THE SOLAR DICTIONARY

Every Solar Term, Explained Honestly

75+ plain-language definitions covering panels, inverters, metering, subsidies, and project finance — India-specific, vendor-neutral, and written so a first-time buyer and a design engineer both leave smarter.

Solar Basics

Photovoltaic EffectThe photovoltaic effect is the physical process by which certain materials, chiefly silicon, convert light directly into electricity. When photons strike a solar cell, they knock electrons loose, creating a direct current (DC).Solar CellA solar cell is the smallest power-generating unit of a solar panel — a thin wafer of treated silicon that produces roughly 0.6–0.7 volts under sunlight. A typical modern panel connects 108–144 half-cut cells in series.Solar Panel (PV Module)A solar panel, formally a photovoltaic (PV) module, is a weatherproof assembly of solar cells that converts sunlight into DC electricity. Residential panels in India today are typically rated 440–575 Wp each.Kilowatt-Peak (kWp)Kilowatt-peak (kWp) is the rated DC output of a solar system under standard test conditions — full sunshine of 1,000 W/m² at 25 °C. A 3 kWp rooftop system in India typically generates 10–13 units of electricity per day.Unit (kWh)One unit of electricity equals one kilowatt-hour (kWh) — the energy consumed by a 1,000-watt appliance running for one hour. Indian electricity bills are calculated in units, making it the common currency between your bill and your solar system.Peak Sun Hours (PSH)Peak sun hours measure a location’s daily solar resource as equivalent hours of full-strength sunshine (1,000 W/m²). Most Indian cities receive 4.5–6 peak sun hours per day averaged across the year.Solar IrradianceSolar irradiance is the instantaneous power of sunlight falling on a surface, measured in watts per square metre (W/m²). Standard test conditions for rating panels assume 1,000 W/m².On-Grid (Grid-Tied) Solar SystemAn on-grid solar system works in parallel with the electricity grid and has no batteries. Excess generation is exported to the grid through a net meter, and the grid supplies power at night — making it the cheapest and most common rooftop configuration in India.Off-Grid Solar SystemAn off-grid solar system operates independently of the electricity grid, storing daytime generation in batteries for use at night. It is sized around the load and required backup hours rather than around a bill.Hybrid Solar SystemA hybrid solar system combines grid connection with battery storage: solar powers the load, surplus charges the batteries or exports to the grid, and the batteries provide backup during power cuts.

Modules & Cell Technology

Monocrystalline PanelMonocrystalline panels are made from single-crystal silicon wafers, giving them higher efficiency (20–22.5%) and a uniform black appearance. They have effectively replaced polycrystalline panels in the Indian market.Polycrystalline PanelPolycrystalline panels are made from silicon cast from multiple crystals, identifiable by a blue, speckled appearance. Their efficiency (15–17%) is well below modern monocrystalline technology, and they are now largely obsolete for new Indian installations.Mono PERCMono PERC (Passivated Emitter and Rear Cell) adds a reflective passivation layer to the back of a monocrystalline cell, capturing light that would otherwise escape. It pushed mainstream panel efficiency past 21% and remains the workhorse of Indian rooftops.TOPConTOPCon (Tunnel Oxide Passivated Contact) is the successor to PERC cell technology, adding an ultra-thin oxide layer that lifts panel efficiency to roughly 22–23%. It also degrades slower and performs better in heat — relevant for Indian conditions.HJT (Heterojunction)HJT (heterojunction) cells sandwich crystalline silicon between thin-film layers, achieving 22.5–24% efficiency with the best temperature behaviour of any mainstream technology. It is the premium tier above TOPCon.Bifacial ModuleA bifacial module generates power from both faces — direct sunlight on the front and reflected light (albedo) on the rear. On elevated structures over reflective surfaces, bifacial panels yield up to 5–11% extra energy.Half-Cut CellsHalf-cut cell panels slice each solar cell in two, halving the current per cell and reducing resistive losses. The design also splits the panel into two halves, so shade on one half does not collapse the whole panel’s output.Module EfficiencyModule efficiency is the percentage of sunlight energy a panel converts into electricity. Current mainstream panels in India run 20–22.5%; higher efficiency means more watts from the same roof area.Temperature CoefficientThe temperature coefficient states how much power a panel loses for every degree above 25 °C, typically −0.30% to −0.40%/°C for modern modules. On a 65 °C Indian rooftop afternoon, that is a real loss of 12–16%.Degradation RateDegradation rate is the small annual loss of a panel’s output over its life, warranted at roughly 0.40–0.55% per year for current technology. A quality panel still delivers 85–90% of its original output after 25 years.ALMM (Approved List of Models and Manufacturers)ALMM is the Government of India’s official list of approved solar module models and manufacturers, maintained by MNRE. Government-supported projects — including PM Surya Ghar subsidised rooftops — must use ALMM-listed modules.DCR (Domestic Content Requirement)DCR modules are solar panels whose cells and modules are both manufactured in India. Central subsidy schemes, including PM Surya Ghar, require DCR modules — using imported-cell panels disqualifies the subsidy.Tier-1 ManufacturerTier-1 is a bankability ranking (originated by BloombergNEF) for module manufacturers whose products have been financed by major banks in utility-scale projects. It signals financial stability, not directly product quality.BIS Certification (IS 14286)BIS certification is the mandatory Bureau of Indian Standards approval for solar modules sold in India, primarily under IS 14286 (design qualification) and IS/IEC 61730 (safety). Uncertified modules cannot legally be sold.

