The commercial solar terms, defined plainly.

Every acronym a finance director or facilities manager meets on a project over 50 kWp, defined in one or two sentences. Precise enough for the engineer, plain enough for the board paper.

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Grid, standards, performance, finance, kit
Over 50 kWp, outside MCS

01 How to use this

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The terms are grouped into five themes. Use the links below to jump, or your browser’s find on the page. Where a term has a guide of its own, the definition links to it.

Grid & connection

Active Network Management (ANM): A DNO control scheme that dynamically curtails a generator's output in real time according to network conditions, rather than fixing a single static limit. An ANM connection is usually offered faster and at lower cost than reinforcement, but accepts that export may be constrained at times of network stress.
Available capacity (kVA): The capacity your connection is authorised to import or export, stated in kilovolt-amperes (apparent power) rather than kilowatts. A site has an agreed import and may have a separate agreed export capacity, and a generation connection cannot exceed the export figure the DNO has granted.
Distribution Network Operator (DNO): The licensed company that owns and operates the regional electricity distribution network your site connects to, such as National Grid Electricity Distribution or UK Power Networks. The DNO assesses and authorises any new generation connection, sets the terms of connection and decides whether the local network can accept your export. Read more
Engineering Recommendation G100 (G100): The standard governing export and capacity limitation, where a control scheme caps the power a site exports (or, more broadly, imports and exports) at the connection point to an agreed value. It lets a larger array connect on a constrained network by guaranteeing the agreed limit is never exceeded, and is the usual route to connecting where headroom is limited, with monitoring and failsafe requirements the DNO will verify. Read more
Engineering Recommendation G98 (G98): The connection standard for small generation up to and including 16 A per phase (about 3.68 kW on a single phase, around 11 kW across three phases), which can be connected on a notify-and-fit basis without prior DNO approval. Commercial systems above 50 kWp sit well outside G98 and are governed by G99 instead.
Engineering Recommendation G99 (G99): The connection standard that applies to generation above the G98 threshold, which covers effectively all commercial solar above 50 kWp. It requires a formal application to the DNO, witnessed or type-tested commissioning, and protection settings agreed before the system can energise and export. Read more
Half-hourly metering (HH): Settlement-grade metering that records consumption and export in 30-minute intervals and transmits the data automatically. It is mandatory for larger commercial supplies and is the basis on which exported solar is measured and paid, and on which time-of-use energy costs are calculated. Read more
Meter Point Administration Number (MPAN): The reference that identifies an electricity supply point on the national system, shown on every commercial bill as a 21-digit Supply Number built around a unique 13-digit core. It identifies the connection in DNO applications, export agreements and supplier contracts, and a separate export MPAN may be issued for the generation.
Network reinforcement: Physical upgrades to the DNO's network, such as larger cables, a new transformer or substation work, needed when the existing infrastructure cannot accept the proposed generation. Reinforcement can carry significant cost and lead time, which is why export limitation or ANM is often preferred where it avoids the need for upgrades.
Point of Connection (POC): The physical point at which your installation joins the DNO's distribution network, typically at the main incoming supply or a dedicated connection. Its location and the available headroom there largely determine the cost and feasibility of connecting generation.

