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Renewable energy glossary

Plain-English definitions of the key terms used across solar PV, battery energy storage, inverters, policy, markets, and grid services. 90 terms.

Solar PV

Albedo: The fraction of sunlight a surface reflects — the ground property that determines how much extra energy a bifacial array's rear side can collect.
ALMM(Approved List of Models and Manufacturers): India's MNRE-maintained list of solar module manufacturers eligible to supply government-supported renewable energy projects.
Back-Contact Cell(IBC): Solar cell with all electrical contacts on the rear, eliminating front-side shading and reaching the highest commercial efficiencies.
Bifacial Module: Solar module that generates power from both front and rear sides, capturing reflected light for 8–12% extra yield.
BoS(Balance of System): All components of a solar installation other than the modules — inverters, mounting, cabling, transformers, monitoring.
Bypass Diode: A diode wired across a group of cells that conducts when those cells are shaded or damaged, letting string current flow around them instead of forcing them into destructive reverse bias.
Electroluminescence (EL) Testing(EL imaging): A diagnostic that pushes current backwards through a module in darkness and photographs the resulting infrared emission, revealing cracks, broken fingers and dead cells invisible to the eye.
EPC(Engineering, Procurement, and Construction): End-to-end project delivery model where a single contractor handles engineering, procurement, and construction of a renewable plant.
Fill Factor(FF): The ratio of a cell's actual maximum power to the theoretical product of its open-circuit voltage and short-circuit current — a direct measure of electrical quality.
Glass-Glass Module(Dual glass): A module built with glass on both front and rear instead of a polymer backsheet — the standard construction for bifacial products and the basis for longer warranties.
Half-Cut Cell(Half-cell module): A module design that laser-cuts each cell in two and wires the halves as parallel sub-arrays, halving current per path and cutting resistive losses by roughly three-quarters.
HJT(Heterojunction Technology): Solar cell technology combining crystalline and amorphous silicon layers, offering higher efficiency at higher capex than TOPCon.
I-V Curve(Current-voltage curve): The plot of current against voltage for a cell, module or string — the diagnostic fingerprint that reveals shading, mismatch, degradation and wiring faults.
IEC 61215 and IEC 61730(Module qualification standards): The paired international standards that qualify PV modules — 61215 for design qualification and type approval, 61730 for safety — and the baseline any credible module must hold.
Irradiance(GHI): The solar power arriving on a surface per unit area, measured in watts per square metre — the input that determines how much a PV array can produce at any instant.
kWp(Kilowatt-peak): Nameplate DC capacity of a solar module or system measured at Standard Test Conditions.
LID and LeTID(Light Induced Degradation): Two distinct degradation mechanisms: LID causes a rapid 1–3% power loss in the first hours of sun exposure, while LeTID develops slowly over months at elevated temperature and can reach 6%.
Mismatch Loss: Energy lost when series-connected cells or modules operate at different currents or voltages, forcing the whole string toward the weakest element's behaviour.
Net metering(Net energy metering): Billing arrangement where solar customers receive bill credits for grid-exported electricity, often at the retail rate.
PERC(Passivated Emitter and Rear Cell): Legacy crystalline silicon cell architecture that dominated solar from 2018–2023; now superseded by TOPCon.
Performance Ratio (PR)(PR): The ratio of a plant's actual energy output to what it would have produced at nameplate efficiency under the measured irradiance — the standard scorecard for system health.
Perovskite tandem(Perovskite-silicon tandem): Solar cell architecture stacking a perovskite layer atop silicon to break the single-junction efficiency limit.
PID (Potential Induced Degradation)(Potential Induced Degradation): Power loss caused by leakage currents driven by the voltage difference between cells and the grounded module frame — capable of costing 20% or more of array output within a few years.
Soiling Loss: Energy lost when dust, pollen, bird droppings, salt or industrial deposits block light from reaching the cells — typically 2–6% annually, but far higher in arid and dusty regions.
Solar module degradation(PV degradation rate): Annual loss of power output from a solar module due to material aging, typically 0.4–0.7% per year for 2026 modules.
Solar Tracker: Mechanical structure that rotates modules to follow the sun, increasing daily energy yield 18–25% over fixed-tilt.
Specific Yield(kWh/kWp): Annual energy produced per kilowatt-peak of installed capacity — the number that lets a 5 kW rooftop and a 500 MW plant be compared on the same axis.
STC (Standard Test Conditions)(Standard Test Conditions): The laboratory reference conditions — 1,000 W/m², 25°C cell temperature, AM1.5 spectrum — at which module nameplate power is rated.
Temperature Coefficient(Pmax temperature coefficient): The percentage change in a module's power, voltage or current for each degree Celsius the cell rises above 25°C — the single most useful datasheet number for hot climates.
TOPCon(Tunnel Oxide Passivated Contact): Solar cell architecture using a tunnel oxide layer to passivate rear contacts — the dominant cell technology in 2026.

