Fundamentals, Terminology, and Core Concepts
What you must understand
A solar site technician works from a small vocabulary that is used very precisely. Separate irradiance (instantaneous power per area, W/m²) from irradiation (energy per area over time, kWh/m²/day). Standard Test Conditions (STC) are 1000 W/m², 25 °C cell temperature, and AM 1.5; NMOT/NOCT describes a realistic operating condition. Know the four points on the I–V curve - Voc, Isc, Vmp/Imp (Pmax) - and fill factor as a shape indicator.
Understand how arrays are wired: series strings add voltage, parallel strings add current. Bypass diodes limit shading losses and hot spots; blocking diodes prevent reverse current. Inverters track the maximum power point (MPPT), and DC/AC ratio above 1.0 produces deliberate clipping at peak sun. Know performance ratio, capacity factor, peak sun hours, tilt, azimuth, and soiling.
Traps that appear in questions
- Treating kW (capacity) and kWh (energy) as interchangeable.
- Assuming current falls with heat: voltage is what drops as cells get hot; current barely moves.
- Forgetting that Voc rises in cold weather - the classic string-sizing error.
- Thinking bypass diodes solve all shading; they only isolate groups of cells.
Worked example
A module has Voc = 40.0 V at STC and a Voc temperature coefficient of −0.30 %/°C. At a record low of −10 °C, ΔT = −35 °C, so Voc = 40.0 × (1 + (−0.003 × −35)) = 40.0 × 1.105 ≈ 44.2 V. Rule of thumb: cold-weather Voc runs roughly 10–15 % above nameplate - always size equipment to that number, not to STC.