What do the specifications on my solar panels mean?

There are a lot of numbers to consider when assembling a solar array. The following section will take a look at the specifications of an Enerdrive 180W solar panel and provide a brief explanation of what each of these figures mean. The image below shows the specifications list from the back of the panel.

Nominal Voltage

Nominal solar panel voltage is important when you are using a PWM solar controller, because the nominal voltage of the panel and battery should match. The nominal voltage is typically listed in the product name or description. If it isn't listed there, it can be identified by examining the panel's open circuit voltage, max power voltage , or counting the number of cells. The table below provides a rough guide to what these values will be for panels of different nominal voltages, though the actual values will vary depending on the panel.

Max Power

Max Power is where the power rating of the panel comes from. It is the maximum wattage the panel will output when it is subject to Standard Test Conditions (STC), which will be explained in greater detail below. Odds are that your panel will produce this figure for only a very short period of time during the day, but you should still use it when calculating figures such as the total amount of power a solar controller can accept.

Power Tolerance

This number shows the possible range of deviation the panel can have from the Max Power. In the case of this panel, the tolerance is up to 3% above or below the max power rating. This means that the real world performance of this panel is between 175W and 185W

Max Power Voltage (Vm, or Vmp)

The Vm is the voltage produced by the panel when it is operating at maximum power. If you have an MPPT solar controller, this is the voltage that power will be drawn out of the panel at when it is performing at it's peak. The Max Power Voltage multiplied by the Max Power Current will give your the Max Power of the panel

Max Power Current (Im, or Imp)

The is the current produced by the panel when it is operating at maximum power. If you have an MPPT solar controller, this is the current that power will be drawn out of the panel at when it is performing at its peak. The Max Power Current  multiplied by the Max Power Voltage will give your the Max Power of the panel

Open Circuit Voltage (Voc)

This is the voltage produced by the solar panel when there is no load attached. This number is used when calculating how many panels can be connected in series to an MPPT solar controller because it represents the highest voltage your panels will produce in test conditions. If the combined open circuit voltage of your solar array is close to the input limit of your solar controller, you might also need to consider that voltage of your array will rise slightly as the temperature drops. More information on how to factor temperature into Voc calculations can be found below.

Short Circuit Current (Isc)

Short circuit current shows how many amps your panels produce when they are not connected to any loads. This number is used when calculating the maximum amount of current a solar controller can handle, since it represents the highest current your panels will produce. Even when using this number, leaving a small amount of headroom for temperature changes can help protect your solar controller. More information on this can be found here.

Max System Voltage (Vdc)

This is the maximum voltage when connecting these panels in series. It is not a relevant number for most off-grid applications because it will usually exceed the maximum input voltage of an MPPT solar controller and require you to connect a massive number of panels. In the case of the Enerdrive 180W panel, you need to connect 40 panels in series for this figure to be relevant.

Testing Conditions

Panel output varies depending a variety of factors, including temperature, the angle of the sun, and how much light is reaching them. To ensure that specifications are comparable between panels, manufacturers test their panels under a specific set of laboratory conditions. These are known as standard test conditions (STC). All of the above values are tested at STC (1000W m², 1.5AM, 25˚c). More information on STC can be found here.

Other Specifications

Panel Efficiency

Another value you might encounter when reading through a solar panel spec sheet is panel efficiency. This is a measure of how much of the sun's energy the panel is able to convert into electrical power. Solar panel efficiency is typically between 15% and 20%. Even the most efficient panels on the market, typically residential grid panels, are only around 22% efficient. If your panel doesn't list the efficiency, you can easily calculate it yourself with the following equation:

Using the Enerdrive 180W panels as an example

• Max Power is 180W
• Area is 1.01m (1.482m long x 0.680m wide)
• Efficiency is 17.8% [180/(1.01 x 1000) x 100]

The 180W panel specification sheet lists a panel efficiency of 17.8%, which matches our calculated efficiency . Panel efficiency is not particularly important when planning your solar array, but it can help you confirm that the specifications listed on the panels you plan to purchase are legitimate. Solar panels sold by less reputable retailers will sometimes boast max power ratings that seem too good to be true. If you are suspicious of the listed ratings of a solar panel, you can quickly work out the panel efficiency to see if the it falls in line with the standard 15% to 20% range. If the panel efficiency exceeds this range, then there is a good chance that the listed output is incorrect.

Nominal Operating Cell Temperature (NOCT)

The STC used to rate solar panels does not usually reflect the real world conditions that most panels are operating under. The NOCT reflects what the operating temperature of the solar cells will be under a more realistic set of conditions.

Temperature Coefficients of Pmax

The amount of power a solar panel outputs has an inverse relationship to the temperature, which means that an increase in temperature causes a decrease in output and vice versa. This relationship is measured with the temperature co-efficient of Pmax. In the case of a the Enerdrive 180W panel, the temperature coefficient is -0.37%/˚c. This means that for every degree the panel cell temperature rises, the panel produces 0.37% less power

If you want to know how much your panel will output at a given temperature, you just need to work out the difference between your chosen temperature and the test temperature (which is usually 25˚c) and multiply by the co-efficient. For example, if our 180W panel was operating at 60˚c, then the output would be:

1. 60˚c - 25˚c = 35˚c, which is the difference between the real cell temperature and the test cell temperature

2. 35˚c x -0.37% = -12.95%, which means that module would lose 12.95% of its output at that temperature

3. 180W x -0.1295 = -23.31W, which means a power loss the panel would only be producing 157W at 60˚c

Keep in mind that there other factors as to consider in solar generation as well, but factoring in cell temperature can give you a rough idea of how much power loss you might experience in very warm environments.

Temperature Coefficient of Voc

The voltage of solar panels also has an inverse relationship to temperature, which means that an increase in temperature means a decrease in voltage and vice versa. This relationship is measured with the temperature co-efficient of Voc. In the case of a the Enerdrive 180W panel, the temperature coefficient is -0.30%/˚c. This means that for every degree the panel cell temperature rises, the panel produces 0.37% less voltage

Working out the change in Voc can be important if your solar array is close to the voltage limit of your solar controller. The voltage rise of solar panels can be significant in extremely cold temperatures, and might even be enough to cause damage to your solar controller if there is not enough room left between the STC Voc and the controller input maximum. .

If you want to know how much your panel will output at a given temperature, you just need to work out the difference between your chosen temperature and the test temperature (which is usually 25˚c) and multiply by the coefficient. For example, if our 180W panel was operating at 0˚c, then the output would be:

1. 0˚c - 25˚c = -25˚c, which is the difference between the real cell temperature and the test temperature

2. -25˚c x -0.30% = 7.5%, which means the Voc of your panel would rise by 7.5% at 0˚c

3. 24.1V x 0.075 = 1.81V, which means that the Voc of your panel would rise to 25.9V

The voltage change of a single panel is unlikely to have significant impact on your system, but the voltage change for several panels connected in series could cause problems. For example, if you had six 180W panels connected in series and the temperature fell to 0˚c, the combined Voc rise would be nearly 11V.

Temperature Coefficient of Isc

The flow of current from your solar panels will rise slightly as the temperature of your solar panels rise. Compared to the change in power output and open circuit voltage, the change in the short circuit current of your panels is very small. For an Enerdrive 180W panel, the current only increases by 0.06% for every 1 degree Celsius. If the panels were operating at 60 this means they would only produce an extra 0.2A. This small increase will generally not be a concern unless you have a large solar array that is operating at the very limit of your solar controller. If this is the case, it may be worth calculating the potential change in current to make sure your array will not damage the solar controller.