Electrical Specifications of PV Modules
PV modules have the ability to produce power, and each module has its own specific current and voltage characteristics. You can find all of these specifications listed on the backside of every module you buy as well as on every specification (spec) sheet from the manufacturers.
The power value is important because it lets you know the wattage the module can produce. From this power value, you can calculate the energy output for the system.
The current is important so you know how many amps are running through the conductors (wires), and the voltage is important because it tells you of the module’s potential to push the current.
In the following sections, I show you the different current and voltage specifications associated with PV modules. I also get you acquainted with some other key PV specifications that are worth knowing.
Current Specifications of PV Module
The current that a module can produce is represented by the ampere, or amp. The amount of current producedby a module is directly proportional to the size of the individual solar cells within the module, so the larger the cells, the more current you get. The amount of current flow is also dependent on the intensity of the sunlight. As the intensity increases, the electrons move faster, but the modules can only ever deliver the amount of current that the sun allows, which means they’re inherently current limited.
Two current values are reported on PV modules and spec sheets: short circuit current (Isc) and maximum power current (Imp).
Short circuit current, abbreviated Isc, is the value achieved if the positive and negative wires on a PV module come into direct contact with each other and there’s essentially no resistance between the positive and negative sides on the module.
The Isc values you see reported by manufacturers all occur at standard test conditions.
You use the Isc value whenever you need to calculate the minimum size of the conductors connected to your system. You must size conductors so as to satisfy the electrical codes, mainly the National Electrical Code (NEC), but not necessarily to ensure maximum system performance.
The Isc value also dictates the ratings of other components you connect to the PV modules and array, such as overcurrent protection devices and disconnects.
Maximum power current (Imp) : It represents the current value when the module is producing the maximum amount of power possible. The value listed by the manufacturer is at the STC. The current value varies greatly based on the intensity of the sunlight striking the module.Use the Imp value when you need help maximizing the performance of the system.
Note that this current value is often used in conjunction with Vmp to size the conductors in the system to maximize the power output from the modules and array.
The Imp value is represented in a number of different ways, including Ipm, Ipp, and Impp. Typically, a manufacturer determines the notation it wants to use for the maximum power values and applies that notation to both the current and voltage maximum power values.
Voltage Specifications of PV Module
The voltage from a PV module is the measurement of how much push or potential is available to move the electrons. PV modules produce voltage as soon as they’re placed in the sun.
Each solar cell contributes some voltage — for example, a mono- or multicrystalline cell produces approximately 0.5 maximum power voltage (Vmp) when it’s in the sun. In addition, it takes very little sunlight to achieve full voltage from a module.
A module that isn’t in the sun still has voltage present, but it can’t produce power because current doesn’t flow (meaning electrons don’t move) unless the module is in the sun.
Two voltage values are on PV modules’ listing labels and in spec sheets: open cir- cuit voltage (Voc) and maximum power voltage (Vmp).
Open circuit voltage, abbreviated Voc, is the voltage value in the absence of current flow i.e. when circuit is open. The Voc value listed on a PV module is at STC; it varies based on environmental conditions.
When calculating the number of modules to use in conjunction with specific equipment, you must use the Voc value to determine the maximum circuit voltage.
Maximum power voltage (Vmp) is, not surprisingly, the value where the PV module produces the greatest amount of power. Just like the Voc, this value moves, particularly in response to the temperature of the cells.
Vmp is often referred to as the operating voltage of the modules, although the true operating voltage is generally lower than this value due to the loss of voltage as temperature increases. The Vmp value you see is always reported at STC.
The Vmp value is important because this voltage is associated with the current flowing from the PV modules.
When thinking about Vmp, you need to keep two things in mind: Voltage drops as temperatures rise, and PV modules always need to have enough voltage (push) to keep the current flowing. These facts mean you must adjust the Vmp value for high temperatures to make sure the modules are arranged such that they always have enough voltage present to push the current, regardless of the temperature.
