On the back of every solar panel is a sticker with a lot of numbers. Ignoring these numbers is the fastest way to blow up your charge controller or start a fire.
Here is the translation guide.
1. Pmax (Maximum Power)
- What it is: The rated wattage of the panel (e.g., 400W).
- Reality Check: This is measured in a lab (STC - Standard Test Conditions: 1000W/m² light, 25°C temp). In the real world, you will rarely see this number. Expect 75-80% of Pmax on a normal sunny day.
2. Voc (Open Circuit Voltage) - CRITICAL
- What it is: The voltage the panel produces when nothing is connected to it (wires are "open").
- Why it matters: This is the maximum voltage the panel can ever produce.
- Danger Zone: Your charge controller has a "Max Input Voltage" limit (e.g., 100V or 150V).
- Rule: The sum of the Voc of all panels in a series string MUST NEVER exceed the controller's limit.
- Cold Warning: As temperature drops, voltage RISES. A panel with Voc 40V at 25°C might hit 45V at -10°C. Always leave a 15-20% safety margin below the controller's limit.
3. Isc (Short Circuit Current)
- What it is: The amps flowing if you touched the Positive and Negative wires together directly (short circuit).
- Why it matters: This is the maximum current the panel can produce. You use this number to size your fuses and wires.
- Rule: Fuse size = Isc x 1.56.
4. Vmp (Voltage at Maximum Power)
- What it is: The voltage the panel runs at when it is actually charging your battery efficiently (under load).
- Why it matters: This is the "working voltage."
- For a 12V battery system using a PWM controller, you need a panel with Vmp ~18V.
- For MPPT, Vmp just needs to be higher than battery voltage (plus a few volts headroom).
5. Imp (Current at Maximum Power)
- What it is: The amps the panel pushes when working at Vmp.
- Why it matters: Pmax = Vmp x Imp. (Volts x Amps = Watts).
Example: Sizing a Controller
Panel Specs:
- Pmax: 100W
- Voc: 22V
- Isc: 6A
Scenario: You have 3 panels in Series.
- Total Voc: 22V + 22V + 22V = 66V.
- Total Isc: Remains 6A (Series keeps amps same).
Controller Needed:
- Must handle > 66V input (e.g., a Victron 75/15 or 100/20).
- Do NOT use a controller rated for only 50V input! It will fry instantly.
Conclusion
Voc kills controllers. Isc melts wires. Always design your system based on the worst-case scenario (Coldest day for Voc, Hottest day for wire ampacity).
Learn more about wiring configurations in How to Wire Solar Panels: Series vs Parallel.
