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As long as you know how many solar panels you need, you can call a solar power provider to install a system for your home and not do any work. But if you want the DIY approach, you have to know the different parts and terms used, like cell, modules, strings and arrays and how they interact.
A solar panel or PV module is made up of several cells, while multiple solar panels wired in a series or parallel is called a solar array. A string consists of solar panels wired in a series set into one input on a solar string inverter.
If you have two or more solar panels wired together, that is a solar / PV array. String sizing refers to how many solar panels can and should be wired to an inverter for best results. This will depend on several factors including the inverter voltage capacity.
What is the Difference between Solar Cell, Panel, Array and Module?
A solar panel is the same as a PV (photovoltaic) module. A solar panel is made up of several semiconductors called cells. There are 36 cells in a typical solar panel like the Sonali 190W 12V. When the sun strikes the cells, the energy is converted into direct current electricity. This power can be used directly by DC powered devices. AC electronics however, require an inverter to convert DC into AC.
Think of the solar panel or module as the housing for the cells. So a 12V solar panel / module has 36 or 72 cells connected in parallel or series. To increase power, several solar panels or modules may be wired together to create a solar or PV array.
What makes solar arrays effective is their modularity. That is, you can add more panels to increase power, or change the wiring to suit your requirements. The cells are covered by a hard plastic/glass sheet. These are heavy duty materials designed to protect the cells from the elements, though solar panel covers are also available.
The term solar array is also used for solar farm, but generally it refers to any group of solar panels wired together for residential use. The term can also be used on RVs or ground mounted systems with multiple solar panels installed.
How Many Solar Panels Should be in an Array?
This depends on how many solar panels you need. Your location, electricity usage and available space on your roof (or ground) will determine how many panels make up your array.
Here is a scenario.
Your house needs 1000 kwh per month. Among the combinations and solar panel sizes you can buy.
- 5 x 250W = 1250W
- 4 x 315W = 1260W
- 3 x 375W = 1125W
Because solar panel output is often lower than its rating (for various reasons), we suggest more than 1000W total in case of cloudy or rainy days. If your system produces more power than you need, you can store it in a battery bank or export it to the power grid and receive credit. When solar production goes down, you can use that credit.
Series vs. Parallel Solar Panel Wiring
Once you have your solar panels, these can be installed in an array. The array may be installed in series, parallel or both depending on your system.
In a series, the voltage is added up but the amperage remains the same. In a series connection, a solar panel’s positive terminal is connected to the negative terminal of the next panel.
In a parallel configuration, the amperage increases but the voltage is not changed. In a parallel configuration, the positive terminals of all the panels are connected in a single wire, and every negative terminal is on another wiring.
The number of parallel cells denotes the current, while the number of series denotes the voltage. Series connections have a bypass diode to protect the cells. Only one bypass diode is needed for every 15 to 20 cells. The diode enables current to move through the solar panel even if there is some shading.
Which is better, series or parallel? Your solar installer will tell you which is best for your home depending on your electricity usage. If you need higher voltage, a series connection is ideal. But if you require more amperage, then parallel is the better wiring configuration. In some cases, a series and parallel configuration may be set up.
Once your solar array has been configured, it has to be connected to an inverter to run AC powered appliances on solar. This is where string sizing comes into play.
How Many Solar Panels are in a String?
A string panel can wire up to 8 solar panels into one inverter input. Most inverters have 3 string inputs so up to 24 solar panels can be connected. The number of solar panels will depend on the inverter operational range.
Inverters run within a particular voltage range, and the solar modules must generate voltage inside that range. If the modules produce too much power it could destroy the inverter, but if the power is too low, the inverter will not run. So it must be the right level.
You can find the voltage range on the inverter itself or the product operating manual. However there are other factors that you have to consider when stringing solar panels. Again you do not have to worry about this if you have a solar panel kit as the components have been configured and matched. But if you are into the DIY stuff, then you have to learn these.
Guidelines For Stringing Solar Panels
How you set the distance between solar panels and batteries is critical to running the system, but so are the following details about your inverter.
Required Inverter Information
- Maximum DC Input Voltage: this is the maximum voltage amount the inverter can handle.
- Start / Minimum Start Voltage: the lowest voltage level needed to run the inverter.
- Maximum Input Current: the highest current level acceptable for the inverter
- Number of MPPTs: MPPTs (Maximum Power Point Trackers) determine the voltage and current to optimize power input. The more MPPTs built in, the more it will be able to maximize solar panel power production.
Required Solar Panel Information
Two pieces of information are required from your solar panels, the short circuit current and open circuit voltage. You can find these details on your solar panel guide.
Keep in mind though, the data is based on Standard Test Conditions (STC), meaning the results were obtained in lab tests. The conditions in real life may vary, so don’t be surprised if the panel voltage and current are different from the values given.
You have to account for your area’s temperature, sunlight availability, string length, roof angle etc. You or your solar provider will have to make the proper calculation adjustments so you have a better idea of what to expect performance wise.
Maximum and Minimum Voltage Must be in Range
If you are going to install the solar array and string the panels yourself, the following conditions have to be met.
- The string voltage must be above the minimum and below the maximum voltage capacity of the inverter
- The string voltage must not be greater than the inverter maximum current
- The maximum voltage must conform to the limits set in your area
To determine if the voltage in your proposed solar array falls within the inverter range, multiply the panels wattage by the series number in a string.
Assume a 300W solar array with a VOC (open circuit voltage) of 40V. Your inverter has a minimum / start voltage of 150V and maximum 600V.
Find the maximum number of solar panels per string: divide the maximum inverter voltage by the solar panel VOC
600V / 40V = 15 maximum panels per string
Find the minimum number of solar panels per string: divide the minimum inverter voltage by the solar panel VOC
150V / 40V = 4 minimum panels per string
These figures are based on the values provided by your solar panel instructions. But as pointed out earlier, real world conditions may lead to vastly different figures which you have to account for. Voltage may be lower at high temperatures, and voltage may go down during cold weather.
Optimize String Conditions
Strings on the same MPPT have to be in the same condition, that is, similar irradiance, azimuth, orientation etc. There could be performance issues or reduced solar array efficiency if the strings have different settings.
If the string conditions have to be different, say, you are installing panels on different roof angles, or some panels have more sunlight than others, connect them to different MPPTs. By connecting strings with similar conditions together, you are assured of optimum performance.
To quickly recap, a solar array consists of two or more solar panels wired together, and a string refers to solar panels wired into one inverter input. The good news is you do not have to be an expert in these to avail of solar power. Solar panel kits and systems already account for this so you don’t have to worry if it will work for your home.