Battery Storage

BESS(Battery Energy Storage System): Stationary battery system providing grid-scale or behind-the-meter energy storage and grid services.
BMS(Battery Management System): Electronic system controlling battery cell-level monitoring, balancing, thermal management, and safety protections.
C-rate(Charge/discharge rate): Ratio of battery power output to energy capacity; a 1C rate fully discharges a battery in one hour.
DoD(Depth of Discharge): How much of a battery's energy is used in each discharge cycle — affects cycle life inversely.
EMS(Energy Management System): Software layer above BMS that orchestrates BESS dispatch decisions — when to charge, discharge, and which markets to bid into.
LDES(Long-Duration Energy Storage): Energy storage technologies discharging for 10+ hours, targeting renewable integration beyond what 4-hour BESS can solve.
LFP(Lithium Iron Phosphate): Battery cathode chemistry — the dominant choice for stationary energy storage due to long cycle life, safety, and cost.
NMC(Nickel Manganese Cobalt): Battery cathode chemistry with higher energy density than LFP — used in premium EVs but losing stationary storage share.
Round-Trip Efficiency(RTE): Ratio of energy out to energy in for a complete charge-discharge cycle of a BESS, typically 85–92% for LFP systems.
SoC(State of Charge): The current available energy in a battery expressed as a percentage of its full capacity.
Sodium-Ion Battery(Na-ion): Emerging battery chemistry using sodium instead of lithium — currently 13–25% premium over LFP, crossover expected 2028–2029.
SoH(State of Health): A battery's current maximum capacity expressed as a percentage of its original rated capacity.
Thermal runaway: Self-sustaining exothermic chain reaction in a lithium-ion cell that can propagate to fire or explosion.

Inverters

Anti-Islanding: Inverter protection that disconnects from the grid when an islanded condition is detected, for safety.
Central Inverter: Large 1–5 MW inverter handling power conversion for a substantial section of a utility-scale solar plant.
Clipping(Inverter clipping): Energy discarded when the array's available DC power exceeds what the inverter can convert, capping output at the inverter's AC limit.
DC/AC Ratio(Inverter Loading Ratio): The ratio of installed DC module capacity to inverter AC rating — deliberately set above 1.0 to keep inverters working near full output for more hours of the day.
FRT(Fault Ride-Through): Inverter capability to remain connected to the grid during voltage disturbances rather than disconnecting immediately.
GaN(Gallium Nitride): Wide-bandgap semiconductor used in solar inverters and power electronics, offering higher switching frequencies than silicon or SiC.
Grid-Following Inverter(GFL): Conventional inverter that senses grid voltage and injects power, requires an existing grid waveform to operate.
Grid-Forming Inverter(GFM): Inverter capable of establishing grid voltage and frequency autonomously, providing synthetic inertia in low-inertia grids.
Hybrid Inverter: Inverter combining solar PV input and battery charge/discharge in a single unit — increasingly the default for residential solar+storage.
Microinverter: Small inverter (typically 250–400 W) installed at each individual solar module, common in residential solar.
MLPE (Module-Level Power Electronics)(Module Level Power Electronics): Electronics attached to individual modules — DC optimisers or microinverters — that give each module its own maximum power point and enable module-level monitoring and shutdown.
MPPT(Maximum Power Point Tracking): Inverter control algorithm that continuously adjusts operating voltage to extract the maximum power from a solar array.
Rapid shutdown(NEC 690.12): US electrical code requirement that residential solar systems de-energize within 30 seconds of an emergency shutoff signal.
SiC(Silicon Carbide): Wide-bandgap semiconductor enabling higher-efficiency, higher-power-density inverters compared to silicon IGBT.
String Inverter: Solar inverter connected to a string (or strings) of modules — the dominant inverter type for utility-scale and commercial solar in 2026.
String Sizing(String design): Choosing how many modules to wire in series so that string voltage stays inside the inverter's operating window at both temperature extremes — the calculation that keeps a design legal and functional.

