Solar Panel Specifications Explained
Solar Panel Specifications Explained
Watts, volts, amps and more — understand what solar panel specifications mean and how they help you choose the right panel for your application.
Understanding solar panel specifications
When comparing solar panels, you will see specifications such as Pmax, Vmp, Imp, Voc and Isc. These figures describe how a panel performs under specified test conditions and help you determine whether it is suitable for your battery-charging system.
You do not need to be an electrician to understand the basics. This guide explains the main specifications in plain English, using an example from our range of solar panels.
The main solar panel specifications
These are the key figures you are likely to see on a solar panel label or product listing.
Maximum rated power
Pmax is the panel's rated maximum power output under standard test conditions. It is commonly used to describe a panel's size, such as a 100W, 170W or 215W panel.
A higher wattage panel can produce more power under comparable conditions, but actual output varies with sunlight, temperature, shading and installation.
Voltage at maximum power
Vmp is the voltage at which the panel produces its rated maximum power under standard test conditions.
This figure is important when matching the panel to a solar controller. The controller must be suitable for the panel's operating voltage and the battery system.
Current at maximum power
Imp is the current the panel produces at its maximum power point under standard test conditions.
Along with Vmp, it helps determine the panel's rated power. Actual current changes with available sunlight and other operating conditions.
Open-circuit voltage
Voc is the voltage measured across the panel when it is not connected to a load and no current is flowing.
This is particularly important when selecting a controller. The controller's maximum PV input voltage must not be exceeded.
Short-circuit current
Isc is the current measured when the panel's positive and negative terminals are short-circuited during a specified test.
It helps describe the panel's electrical output and is relevant when checking controller, wiring and protection requirements.
Dimensions and weight
Dimensions tell you how much space the panel needs, while weight helps you assess mounting and handling requirements.
For caravan, motorhome, vehicle and marine installations, allow for mounting hardware, clearances, cable routing and the suitability of the mounting surface.
Worked example: reading a 215W solar panel
Consider a Curtech 215W monocrystalline solar panel. The following figures illustrate how the specifications work together.
Example panel specifications
| Specification | Example rating | What it tells you |
|---|---|---|
| Pmax | 215W | Rated maximum power |
| Vmp | 18.2V | Voltage at maximum power |
| Imp | 11.81A | Current at maximum power |
| Voc | 21.5V | Open-circuit voltage |
| Isc | 13.07A | Short-circuit current |
| Dimensions | 1300 × 820 × 35mm | Space required for the panel |
| Weight | 13.4kg | Panel weight before installation |
How do volts and amps relate to watts?
Electrical power is calculated by multiplying voltage by current.
This shows how the panel's Vmp and Imp combine to give approximately its rated maximum power under the specified test conditions.
These example figures are for explaining the specifications. Always check the current specifications on the exact product listing and the manufacturer's label before designing or connecting a system.
What are standard test conditions?
Solar panel wattage is normally rated under a defined set of laboratory conditions known as Standard Test Conditions (STC). These generally use:
- Irradiance: 1,000 watts per square metre of solar radiation.
- Cell temperature: 25°C.
- Reference spectrum: a defined solar spectrum used for testing.
These standard conditions allow panels to be compared on a consistent basis. They do not mean a panel will continuously produce its rated wattage outdoors.
In real use, output depends on sunlight intensity, cloud cover, shading, panel angle, orientation, cell temperature and the electrical equipment connected to it. Hot solar cells generally produce less voltage than cooler cells.
Choosing a solar panel for 12V battery charging
A panel described as suitable for 12V battery charging is intended to be used with a compatible charging setup. The panel's nominal application is not the same as its exact operating voltage.
1. Check the panel's operating voltage
For a conventional 12V battery-charging system, a suitable panel and controller combination must provide enough voltage for the controller to charge the battery correctly. The required panel voltage depends on the controller design and battery chemistry.
2. Choose a compatible solar controller
The controller connects the solar panel to the battery and regulates charging. Check all of the following against the controller manufacturer's specifications:
- Maximum solar input voltage.
- Maximum allowable solar panel power for the intended battery voltage.
- Maximum input current and the controller's charging-current rating.
- Compatibility with the battery type and its required charging settings.
3. Consider your available space and power needs
Think about the appliances you use, how much energy you consume each day, available sunlight, mounting space and whether the panel will be permanently mounted or portable. A larger panel may provide more energy when conditions are suitable, but it still needs to fit your installation and charging system.
What does maximum system voltage mean?
Maximum system voltage is the manufacturer's specified limit for the panel's intended electrical system installation. It is especially relevant when panels are connected in series, because their voltages add together.
This rating does not mean the panel always operates at that voltage, nor does it tell you the maximum voltage your controller can accept. The controller has its own maximum PV input voltage, which must not be exceeded under expected operating conditions.
Common questions about solar panel specifications
Does a 200W solar panel always produce 200 watts?
No. The wattage is a rated output under standard test conditions. Outdoor output varies with sunlight, temperature, shading, orientation and other factors. A panel may reach its rated output under favourable conditions, but it will not produce that output continuously.
What is the difference between Vmp and Voc?
Vmp is the voltage when the panel is operating at its maximum power point. Voc is the open-circuit voltage when no current is flowing. Voc is particularly important when checking the maximum input voltage of a solar controller.
What is the difference between Imp and Isc?
Imp is the current at the panel's maximum power point. Isc is the current measured under a short-circuit test. They describe different operating conditions and should not be treated as interchangeable.
Can I connect any solar panel to a 12V battery?
No. You need a suitable solar controller and a compatible panel configuration. Check the panel voltage and current, controller limits and battery charging requirements before connecting the equipment.
Is a higher-wattage panel always the better choice?
Not necessarily. The best panel depends on your energy needs, available space, weight limits, budget, sunlight conditions and controller compatibility. A panel that suits your application is more useful than one chosen on wattage alone.
Do portable solar panels use the same specifications?
Yes. Portable and folding solar panels can have wattage, voltage and current ratings just like fixed panels. When comparing a portable kit, also check its folded dimensions, weight, leads, connectors and whether a solar controller is included.
Need help choosing a solar panel?
Use our panel selection calculator as a starting point, or browse our solar panels and kits to compare available options and product specifications.
For product enquiries, contact [email protected] or call 07 4124 9394.
