Solar Panel / Module
Power
Solar cells laminated in series behind glass: a '12 V' panel really peaks around 18 V so it can charge batteries through a controller. Watts are rated at full noon sun — plan on much less on average.
Schematic symbol
Photodiode
How solar panel / module appears in a circuit diagram.
Types & variants
Key specs
Pmax
WPower at standard test conditions (1000 W/m², 25 °C).
Vmp / Imp
V / AVoltage and current at the max-power point — what an MPPT tracks.
Voc / Isc
V / AOpen-circuit voltage (cold mornings push it higher!) and short-circuit current.
Temperature coefficient
%/°COutput drops as panels heat — roughly −0.4%/°C.
Laws that govern it
A panel is series-connected diodes under illumination: its I–V knee, and the way voltage falls as the panel heats, are the diode equation.
MPPT exists because there is exactly one operating point per irradiance that yields the most watts — a direct application of this theorem.
- Lambert's cosine lawE = (I / d²) × cos θ
Output follows cos θ between the sun and the panel normal — the physics behind tilt angle, seasonal output and why trackers pay for themselves.
- Kirchhoff's voltage law (KVL)ΣV(loop) = 0
Panels in series add voltage (watch the inverter's Voc limit on cold mornings); in parallel they add current. The array design is loop arithmetic.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
Markings
Rear label: all electrical specs + series fuse rating. Junction box with MC4 connectors.
Standard values
10–50 W hobby panels (36-cell, Vmp ≈ 18 V), 400–600 W residential modules.
How to choose
Size from daily energy need ÷ realistic sun-hours, match Vmp to your charge controller (MPPT tolerates ranges; PWM wants Vmp ≈ battery + 5 V), and mind Voc stays under the controller's max on cold days.
Pinout & package
MC4 + / − leads from the junction box. Series strings raise voltage; parallel raises current (fuse each string).
Example circuits
- 20 W panel + MPPT controller + 12 V battery for a gate opener
- Balcony micro grid-tie setup
- Solar water-pump direct drive
Common failures
Hot-spots and burnt cells under partial shading, cracked cells from hail/flexing (visible in IR), delamination and yellowing, corroded MC4s, bypass-diode failures.
How to test
In sun: Voc ≈ label (e.g. ~21 V for a '12 V' panel), then briefly Isc with the meter's 10 A range. Big shortfall in full sun = damaged string or dead bypass diode.
Substitutes
Any panel matching your controller's voltage window; two smaller series panels replace one large one if Voc total stays within limits.