Sizing and selecting solar controllers for Boats
by Tom Whitehead
With the rising awareness of energy use and conservation, and the expanding demand for power on both day-use recreational and offshore cruising boats, many boat builders and boat owners are turning to alternative sources of power, including solar. The development of lightweight, marine grade solar panels that can be mounted on canvas, or fully integrated into the decks has made this technology more available to all. The choice of what panels to install is often straightforward and decided by their size and where/how they can be mounted. The decision of how to wire the system and choose the best type and number of controllers is more complicated, and is influenced by several factors including – shading and panel configuration; panel voltage, current and wire runs; buck vs. boost controllers; battery type and voltage; temperature sensing capabilities and methods. You need to consider all the variants of the system to make the best choice for a controller.
The sophistication and efficiency of solar controllers has improved greatly in recent years. The MPPT (Maximum Power Point Tracking) controllers have essentially eclipsed the older PWM (Pulse Width Modulation) versions, offering much greater efficiency at harvesting power from the sun.
A quick summary of the differences between PWM and MPPT
The earlier designs of solar panels were made from smaller cells and cut cells put in series, resulting in voltages that were often higher than was safe for applying directly to a battery for charging. The PWM (Pulse Width Modulation) solar controllers were developed to keep voltage at a safe charging level. These controllers feed the power straight to the battery until it reaches a predetermined acceptance level. They then keep the voltage constant and safe by ‘pulsing’ the panel voltage on and off, but this results in significant losses in actual charging current to the battery.
The MPPT (Maximum Power Point Tracking) controllers register the optimal combination of amps and volts in any set of conditions, and then track this ‘power point’ as conditions of light intensity and air temperature change. The DC (direct current) panel output is inverted to high frequency AC (alternating current) and then converted back to DC to charge the battery. The result of this inversion/conversion process is that more amps can be delivered to the battery than were produced by the panel. These controllers are also more efficient in low light angles and soft shading conditions.
This article will focus only on MPPT controllers.
Defining electrical terms for Solar Panels
We will assume the panels have already been chosen. What information do you need about the panels to help you choose the appropriate controller? All panels are given ratings under Standard Test Conditions (STC). The STC are: 1000 watts per square meter (W/m2) of irradiance; a cell temperature of 25C(77F); and an air mass coefficient of (AM) 1.5. This is a combination of ideal conditions that will rarely occur, but nevertheless it enables important information to be provided so that cables and fuses can be sized safely. Depending on the type and number of cells in the panel (cut cell, polycrystalline, monocrystalline), each will put out different amounts of voltage and current (amperage) in varying conditions.
Each rating is defined here:
- Max Power Voltage-Vmp (V) – This is the voltage that occurs when the module is connected to a load and is operating at its peak performance output under standard test conditions (STC). This is determined by the number of cells in series, their basic voltage and the temperature of the cells.
- Max Power Current – Imp (A) – Maximum current the panel will produce when under load. This is dependent on the size, type and quality of the cells, and the strength and quality of the available sunlight.
- Watts – The rated power of a panel under STC. This power rating is equal to the Vmp (max power voltage) X Imp (max power amps). For example, the SP 104 has a Vmp of 18.22v X Imp of 5.71a = 104 watts
- Open Circuit Voltage-Voc (V) – Maximum voltage the panel produces when not connected to a load. Use Voc when calculating voltage limits for series set ups, as this is the maximum possible voltage the panel can put out – even if only for a moment.
- Short Circuit Current Isc (A) – This is the current through a solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited). Panels built with the same type of cell, will all have the same Isc or short circuit current.
Shading and Panel Wiring Choices
Effects of shading
Solar panels perform best in bright sunlight at solar noon with little or no shading. Most boats, either sail or power, will create some type of shade for a solar installation unless the panels are on the very top of a fly-bridge, or extremely far aft on a set of davits or a bimini.
Soft Shading, essentially shadows, will reduce the output of an individual cell if it is completely covered. Because solar panels are made up of strings of individual cells wired in series, the shaded cell or cells consume power from the non-shaded cells in the series string. Most panels have bypass diodes built into the panels to help alleviate the problem, but often the panel output is reduced by half.
Hard Shading, or when a totally opaque object covers a cell, greatly increases the losses. A hard shadow “…has the effect of knocking out a percentage of the panel’s output equal to the percentage shading of a single cell. ” (Calder, Professional Boatbuilder, #182) Hard shading can also cause the other cells in a string to back feed the covered cell, creating a hot spot, and possibly a fire, so never leave boat cushions or other similar objects on a solar panel!