Green Hydrogen

Alkaline electrolyser(ALK electrolyser): Mature hydrogen electrolyser technology using a liquid alkaline electrolyte (typically KOH), lowest capex per kW.
Green hydrogen: Hydrogen produced by electrolysis using renewable electricity, with near-zero lifecycle emissions.
PEM electrolyser(Proton Exchange Membrane electrolyser): Hydrogen electrolyser using a solid polymer electrolyte, offering fast response and high current density at higher capex than alkaline.

Wind

Capacity factor: Ratio of actual energy generated over a period to the maximum possible at nameplate capacity, expressed as a percentage.
Monopile: Single large-diameter steel cylinder driven into the seabed, the dominant offshore wind foundation type up to ~50m water depth.
Offshore wind: Wind turbines installed in marine environments — typically larger, with higher capacity factors and higher cost than onshore wind.

Policy

CBAM(Carbon Border Adjustment Mechanism): EU regulation imposing carbon pricing on imports of carbon-intensive goods to prevent carbon leakage.
CERC(Central Electricity Regulatory Commission): India's central regulator setting tariffs, grid codes, and ancillary services rules for interstate electricity.
DISCOM(Distribution Company): Electricity distribution utility that buys power wholesale and sells to end consumers.
Domestic Content Requirement(DCR): Policy mandating that a minimum percentage of equipment in a renewable project be sourced from domestic manufacturers.
IRA(Inflation Reduction Act): US legislation passed in 2022 providing tax credits and incentives for renewable energy projects and US-domestic manufacturing.
IRA 45X(Section 45X Advanced Manufacturing Production Credit): US Inflation Reduction Act per-unit production tax credit for domestic manufacturing of solar modules, cells, wafers, batteries, and wind components.
MNRE(Ministry of New and Renewable Energy): India's central government ministry overseeing renewable energy policy and implementation.
NZIA(Net-Zero Industry Act): EU regulation (2024) targeting 40% of EU demand for strategic net-zero technologies to be met by EU manufacturing by 2030.
Open Access(Green open access): A regulatory right allowing large electricity consumers to buy power directly from a generator of their choice using the incumbent utility's network for a fee.
RPO(Renewable Purchase Obligation): Indian regulation requiring DISCOMs and large consumers to source a minimum percentage of electricity from renewable sources.
SECI(Solar Energy Corporation of India): India's central PSU that conducts renewable energy auctions and signs power purchase agreements on behalf of DISCOMs.

Markets

PPA(Power Purchase Agreement): Long-term contract for the sale of electricity from a generator to a buyer at agreed pricing.
REC(Renewable Energy Certificate): Tradable certificate representing the green attributes of one MWh of renewable electricity generation.
Resource Adequacy(RA): Regulatory requirement that load-serving entities have enough firm capacity to meet expected peak demand plus a reserve margin.
RTC(Round-the-Clock): Power supply that's available 24/7 from renewable + storage combinations — emerging tender structure in India.
Time-of-Day Tariff (ToD)(ToD): An electricity tariff that charges different rates by time block, pricing peak hours higher and solar-surplus hours lower to shift consumption.
VPPA(Virtual Power Purchase Agreement): Financial contract where a buyer pays a fixed price for renewable energy without physical electricity delivery, settled against market prices.

Grid

Ancillary Services: Grid stability services — frequency regulation, reserves, voltage support — provided by generators or storage to grid operators.
Behind-the-Meter (BTM)(BTM): Generation or storage installed on the customer's side of the utility revenue meter, serving on-site load before any surplus reaches the grid.
Curtailment: Forced reduction of renewable energy output below capability, typically due to oversupply, transmission constraint, or negative pricing.
Demand Response (DR)(DR): Deliberate reduction or shifting of electricity consumption in response to a grid signal or price, used as an alternative to dispatching additional generation.
Interconnection: Process and infrastructure required to connect a new generator to the transmission or distribution grid.
Virtual Power Plant (VPP)(VPP): A software-coordinated aggregation of customer-owned distributed energy resources that is bid into electricity markets or dispatched for capacity as if it were a single power plant.

Finance

ITC(Investment Tax Credit): US federal tax credit equal to 30% of qualifying solar and storage installation cost, extended through 2032 under the IRA.
LCOE(Levelized Cost of Electricity): Per-unit cost of electricity that recovers all lifetime costs of a project — the standard cost metric for energy projects.