Solar Power

Solar power

Estimated study time 1 hour

The learning outcomes for this section are:

  1. Introduction to solar
  2. Solar Panel safety
  3. Solar panel Specification Terminology
  4. Interconnecting panels
  5. Solar Charge Controller (regulator)
  6. Panel Types
  7. Recommendations

1 Introduction to solar

There are many factors involved in order to achieve an efficient Solar installation, it's not just a simple case of wiring a few panels to a battery, a good system will need to consider all the points below:

  • Location - Clear sky or mostly patchy cloud
  • Use of boat - Weeks at anchor, or the odd night?
  • Shading - Rigging, or spars causing shadows, or clear view to the sun.
  • Rating (Watts) - is there enough power from the panels
  • Panel Type - There are 2 main types of panel to consider.
  • Cabling - voltage drop, circuit protection and safety
  • Panel voltage - wiring configurations - series or parallel
  • Regulator - Type, maximum voltage, maximum current
  • Boat voltage - compared to panel voltage, compatible with regulator..

Maintenance, faultfinding, repairs and testing is difficult if a set of baseline figures are not available, so a good knowledge of the specification of the equipment and how the system is configured essential.


2 Solar Panel Safety

Solar panels are producing electricity all the time there is light falling on the panel, so you are working with live DC, this is not much of a problem for single panels, but voltages can add up as you connect multiple panels together, many systems run voltages of up to 100V dc. Read all the safety guidelines in the manufacturers literature before working on any equipment, the list below is not exhaustive.

  • Mask off panels with cardboard or a blanket on larger series connected arrays during maintenance, repairs and installation.
  • Tape over any exposed panel terminals with insulation tape until they are connected.
  • Most importantly, thoroughly read the manual for the specific equipment you are working on, most manufacturers have this online if it's an older system. different technologies and configurations require different considerations so there is no one size fits all approach.

3 Panel Specification Terminology

The video lessons in this lecture cover all the terminology below in context and in much more detail, however it's good to give these a quick once over beforehand.

3.1 Extreme values

The extreme values of voltage and current that a panel can produce are quoted in the literature, we can test the performance of individual panels easily using these simple specifications.

  • Isc - Short circuit current (Maximum Amps the panel can produce under a dead short)
  • Voc- voltage open circuit (Maximum volts the panel can produce with no load)

By definition - the Isc and Voc can never occur at the same time, but they are essential values to consider when it comes to testing, wiring configurations, cable sizing, circuit protection and charge controller selection. They are the extreme values the panels can produce in either voltage or current, but as the panel cannot deliver both these values at once, they are of less use when it comes to calculating potential power output.

3.2 Working values

All panels have a 'sweet spot', this is a current and voltage balance that produces the maximum power, this is referred to as the Mp or Mpp

  • Mpp - Maximum Power Point

The Mpp is the working power the panel can produce and the table below shows values attained based on:

  • STC - Standard Test Conditions, more information on STC below.

The figures in the table below help us calculate how much actual power we can harvest. Let's look at some more terminology:

  • Pmpp - Power at Maximum Power Point (Actual Max Power Watts)
  • Impp - Current at maximum power point (Actual working Amps)
  • Vmpp - Voltage at maximum power point (Actual working voltage)

Note that nominal power (Pmpp) is simply Impp X Vmpp

The video lessons below explain all the above terms in context, simply knowing the definitions at this stage is all that's required.

3.1 How they are measured

There are 2 standards when it comes to specifications of solar panels; STC and PTC.

3.1.1 STC

Most manufacturers quote their figures based on STC - this stands for Standard Test Conditions. The standards are based on a theoretical 1000W of Sunlight per m2 on the panel, and many other conditions, such as temperature and wind.

In order to reach the specification figures quoted by the manufacturers, you must test on a bright sunny day with the panels facing the sun without shadows.

3.1.2 PTC

PTC stands for PVUSA Test Conditions, PVUSA stands for Photovoltaics Utility Scale Applications. It is very similar to the above standard, but provides slightly lower values, that many consider to be more realistic.


4 Interconnecting Panels

How panels are interconnected defines how the power is delivered, knowledge of how panels are wired is essential when it comes to fault finding and repairs. Panels, just like batteries can be wired in parallel or series.

