When designing an off-grid energy system that includes solar, properly sizing the solar panel array is vital to overall system performance and satisfaction. Ensuring the proper design, layout, and size will allow for the best operation of the off-grid system. Understanding the components and wiring options is fundamental to being able to be successful. No two systems are necessarily the same, with a lot of differences coming from system utilization and expected goals.
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Reading Solar Panel Specifications
When reading a panel label there are some key specifications that, when calculated based on how the solar panels are configured, will help steer the solar charge controller decision. I will go down the list and explain each data point and why it is important.
This is the panel’s maximum wattage output. The maximum wattage is found on a curve between the open-circuit voltage and the short circuit current. It is the optimum power output of the panel. When sizing a solar charge controller, this number is always additive when calculating maximum wattage, regardless of the wiring layout. In some situations, it could be beneficial to exceed the solar charge controller’s maximum wattage, ensuring adequate charger output in low light conditions.
Open Circuit Voltage(Voc)
This voltage is the maximum no-load voltage output of the panel. This number will fluctuate with temperature. When considering the size of a solar charge controller, this voltage needs to be calculated based on the wiring plan. MPPT solar charge controllers will have a maximum voltage that the solar array should not exceed. This is found by multiplying the open-circuit voltage by the number of panels in series.
Voltage at Pmax(Vmpp)
The max voltage is the voltage that the panel will produce its maximum wattage at. When evaluating your panel performance, you will take this number and multiply it by the number of panels in series. This will give you your working voltage going into your solar charge controller.
Short Circuit Current(Isc)
The short circuit current represents the maximum amperage that the panel will produce. However, it is the maximum amperage at zero volts. This current comes into play when wiring panels in parallel. Solar charge controllers will list a max current that the charger can handle from the panel array. Therefore, to find this number, multiply the short circuit current by the number of panels in parallel.
Current at Pmax(Impp)
Current at Pmax is the working current of the panel. This is the current that one would expect to see when the panel is putting out its max power. When evaluating your panel performance, you will take this number and multiply it by the number of panels in parallel. This will give you your working current being going into your solar charge controller.
Solar Panel Array Wiring
Before deciding what size solar charge controller you need, there needs to be some thought dealing with solar panel layout and wiring. How the panels are mounted to the roof, solar panel layout, and panel wiring can influence the number of solar charge controllers and wiring configuration. Therefore, let’s take a look at the various wiring methods.
Parallel Wiring Solar Panels
Wiring panels in parallel at the beginning was the only option. Panels were built to produce voltages around 18 volts. When using a PWM solar charge controller, they match the panel voltage to the battery voltage. As the battery state of charge rises, the voltage will increase. If one panel is in shade, the others in parallel will continue to provide output. This configuration will also result in the wires to the solar charge controller carrying high current levels.
The use of an MPPT charge controller allows for the more efficient harvest of solar power when panels are in parallel. They will allow the panel to produce energy at its maximum Power Point. This also allows the use of higher voltage panels such as 60 and 72 cell configurations.
When wiring panels in parallel, panels should be the same voltage. The solar charge controller is going to regulate them all to a common voltage. When calculating potential output during the design phase, remember that voltage will stay constant but amperage will be additive. For example, the panels above have a Pmax of 200 watts, VOC of 22 volts, an ISC of 9.7 amps, and Vmpp of 18 volts. Three panels in parallel will produce 600 watts total. The VOC for three panels will be 22 volts. These three panels will have an ISC of 29.1 amps. The working voltage of this array will be close to 18 volts Vmpp.
Series Wiring Solar Panels
Wiring solar panels in series should only be considered with MPPT solar charge controllers or PWM controllers for 24 volt or 48-volt battery banks. When the panels are wired in series, the voltage becomes additive and the amperage stays the same. This can reduce the size requirements and ultimately the cost of the required conductors going to the controller.
Using the same example as above, the panels have a Pmax of 200 watts, Voc of 22 volts, an ISC of 9.7 amps, and Vmpp of 18 volts. Three panels in parallel will produce 600 watts total. The VOC for three panels will be 66 volts. These three panels will have an ISC of 9.7 amps. The working voltage of this array will be close to 54 volts Vmpp.
