Every solar panel datasheet lists the same handful of values. They decide which charge controller fits, how panels can be wired and how much power you really get on a hot day.
The five key values
- Pmax: the rated power – the “100 W” in the name.
- Voc: open-circuit voltage, the highest voltage the panel can produce.
- Vmp: the voltage at maximum power – where an MPPT controller wants to hold it.
- Isc: short-circuit current, the highest current it can deliver.
- Imp: the current at maximum power. Vmp × Imp = Pmax.
The test conditions
These values are measured under Standard Test Conditions: strong sunlight and a cell temperature of just 25 °C. On a real bike in summer, cells easily reach 60 °C or more – especially flexible panels glued flat. Many datasheets also list more realistic NOCT/NMOT values.
Heat and cold change everything
Heat lowers voltage and power – typically by 0.3–0.4 % per degree. Cold raises the voltage, so on a frosty morning a panel can exceed what your controller accepts. That is why the same panel can be perfect for one controller and dangerous for another – especially with a boost controller, where the panel voltage must always stay below the battery’s.
Quick checks before you buy
- Does the datasheet list all five values and a temperature coefficient? If not, be wary.
- Is the claimed power plausible for the panel’s size? Unrealistic ratings are common in cheap listings.
- How many bypass diodes does it have? That decides how it copes with partial shading.
How voltages and currents add up when you combine panels is explained in series or parallel.