Some cells are small or cut, and others larger. The panels made up of smaller cells do poorer in shade, as it is easier to completely cover the individual cells. Larger, 5 – 6” mono cells are harder to completely cover and therefore do better with soft shading.
Panel Wiring Configurations
Solar panels can be wired three different ways – Isolated, in Series or in Parallel.
Isolated – By far, the best way to manage potential shading of panels is to give each panel its own, individual solar controller. If a cell on one of the panels is shaded, it only effects the output of that panel, while the other panels continue to function independently. The drawback to this is the number of wires and controllers to install and manage.
Series – By putting panels in series you essentially create one large panel and this allows you to run a single set of wires down below to a single, larger controller. This is only recommended when the risk of shading is nonexistent or very low, and wire runs need to be minimized. Only panels with identical cells of equal amperage can be put in series. Remember the result of putting panels in series is that the voltage of each panel is added together, but the amperage stays the same. This information is important when choosing a controller. For example: Three SR 160 watt panels in series each have a Vmp of 18.6v, but a Voc of 23v. If they are put in series, the controller needs to be rated for at least 23v x 3 = 69v, not 3 x 18.6 = 55.8v. Also, the total wattage is now 480 watts. However, the Imp 8.6a and the Isc 9a, stays the same. The downside of large series systems is the high DC voltage. Even though the wires size can be relatively small, some builders and boat owners are not comfortable with 60 – 80 volts, DC running through the boat.
Parallel – Panels are sometimes wired in parallel to avoid increasing voltage and to minimize wire runs. In this situation, blocking diodes must be installed between the panels to prevent a higher producing (non-shaded) panel from back feeding a lower producing (shaded) one. These blocking diodes also create a voltage drop of approximately 0.7v. Only identical panels with the same voltage should be put in parallel. The result of paralleling panels is that the voltage stays the same, but the amperage is multiplied. For example, using the same three SR 160 panels mentioned above, The Vmp and Voc would stay the same but the amperage would be three times that of a single panel, or Imp 8.6a X 3 = 25.8a, total Imp. This higher amperage creates the need for larger cables to run from the panels to the controllers, and if the run is long, it could be a deterrent to this type of configuration.
Panel Voltage and Current
Buck vs. Boost
As noted above the total voltage and current ratings of panels combined in either series or parallel or even series/parallel combinations vary based on the wiring configuration. All solar controllers are rated to handle differing amounts of voltage, amperage and total watts. Most controllers bring voltage down to the acceptable charge level (Buck controller), but some raise voltage up (Boost controller).
With the advent of mono crystalline panels, using large cells in small configurations, it is now more common to have a panel that produces less than 12v, but with greater amounts of current. In fact, most of the more popular panel sizes under 100 watts produce less than the required charging voltage of a 12v battery (13.8v – 14.4v volts). Therefore, it is important to determine whether the output voltage of the panel (or panels) will be greater or less than battery voltage. If the max power voltage (VMP) is equal to or less than battery voltage, you need a boost controller. If the voltage is greater than battery voltage, you need a buck controller. At this time, only Genasun and Western make boost controllers. (Western also makes a versatile controller, the WM 10, that will do either buck or boost.) If the panel voltage is over the battery charge voltage of 14.4v, you need a buck controller.
Total Voltage & Watts
Once you have determined whether you need a ‘buck’ or ‘boost’ controller, you next need to compare the total wattage, voltage and amperage ratings of the panel or panels with the ratings of the controller. For example: The SR+78 watt panel at 9.6v Vmp requires a boost controller and the Genasun GVB-8-12-Pb has a max panel power rating of 105w. 78 watts is well within the 105 watt rating so you are good to go. The Voc of this panel is only 11.7v, and the controller is rated up to 63v. Finally, if the panel is operating at its maximum power (Vmp) at 9.6v it would be producing 8.1a. The max input current of the GV-B is rated at 8a, but we know from the manufacturer that max current up to 9a is fine.