4.1 Parallel

Solar panels should ideally located where there will be no shadowing, even a small shadow across a panel reduces the output significantly. If shadows are unavoidable, then the panels should be wired in parallel. That is the positive of one panel to the positive of another, the negative of one panel to the negative of the other.

Parallel: Voltage remains the same but current adds up.


4.2 Series Connected

If you can be sure there will be no shadowing, say on at the highest point on a motorboat where there is absolutely nothing that can block the sunlight, then series wiring is a better option as this increases the voltage, thereby reducing the current, the voltage drop and cabling requirements. Your solar charge controller must be able to cope with the higher voltages though. Series wiring is the positive of one panel to the negative of the next.

Series: Voltage adds up but the current stays the same

Watch the lesson below.

5 Solar Charge Controller (Regulator)

Solar panels require a dedicated solar charge controller, often these are referred to as solar regulators. The panels connect to the Solar Charge Controllers Input terminals and the controllers output is connected to the yachts' un-switched (always on) domestic busbars via a fuse. Solar panels are an expensive investment, so it's worth investing also in a compatible regulator. A good upgrade to an existing older solar system would be to replace the charge controller.

There are 2 main types of regulator:

5.1 PWM

  • PWM - Less efficient/ Less Expensive

Pulse width modulation controllers are still available, but they are not very efficient by modern standards. They work by pulsing the panel voltage so the battery sees an average lower voltage, effectively turning on and off very quickly. The controller varies the width of the pulses to control the voltage, and some older versions can introduce interference on the dc system, causing problems with audio and visual equipment.

5.1 MPPT

  • MPPT - More efficient / More Expensive

The MPPT stands for Maximum Power Point Tracker and is basically an algorithm that is implemented in solar charge controllers that can squeeze as much power from the panels as possible. The factors that affect this point are

  1. The ambient temperature
  2. The solar cell temperature
  3. The solar radiation

The MPPT controllers can extract the maximum available wattage from solar panels by making them operate in most efficient voltage. More simply, the MPPT will compare and adjust the voltage that comes from the solar panels with the voltage of the batteries. This helps to get the maximum current to the batteries. The most up to date 'Fast' MPPT controllers are 30% more efficient than an equivalent modern PWM controller.


6 Panel Types

There are 2 main types of solar panel:

  • Polycrystalline - Less efficient / less expensive.
  • Monocrystalline - More efficient / more expensive.

There used to be a bigger disparity in performance between these two main contenders, this gap has narrowed somewhat over the last few years. Most people prefer Monocrystalline as watt for watt the panels are slightly smaller, but the gap is closing.

If a vessel has an existing array of older Polycrystalline panels, in good condition, there would be little to gain from upgrading the panels, a much better return on investment would be to upgrade to a more modern Solar Charge Controller, such as a fast MPPT.

7 Recommendations

With current technology (2020) the best system for:

7.1 Sailing yachts

Or any vessel that throughout the day would often cast shadows on the panels would be:

  • Parallel wired panels
  • Monochrystaline panels
  • Fast MPPT controller.

7.2 Motor Yachts

Or any vessel with, enough space, stability, and structural integrity to mount panels high away from all shadowing,

  • series wired
  • Monochrystaline
  • Fast MPPT Controller

7.3 Existing system upgrades / testing

  • 7.3.1 Controller upgrade

For established systems, test the Isc and Voc of the individual panels on a bright sunny day, and compare these to the original specifications, if they are within 15% and in otherwise good condition, keep using the panels, and upgrade the controller to a fast MPPT.

  • 7.3.2 Re wire - Parallel to series

If your panels are wired in parallel yet there is a clear un shadowed view of the sky, with the new controller, you can rewire the panels in series, raise the solar panel voltage, and gain much more from your system. Converting from parallel to series will not require new cabling, as this configuration will result in lower current, but still I'd do a quick voltage drop test from panels to controller, under full load.

  • 7.3.3 Re wire - Series to parallel

If your panels are wired in series yet there is regular shadowing, they'd probably perform better in parallel. But this can be problematic, if the individual panel voltage is less than the battery voltage, it's not possible without a boost regulator, or your current regulator might not be able to cope with the lower voltage / higher current. The other consideration is the panel to controller cabling and circuit protection will need re-sizing as the current will now be higher, remember the Isc from each panel will add up in parallel.

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