effort and resources.

Solar energy: all components and their units

Previously I've introduced you to the parts from the picture above, but we haven't discussed them in detail yet. I'll include them in a list below, and then discuss them briefly.

• Solar panel
  A solar panel has a number of properties and dimensions that are important for calculations:
  • a maximum power (in Watt-peak, or Wp)
    What is the maximum yield of the panel in ideal conditions? That means: if the sunlight falls on it completely, straight from top and there is no shade. This value is often an upper limit, but nevertheless important to know, because based on this we are going to calculate the thickness of the cables (size).
  • a maximum open clamping voltage (in Volt)
    This is the voltage between the plus (+) and the minus (-) of the solar panel. This voltage will drop as soon as a load is connected to it. That can be a direct user, or a solar charger. More on this later.
  • a efficiency factor (in %)
    How well the panel converts sunlight into electricity. If the efficiency is higher, then an equally sized panel will generate more electricity under the same conditions. Common values are 15-25% and these values depend on the age and type of solar panel.
  • temperature sensitivity
    Solar panels work less well when they get too hot. Just like plants grow less well when it's too hot. But since all the solar panels suffer from this, I leave them out of the equation.
• Solar charger or charge controller
  A solar charger or charge controller converts the varying output of a solar panel to a constant output voltage. That way, a battery can be charged. Here are two types of: PWM and MPPT. The difference between these two is in how they convert the varying yield to a stable output voltage.
  • PWM: Pulse Width Modulation
    Works by adjusting the input voltage of the solar panel downwards, so it can charge the battery in a good way. A disadvantage is that this system works less efficient if the output voltage of the panels is lower or becomes lower than the charging voltage of the battery. This can occur because there is a lot of cloud cover, or shade. This system works well if the connected panels give a voltage slightly above that of the battery. Compare a large container of water that overflows. Everything you don't use flows over and flows away. If the water flows a little slower, it will take longer for your large bowl of water to be full.
  • MPPT: Maximum Power Point Tracking
    This charge controller is more advanced than the PWM charge controller and therefore, as a rule, more expensive. It works by adjusting its input voltage to provide as much power as possible to the battery. It does this by following the Maximum Power Point. That is the point where the transferred power is maximum. I won't go into the math of it, but it works better if the solar panel has a (much) higher voltage than the battery to be charged. Compare it to running to the bus without sweating or running out of breath. On hot days, you can run less fast to the bus than on cold days, and if you're rested, you can run a little bit faster than if you're already tired.
• Battery
  In which the electricity can be stored. The relevant value is Ampere Hour (Ah). That stands for how much power (Ampere: A) a battery can supply for an hour. A battery that you can use 100Ah can supply a power of 1 Ampere for 100 hours. That's not so much power (power in W = current in A * voltage in V). A flashing light of a car is 21 Watts. So if we calculate how much current that draws: 21W/12V = 1.75A needed. This battery is finished  after 100/1.75=57 hours. Fortunately, a car's flashing light is not on all the time. :)
• Invertor
  Converts the stored energy from a battery to 230V AC voltage, just as we also get it out of the wall socket at home. Depending on what you want to connect, it needs a certain peak power. You can find invertors with power between 150 and 4000W. That matters a lot in the cable thickness and how quickly your battery will be discharged. The higher the power you want to use, the thicker your cables need to be and the faster your battery will run out.
• Cables
  I assume copper cables. These are the most common cables. These can be obtained in different thicknesses for this type of application, from 3 square mm (3mm2) up to 175mm2.

Calculate cable thicknesses

If you use a cable to transfer current, this cable will resist it and start to counteract by getting hot. This resistance is normally not a problem when working with alternating current, but when we work with solar energy, it is often direct current combined with a not too high voltage. If you want to draw a lot of power, then the current will be very high. There are a few important formulas that we have in the triangle current/voltage/power. I'll teach you some tricks.

Below is a picture: P/VI, where P is the power in Watt, V the voltage in Volt and I the current in Ampere.

Now you have learned a piece of the puzzle, but not yet how to get from a current to a thickness of a cable. If you let power pass through a copper cable, that cable has a resistance. The resistance depends on the length of the cable and the thickness of the cable. If you want a safe margin, this is your formula: the thickness of a cable in mm2 is current (in Ampere) x the distance (in m) x 02. That's why it was so important above that we could calculate the amperage.