One important point to note about the maximum power voltage is the incon- sistency in the terminology. Somehow the PV manufacturers of the world can agree on testing conditions, but they can’t agree on the terms used to report the data they collect.
Maximum power voltage is commonly represented as Vmp, but you’ll also see it listed as Vpm, Vpp, Vmpp, and Vp. All of these variations really say the same thing; they just use different notations to say it. (Thankfully, I’ve never seen a spec sheet list the open circuit with anything other than the Voc notation, so regardless of what the various manufacturers use for the maximum power voltage notation, you have one constant when checking a module’s electrical specifications.)
Maximum Power Point
The maximum power point (MPP) is the product of Vmp × Imp
MPP is listed by every manufacturer at the STC. The MPP of an individual module is often referred to as the rated power output because that’s the amount of power the module can produce at the environmental conditions at which the modules are tested.
The MPP is important because it’s used to determine the following:
The rated power output of the entire array and the components you’re going to connect to: The array’s rated power output is the number of modules used in that array multiplied by the individual module’s MPP value. You then use this value in relation to the inverters you connect to the array.
For example, if you have ten modules that are each rated at 200 W, the array’s rated power input is 10 × 200 = 2,000 W, or 2 kW.
The expected energy production of the PV array: You start with the MPP value for all the modules and apply system losses and local environmental conditions to help compute the number of kilowatt-hours the PV array will generate.
Because the MPP is a product of voltage multiplied by current, it represents a variable value. The value listed by the manufacturer will rarely, if ever, be seen on operating meters because the module’s voltage changes based on temperature and because the current changes based on irradiance. The likelihood of both the current and voltage being at STC when the modules are operating in the field is slim.
Nonetheless, MPP is an important value because it provides a reference point when you’re comparing multiple modules.
Voltage Temperature Coefficient
As I note in the earlier “Vmp” section, a PV module’s voltage is related to the temperature of the cells within the module. This relationship is considered to be inversely proportional because as the temperature increases, the voltage decreases.
Likewise, as the temperature decreases, the voltage increases. This is an important consideration because PV modules are exposed to some of the most extreme temperatures, and their voltages react based on those temperatures.
The change in voltage due to temperature is a linear relationship, meaning the change happens in the exact same increments regardless of the temperature. The amount of change is referred to as the voltage temperature coefficient.
This value is often reported in terms of a percentage per degrees Celsius (%/oC), and it tells you that for every degree change in Celsius, the module’s voltage changes by a corresponding percentage.
Ideally, the modules you’re working with have a small voltage temperature coefficient, meaning the voltage changes very little with changes in temperature. You can’t do anything to change this value, so you need to consider it when designing your systems and specifying modules.
Most crystalline-based modules (mono- and multi-) have very similar voltage temperature coefficients with most manufacturers reporting values that are comparable to their competition’s. This is due to the nature of the cells and how they react to temperature.
Thin film modules, on the other hand, typically have smaller voltage temperature coefficients than their crystalline counterparts, so they don’t lose as much voltage in high-heat conditions.
All module manufacturers share what the power output of their modules are under STC. They also tell you what the power tolerance of that module is, which is basically how close they guarantee to come to hitting that value.
The manufacturers monitor the production process so they can accurately predict each module’s power output. Because the manufacturing process inevitably creates some inconsistencies, the manufacturers guarantee that their modules’ output will be within a certain percentage of the rated power output.
Many manufacturers offer power tolerances that are 0% to +3%. This says to the consumer, for example, that a module rated at 100 W will produce 100 W to 103 W. Again, this is at STC, so it’s highly unlikely you’ll ever see that 100 W.
Series Fuse Rating
The last major specification you should be aware of is the series fuse rating; it represents the largest overcurrent protection device (the fuse or circuit breaker) that can be placed on any series string (which is made by connecting the positive wire from one PV module to the negative wire of the next PV module).
The series fuse rating for PV modules is typically 10 A or 15 A (occasionally it’s even 20 A). The exact value has an effect on the wiring methods and overcurrent protection devices used.