Solar terminology mixes instantaneous power with interval energy, laboratory ratings with field output, and controller states with promises about your battery. Keeping them straight is what stops "a 400 W panel" from being mistaken for "400 W all day," or an STC nameplate from being read as a daily-yield forecast. These definitions tag each term by what it actually describes and link to the fuller articles. For general electrical quantities see the main glossary; for storage, battery storage terms.

Term selection follows the current IEC photovoltaic terminology reference (IEC TS 61836:2025); the definitions below are supported by accessible primary sources and Viherion's canonical articles.

Solar resource and radiation

  • Irradiance — instantaneous solar power per unit area, in W/m². This is a rate.
  • Irradiation / insolation — solar energy per unit area over an interval, in kWh/m². This is an amount.
  • Solar resource — the broader, location- and time-dependent availability of sunlight, driven by location, season, time of day, landscape, and weather.
  • Equivalent peak-sun-hours (PSH) — a way of expressing a day's irradiation as the number of hours at 1,000 W/m². It is an energy normalization, not hours of daylight.

→ how solar panels produce electricity. Do not confuse irradiance (W/m²) with irradiation (kWh/m²).

Cell, module, string, and array

  • Cell — the semiconductor conversion unit.
  • Module (panel) — cells electrically combined behind glass.
  • String — modules connected in series. Array — the full assembly of modules and strings.

A rating must say whether it applies per cell, module, string, array, or complete system — they are different scopes.

Nameplate and test conditions

  • Standard Test Conditions (STC) — the reference rating point: 1,000 W/m² irradiance, 25 °C cell temperature, and the reference AM1.5 spectrum. A module's STC watts are a comparison rating.
  • Nameplate power — the STC rating printed on the module.

An STC nameplate is a peak reporting figure, not a forecast of field output or daily energy — real cells run hotter and under changing light, and the actual operating power is usually below nameplate. "400 W" is a rating condition, not an all-day output.

Voc, Isc, Vmp, Imp, and MPP

On a module's current-voltage (I-V) curve:

  • Open-circuit voltage (Voc) — the voltage at zero output current.
  • Short-circuit current (Isc) — the current at zero terminal voltage.
  • Maximum power point (MPP) — the point between them where power (V × I) is greatest; Vmp and Imp are the voltage and current there, at stated conditions.

Power is zero at both endpoints and maximum in between. Field design must use temperature-adjusted voltages and currents and the exact equipment limits — see how to read an energy-system data sheet. A "zero output" or open-circuit condition does not mean a conductor is isolated or safe to touch.

Controllers, chargers, PWM, and MPPT

  • Solar charge controller — regulates energy from a PV source into a battery/system.
  • MPPT (maximum power point tracking) — actively holds the PV near its maximum-power point and converts the surplus voltage into charge current.
  • PWM (pulse-width modulation) — a simpler control architecture that connects the array closer to battery voltage.

Neither acronym alone states efficiency, the compatible array, safe limits, the charge algorithm, or daily yield — those are separate specifications for the exact product. → inverters, chargers, controllers, and distribution.

Charge stages and control states

Bulk, absorption, float, storage, equalization, and the constant-current / constant-voltage phases are charge-control terms whose sequence and meaning depend on the charger, the battery chemistry, and the settings. A displayed stage is the controller's state — not independent proof of SOC, health, or correct configuration, and under wrong settings a stage indication can mislead. Equalization in particular can damage batteries that are not suited to it, so it is chemistry- and product-specific. → combining solar, alternator, shore, and generator charging.

Power, energy yield, and performance metrics

  • PV power — an instantaneous rate (W).
  • Energy yield — output accumulated over an interval (Wh or kWh).
  • Specific yield — interval energy normalized by rated array power (e.g. kWh/kWp).
  • Modeled yield vs. measured yield — a model estimate is not a meter reading.

Every yield figure must state its AC or DC boundary, interval, weather/resource dataset, losses, availability, and whether it is modeled or measured. Estimators like PVWatts are explicit that their outputs carry inherent assumptions and site-specific uncertainty. → how to design and size a small energy system.

Losses, derating, and availability

Soiling, shading, snow, mismatch, wiring, connections, temperature, conversion, nameplate variation, aging, and availability are different reduction mechanisms. A single derate factor is useful for preliminary arithmetic only when the mechanisms it bundles and the boundary it applies to are stated. Do not reuse a model's default percentages as a universal derate, and do not double-count a loss that a boundary already includes.

Definitions only — no installation actions. These entries help you read labels and manuals. Array configuration, controller settings, protection, isolation, and any electrical test involving exposed PV or battery conductors are qualified work; see the safety fundamentals.

Key takeaways

  • Irradiance (W/m²) is a rate; irradiation/insolation (kWh/m²) is an amount; peak-sun-hours is an energy normalization, not daylight hours.
  • STC nameplate power is a lab comparison point, not a field or daily-energy forecast; actual operating power is usually lower.
  • MPPT and PWM name control architectures, not guaranteed efficiency or yield; a charge stage is a controller state, not proof of SOC or health.
  • Every yield figure needs its boundary, interval, dataset, losses, and modeled-vs-measured status; a derate is only meaningful with its included mechanisms named.

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