Inverters & Electrical

Solar InverterA solar inverter converts the DC electricity produced by panels into the 230/415 V AC electricity that homes and the grid use. It is the brain of the system — handling MPPT tracking, grid synchronisation, safety shutdowns, and monitoring.String InverterA string inverter converts power from panels wired in series ("strings") at a single central point — the standard architecture for Indian rooftops from 3 kW homes to megawatt plants, prized for cost and serviceability.MicroinverterA microinverter is a small inverter mounted under each panel, converting DC to AC at module level. Each panel operates independently, so shading or failure of one panel does not affect the rest.Hybrid InverterA hybrid inverter manages solar panels, a battery bank, and the grid in one unit — directing solar power to loads, storage, or export by priority, and providing backup during outages.MPPT (Maximum Power Point Tracking)MPPT is the algorithm by which an inverter continuously finds the voltage-current combination that extracts maximum power from the panels as sunlight and temperature change. Inverters have one or more independent MPPT inputs.DC/AC Ratio (Inverter Loading Ratio)The DC/AC ratio compares array capacity (kWp) to inverter capacity (kW). Indian designs typically oversize DC to 1.1–1.3× the inverter, because panels rarely deliver nameplate power — this fills the inverter for more hours daily.Voc (Open-Circuit Voltage)Voc is the voltage a panel produces with no load connected — its maximum possible voltage, occurring on cold, bright mornings. String design must keep total cold-condition Voc below the inverter’s maximum DC input (commonly 1,000–1,100 V).Vmp (Maximum Power Voltage)Vmp is the voltage at which a panel delivers its maximum power — typically 41–45 V for current modules. Hot afternoons pull Vmp down, and the string’s combined Vmp must stay inside the inverter’s MPPT window.String SizingString sizing determines how many panels can be safely wired in series: enough that hot-weather voltage stays above the inverter’s MPPT minimum, but few enough that cold-morning Voc stays below its DC maximum.DCDB / ACDBDCDB (DC Distribution Box) and ACDB (AC Distribution Box) are protection enclosures on either side of the inverter, housing fuses, breakers, and surge protection devices. They isolate faults and protect the inverter and home wiring.SPD (Surge Protection Device)An SPD diverts lightning-induced and switching voltage surges safely to earth before they damage the inverter or panels. Solar installations use DC-rated SPDs in the DCDB and AC SPDs in the ACDB.Earthing (Grounding)Earthing connects the solar structure, panel frames, and electrical system to the ground through low-resistance earth pits, so fault currents and surges flow safely into the earth instead of through people or equipment.Lightning Arrestor (LA)A lightning arrestor is a rooftop air terminal that intercepts direct lightning strikes and conducts them to a dedicated earth pit, protecting the array and the building. It is standard on commercial plants and tall buildings.Solar DC CableSolar DC cables are UV-resistant, double-insulated copper cables (commonly 4 or 6 mm²) rated for outdoor DC service between panels and inverter. Undersized cable wastes generation as heat every day for 25 years.MC4 ConnectorMC4 is the industry-standard locking connector for joining solar DC cables — weatherproof, touch-safe, and rated for outdoor service. Every modern panel ships with MC4 leads.

