A panel label is full of numbers: power, voltage, current and temperature figures. You do not need to be an engineer to understand them, and knowing the basics lets you compare panels fairly and understand what the inverter will see.
Key takeaways
- Power in watts (W) equals voltage in volts (V) multiplied by current in amps (A).
- The panel rating, for example 600 W, is measured in standard test conditions, which are close to ideal.
- Open-circuit voltage and short-circuit current are the highest values; the working values are lower.
- Real output is usually lower than the label because of heat, dirt, angle and system losses.
1. Watts, volts and amps
Think of a water pipe. Voltage is like the pressure, current is like the flow, and power is the pressure times the flow. For a panel, power (W) = voltage (V) × current (A). Two panels with the same rating can reach it in different ways, for instance a higher voltage with lower current.
2. The rated power and "STC"
The headline figure, such as 580 W or 620 W, is the maximum power under standard test conditions (STC): a strong light level, a panel temperature of 25 °C and a defined light spectrum. Real roofs are hotter and the light is rarely perfect, so the output at the rated moment is lower. The label is for comparison, not a promise of daily output.
3. Voc and Isc
Open-circuit voltage (Voc) is the voltage with nothing connected. Short-circuit current (Isc) is the current if the output is shorted. Both are the highest values. They are used to check that a string stays within what the inverter and the cables can safely handle, especially in cold weather when voltage rises.
4. Vmp and Imp
Voltage at maximum power (Vmp) and current at maximum power (Imp) are the values at which the panel actually works when the inverter tracks the best point. Multiplying them gives the rated power. These are the numbers that matter for normal operation, while Voc and Isc matter for safety limits.
5. Temperature coefficient
Panels lose a small share of power for every degree they get hotter than 25 °C. The datasheet gives this as a percentage per degree Celsius. A lower (less negative) figure is better in a hot climate. Our guide to why a system produces less power shows how heat shows up in real output.
6. Reading the label sensibly
When comparing two panels, look at the rated power, the efficiency, the Voc and Isc, the temperature coefficient and the warranty, not only the watts. Our guide on how to read a solar panel datasheet goes through the whole sheet.
Next step
Not sure which panel suits your roof? Tell us about the site on the quotation page and we will recommend panels and an inverter that match.
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