Reading the nameplate
Six numbers on the back of the module drive every design decision you will make. Learn them and you can size a system in your head.
| Value | What it means | Where it bites you |
|---|---|---|
| Pmax | Rated watts at STC | Marketing number. Real output is 15–25% lower. |
| Voc | Open-circuit voltage | Rises when cold. Governs max string length. |
| Vmp | Voltage at max power | Falls when hot. Governs min string length. |
| Isc | Short-circuit current | × 1.25 for conductor sizing (690.8). |
| Imp | Current at max power | What the MPPT actually pulls. |
| Temp coeff | %/°C change in Voc | Typically −0.25 to −0.35 %/°C. |
Cell technology
- Monocrystalline PERC — the volume standard. 19–21% efficient, uniform black cells, ~0.30 %/°C.
- TOPCon / heterojunction — 21–23%, better temp coefficient (~−0.24 %/°C), so they lose less on hot roofs. Costs more per watt, wins on hot-climate annual yield.
- Bifacial — glass on both faces, picks up albedo off the surface below. Real gain is 3–5% on a shingled roof, 8–15% on a white commercial roof or ground mount over gravel. Do not let a salesman quote you 25%.
- Thin film (CdTe, CIGS) — low efficiency per square foot, shade-tolerant, mostly utility scale now.
Field ruleTemperature is the enemy of voltage and the friend of nothing. STC is 25°C cell temperature. A black roof in July puts cells at 65–75°C. That is a real 12–15% haircut on output that no spec sheet advertises.
Shade and mismatch
A single shaded cell throttles the whole string it sits in. Bypass diodes — usually three per module — route current around the shaded sub-string, costing you a third of that module rather than all of it. That is also why module-level electronics (microinverters, optimizers) win on complex roofs and lose on clean south-facing ones.
What actually fails
- Junction box — potting cracks, water gets in, diode cooks. Most common warranty claim.
- Microcracks — from walking on modules or shipping abuse. Invisible until year 5, shows as hot spots on IR.
- Backsheet delamination — older polyamide backsheets. Look for chalking and cracks.
- PID (potential induced degradation) — high system voltage plus humidity. Rare on modern modules, real on 1000 V commercial strings.