Grid & Metering

Net MeteringNet metering credits solar exports against grid imports on your electricity bill: the meter runs both ways, and you pay only for net consumption. It is the billing arrangement that makes Indian rooftop solar economics work.Gross MeteringUnder gross metering, all solar generation is exported and sold to the DISCOM at a fixed feed-in tariff, while you buy all consumption at retail rates on a separate meter. It suits generation-first installations rather than self-consumption.Bidirectional (Net) MeterA bidirectional meter records energy flowing in both directions — imports from the grid and exports from your solar system — enabling net metering. The DISCOM installs it after commissioning inspection.DISCOMA DISCOM (Distribution Company) is the utility that delivers electricity to your premises and bills you — MSEDCL in most of Maharashtra, BESCOM in Bengaluru, and so on. Every rooftop solar application is processed and approved by your DISCOM.Technical Feasibility ApprovalFeasibility approval is the DISCOM’s confirmation that your connection and local transformer can accommodate the proposed solar capacity. It is the first gate in the rooftop application process, before installation may begin.Sanctioned LoadSanctioned load is the maximum power (in kW) your electricity connection is contractually approved to draw, printed on your bill. Many states cap rooftop solar capacity at or near the sanctioned load.Anti-Islanding ProtectionAnti-islanding is a mandatory safety feature that shuts down a grid-tied inverter the moment grid supply fails, preventing your system from energising lines that utility workers believe are dead.

Policy & Subsidy

PM Surya Ghar: Muft Bijli YojanaPM Surya Ghar is the Government of India’s flagship residential rooftop solar scheme, launched in February 2024 with a target of one crore solar homes. It provides central subsidy of ₹30,000/kW for the first 2 kW and ₹18,000 for the third — up to ₹78,000.CFA (Central Financial Assistance)CFA is the formal name for the central government subsidy paid on rooftop solar under schemes like PM Surya Ghar. It is a fixed rupees-per-kW amount credited to the consumer, not a percentage of the quote.MNREThe Ministry of New and Renewable Energy (MNRE) is the Government of India ministry that frames solar policy, administers subsidy schemes, and maintains quality gates like the ALMM list.State Nodal Agency (SNA)A State Nodal Agency is the state-government body that implements renewable energy programmes locally — for example MEDA in Maharashtra or GEDA in Gujarat. SNAs run state incentives, empanelments, and awareness programmes.Open AccessOpen access lets large consumers (typically ≥100 kW–1 MW contract demand, state-dependent) buy power directly from a generator — such as an off-site solar farm — using the grid as a carrier, instead of buying from the DISCOM.Captive Power PlantA captive solar plant is one where the consumer owns at least 26% equity and consumes at least 51% of the generation, qualifying for exemption from cross-subsidy surcharge under Indian electricity rules.Accelerated Depreciation (AD)Accelerated depreciation lets businesses depreciate solar assets at 40% per year (WDV) for income-tax purposes, front-loading tax savings and improving project returns. It applies to commercial and industrial owners, not homes.BCD (Basic Customs Duty) on SolarIndia levies Basic Customs Duty on imported solar modules and cells (introduced April 2022) to promote domestic manufacturing. The duty materially raises the cost of imported modules relative to Indian-made ones.Empanelled / Registered VendorAn empanelled vendor is an installer registered with the DISCOM or national portal to execute subsidised rooftop systems. Subsidy claims are valid only when installation is done by a registered vendor.

Performance & O&M

Performance Ratio (PR)Performance ratio measures how much of a plant’s theoretically possible generation is actually delivered, after temperature, soiling, wiring, and inverter losses. Well-run Indian rooftops achieve 75–80%.Specific Yield (kWh/kWp)Specific yield is annual generation divided by system size — units produced per kWp per year. Indian rooftops typically deliver 1,400–1,700 kWh/kWp/year depending on region and design quality.CUF (Capacity Utilisation Factor)CUF expresses annual generation as a percentage of what the plant would produce running at full rated power 24×365. Indian solar plants typically achieve 16–20% CUF — normal for a resource available only in daylight.Soiling LossSoiling loss is generation lost to dust, bird droppings, and pollution settling on panels — commonly 3–7% in Indian conditions, and far worse near construction sites or on low-tilt panels left uncleaned.Inverter ClippingClipping occurs when a DC array produces more power than its inverter’s AC rating, and the inverter caps output at its limit — discarding the excess. It appears as a flat-topped midday generation curve.HotspotA hotspot is a localised overheating point on a panel, usually caused by persistent shading, cell damage, or a failed solder joint forcing one cell to dissipate the string’s power as heat. Severe hotspots can crack glass or start fires.Thermography (IR Inspection)Thermography uses infrared cameras — increasingly drone-mounted — to photograph the heat signature of an operating array, revealing hotspots, failed diodes, and dead strings invisible to the eye.PID (Potential Induced Degradation)PID is a gradual power loss caused by voltage stress between solar cells and the grounded frame, historically capable of degrading affected panels by tens of percent. Modern PID-resistant modules largely prevent it.Remote MonitoringRemote monitoring streams your plant’s generation data to a phone app or web dashboard via the inverter’s Wi-Fi/GSM link, showing daily units, alerts, and fault codes in near real time.