Now let’s look at a ‘buck’ controller situation. The chosen panel is an SR+ 175 watt, with a Vmp of 21.6v. A Genasun GV-10 has a limit for max panel voltage (Vmp) of 34v, so it looks promising; but the rated watts limit is only 140w, and max output current is only 10.5a, so we need to move to a larger controller such as the Victron 75/15 which has a rating of 200 watts, max panel Voc of 75v and a max output current of 15a. One point of confusion is the difference between the rated current out put of the panel, and the max current output of the controller. Remembering that the MPPT controllers can actually produce more current than the panel itself. In this case, the SR+ 175 has a Vmp of 21.6v. Since Amps = Watts/Volts — 175/21.6 = 8.1a. So one might think that this panel is OK for the GV 10 since the Voc is fine and it outputs 8.1a. However, the controller is rated for Max Power Output, and If you divide 175 watts by the battery voltage of 12v you get 14.5a. This output current exceeds the 10.5a rating of the controller. It is always important to check the total wattage rating of the controller, as this usually will guide you correctly.
When putting panels either in series or parallel, remember to not only add the wattage, but check the total Voc of the panels when in series, and the total Imp of the panels when in parallel. Suppose we decide to put four SR 108 panels in series on the top of a flybridge where there is no shading. Total wattage – 432w; total Voc – 61.2v; panel output current is 8.6a; but maximum current to a 12v battery bank is (432/12=36a.) You might think the Victron 100/30 would work as the wattage rating is 440w, the max panel Voc is 100v, BUT- the max output current is only 30a, slightly less than the 36a you need. Most panels will never quite reach their peak performance, so the 30a might be close enough, but to be safe you need to move up to the 100/50 with the 50a rating.
Putting those same panels in parallel, the total wattage is still 432w, the Voc is now only 15.3v, but the panel current is now 34.4a. (The panel current has a direct effect on the wire size running from the panels to the controller, and for this parallel set up this wire will need to be a much larger diameter, depending on the distance there and back.) The wattage of the parallel set up should still be the primary guide, but again, the power output to a 12v battery system is larger than 30 amps, so always check the max output amps of the controller as well.
Minimum panel voltage
Controllers have a minimum voltage to turn on. This important rating is often overlooked. With the advent of monocrystalline, lower voltage panels, it is important to check for all buck controllers to ensure the panel V is high enough. Even boost controllers have a minimum amount of voltage needed to turn on, so check the fine print! A GV-10 requires 2V over battery voltage (14v) to function; the Blue Sky only needs 0.2v over battery voltage; the Western requires 2.5v over battery voltage and the Victron needs 5v over battery voltage.
Battery Voltage & Type
Not all battery systems are 12v! Many larger boats are moving to 24v systems, and for electric propulsion, there is a wide range of voltages from 36v, 48v and on up. Make sure of the voltage of the battery bank that you are going to charge.
The days of flooded lead acid being the only battery type on a boat are long gone. The various types of lead batteries can include – Flooded, Gel, AGM (several variations), TPPL(thin plate, pure lead), Carbon Foam (Firefly), Lead Crystal, etc.,etc. Then there are the increasing Lithium-ion based chemistries.
All of these various types and manufacturers can require different charge parameters. When picking a solar controller for a specific battery type, make sure of what the charge parameters are, and if the stock settings do not suit your battery bank, you will need to find a controller that is fully programmable for the specific needs of your battery chemistry and construction.
Most solar controllers use a form of temperature compensation to moderate the target charge voltages based on battery temperature. A cold battery can be safely charged at a significantly higher voltage than a warm battery. Most published charging voltages are for a standard temp of 77 F. The various controllers get temperature information in different ways. Some, like the Genasun and original Victron use internal temperature sensing. This means that they need to be positioned within 12” of the batteries to sense the ambient temperature, or at least be in a compartment that has the same temperature as the battery compartment. Others like the Blue Sky and the Western use a wire that runs from the battery post to the controller. The newer, “Smart” Victron controllers now use Bluetooth to both send and receive temp and voltage information. They also allow you to view their performance with an app that you can download to your smartphone, tablet or laptop computer. The simpler controllers either have LED indicator lights to let you know what they are doing or a small readout screen. What ever controller you choose, it is important to decide where it will be located for viewing or not, and how it gathers the temperature information it needs.
When installing solar panels, you need to estimate the amount of shading. This determines how multiple panels will be wired – either isolated, in series or in parallel. This defines the system voltage and whether you need a buck or a boost controller. The panel configuration determines the total wattage, voltage and power output that need to be matched to the controller’s ratings. You also need to consider the battery voltage and the type of battery chemistry and charge parameters. Finally, remember the need for temperature compensation, and if/how the controller needs to be read or monitored.