Series/Parallel Wiring Solar Panels
The last method of wiring solar panels is a combination of the first two. When done correctly you can get the benefits of both methods. This will allow you to work with a layout that will fit your needs. There are a few practices that need to be kept in mind when creating your solar array design. When assembling strings of panels, each string must be the same voltage. As you are planning the layout of the roof, try to keep panels in strings aligned in the same direction. Do not think that you have to get all of the panels onto a single controller.
Determining the Sizing of the Solar Panel Array
There are a few different ways to approach sizing a solar panel array. System expectations, space availability and mounting options, and budget are key points to evaluate when sizing the solar panel array. But in the end, it will be a personal decision on what will take priority.
Expectations When Sizing a Solar Panel Array
At some point in the designing process, a decision on the overall role of solar panel array is going to play into the design process of the energy system. Is it going to be a primary means of providing energy or just a method to slow the draw on the battery bank? A general goal is to have approximately 200 watts of solar panels for every 100 usable amp-hours of battery. As the cost of solar panels has dropped considerably in the last few years, people are starting to exceed that estimate or to fill the roof to capacity.
Space Availability and Mounting Options for a Solar Panel Array
When evaluating a solar panel array, panel layout and mounting techniques can be a key factor in overall performance. The available roof space of an RV can dictate a solar panel array size and limitations. Flat mounting solar panels on the roof will result in performance loss because of the low angle of sunlight contacting the solar panel face. Tilting brackets allow for the proper angling of panels, however, most will require manually tilting which will involve time on the roof.
There has been some pretty creative racking systems custom designed and built by owners to help raise the panels in order to avoid roof obstacles. Attention to the mounting hardware, panel orientation, and the height and weight of the mounts and panels is just as important. A standard 60 cell panel can weigh between 40 to 50lbs each. They all will add to the total weight of the RV.
In order to make up for deficiencies in the mounted solar panel array, the use of a ground deploy solar panel array can be a viable option. These could consist of either an assemble suitcase set of solar panels or a home built, combination of panels, and a quality solar charge controller. Planning the system to be adequate in less ideal conditions will generally result in better performance overall.
Budgeting for a Solar Panel Array
The cost of solar panels has been becoming cheaper over the last few years. As technology has been improving, higher wattage panels are becoming cheaper to manufacture. Along with that, the availability of used or excess panels leftover from installations are also options to lower the costs. Bundled kits from some of the big-name manufactures tend to be some of the more expensive routes to take.
On the other hand, cheap equipment and panels should also not be the sole factor in gear selection. Sometimes you do get what you pay for. There are deals that are too good to be true. Knowing how to tell the difference will help save a lot of extra work down the road. Finding equipment with a good reputation and a long warranty period is a good indicator of quality equipment.
The cost of solar panels for the array can seem to be all over the spectrum. For a new standard size aluminum frame 36 cell solar panel, expect prices around $1 per watt. For quality flexible panels of this size, prices start at $1.50 to $2 per watt. Finding quality used or leftover solar panels from local solar installers can produce prices of $.30 to $.50 range for 60 and 72 cell residential panels. However, with the larger panels, layout or regular handling becomes more difficult.
Still looking for some assistance?
If you would like a consultation to discuss your specific needs, CLICK HERE to get signed up for Justin’s solar consultation. We can get a meeting scheduled. I do have a list of recommended vendors that do pay an affiliate fee. I could get credit based on people following links in my blogs or working with those vendors to quote packages for consultation projects. If you would like to source your own parts and equipment, consulting fees may apply depending on complexity and involvement.
A complete list of parts and equipment that we utilize in our system can be seen at Kit.co/optingoutofnormal.
Check out our blog “How to size a battery bank to fit your needs“. We show how we get the most benefit out of our Battle Born battery setup. If you are boondocking in the National Forest or plugged into an outlet, you can harness the great features of the batteries. This allows you to maintain your energy loads while still living normally in an RV.
Check out some of our other blogs to see how we use our system.
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