Standards, compliance & assurance

BS 7671 Requirements for Electrical Installations (IET Wiring Regulations) (BS 7671): The UK national standard for the design, installation and testing of electrical installations, including the dedicated solar PV provisions in Section 712. Commercial PV work is expected to be installed and certified to BS 7671, with an Electrical Installation Certificate issued on completion. Read more
Construction (Design and Management) Regulations 2015 (CDM 2015): The UK regulations governing health and safety on construction projects, which apply to commercial rooftop and ground-mount PV installations. They define duties for the client, principal designer and principal contractor, including risk management, competence and the production of site documentation. Read more
Energy Performance Certificate (EPC): A certificate rating the energy efficiency of a building from A to G, required when most commercial properties are built, sold or let. On-site solar generation can improve the rating and the underlying energy use figures that the certificate reflects.
Energy Savings Opportunity Scheme (ESOS): A mandatory energy assessment scheme for large UK undertakings, defined as those with 250 or more employees, or annual turnover above £44m together with a balance sheet total above £38m. Qualifying organisations must audit their energy use every four years and report identified savings, of which solar generation is often one. Read more
IEC 62446-1 PV system test, documentation and maintenance requirements (IEC 62446-1): The international standard setting out the minimum documentation, commissioning tests and inspection records required for a grid-connected PV system. Compliance gives the client a defined handover pack and a baseline against which future performance and faults can be measured.
Insurance-backed guarantee (IBG): A guarantee underwritten by an independent insurer so that the workmanship warranty remains valid even if the installer ceases trading. For commercial PV it is a core part of the assurance stack, since the asset is expected to operate for decades beyond any single contractor.
Microgeneration Certification Scheme (MCS): A UK certification scheme for small-scale, mainly domestic renewable installations that does not apply to commercial solar above 50 kWp. For larger systems the relevant assurance comes from BS 7671, IEC 62446-1, G99 connection approval, CDM 2015 and an insurance-backed guarantee rather than MCS. Read more
Minimum Energy Efficiency Standards (MEES): The regulations that make it unlawful to let most commercial properties below a minimum EPC rating, currently band E. Reducing a building's energy demand through measures such as on-site solar can help a landlord meet the standard and avoid letting restrictions. Read more
RC62 Recommendations for fire safety with PV panel installations (RC62): A freely available joint code of practice from the RISCAuthority, MCS and Solar Energy UK addressing fire safety for rooftop PV, with particular focus on commercial buildings. It covers fire risk assessment by a competent person, roof access for the fire service and DC isolation, and is increasingly referenced by property insurers. Read more
Risk Assessment and Method Statement (RAMS): The paired site document setting out the hazards of a task and the safe step-by-step method for carrying it out, produced before work begins on a commercial PV project. RAMS are a practical expression of CDM 2015 duties and are routinely required by clients and their insurers.
Streamlined Energy and Carbon Reporting (SECR): The framework requiring large UK companies to disclose energy use and carbon emissions in their annual reports, applying to organisations that meet two of three tests: turnover above £36m, balance sheet above £18m, or 250 or more employees. On-site solar generation reduces the reported figures and demonstrates action on emissions. Read more

Sizing & performance

Annual degradation: The gradual, predictable decline in a panel's output as it ages, commonly around 0.4 to 0.5 per cent per year for current modules after a slightly larger first-year drop. Manufacturers warrant a guaranteed output (often around 85 to 90 per cent of nameplate at year 25), and degradation should be built into any long-term yield and return modelling.
DC:AC ratio and inverter clipping: The DC:AC ratio is the array's DC rating divided by the inverter's AC rating; deliberately oversizing the array (a ratio above 1) raises output in weaker light. Clipping is the energy lost when array output briefly exceeds the inverter's limit at peak times, and a sound design balances a small, accepted amount of clipping against better all-round generation and lower cost per unit.
Irradiance and insolation: Irradiance is the instantaneous solar power falling on a surface, measured in watts per square metre; insolation is that energy accumulated over time, measured in kWh per square metre. Insolation across a site's roof or land is the primary input to any honest generation estimate.
Kilowatt-hour (kWh): A unit of energy: one kilowatt sustained for one hour. Where kWp describes how big the array is, kWh describes how much electricity it actually generates or a site consumes over a period, and is the unit your bills and savings are measured in.
Kilowatt-peak (kWp): The rated DC output of a solar array under standard test conditions (1,000 W/m² irradiance, 25°C cell temperature). It measures the size of the array, not the energy it produces, and is the figure used to set the MCS 50 kWp boundary that separates domestic-scale from commercial work.
Megawatt-peak (MWp): One thousand kWp, used to describe the rating of larger ground-mount and multi-roof installations. A site at or above 1 MWp is handled under the G99 process and, depending on the network, may involve a formal connection offer and a longer assessment by the Distribution Network Operator.
Performance ratio (PR): The ratio of the energy a system actually delivers to the energy it would deliver if it ran at its full rated efficiency in the available sunlight, expressed as a percentage. It captures real-world losses (temperature, wiring, inverter conversion, soiling, downtime) and is the cleanest single number for judging how well a system is engineered and maintained, independent of location or weather.
Self-consumption: The share of generated electricity used on site rather than exported to the grid. Because avoided import (the full retail unit price) is worth considerably more than export payments under the Smart Export Guarantee, maximising self-consumption is usually the largest driver of a commercial system's financial return. Read more
Shading loss: The reduction in output caused by obstructions such as parapets, plant, flues, neighbouring buildings or trees casting shadow across the array. Because panels are wired in series, partial shade can drag down a whole string, so it is assessed at design stage and mitigated through layout, string design, or module-level electronics such as optimisers.
Specific yield and capacity factor: Specific yield is the annual energy a system produces per unit of installed capacity (kWh per kWp per year), driven mainly by location, orientation and shading; a well-designed UK commercial array typically falls in the region of 900 to 1,000 kWh/kWp per year. Capacity factor expresses the same productivity as a percentage of theoretical maximum output, typically around 10 to 11 per cent for UK solar, which is normal for the technology rather than a sign of underperformance.

Finance & tax

Annual Investment Allowance and capital allowances (AIA): Capital allowances are the mechanism by which a business writes down the cost of capital assets against taxable profits; a commercial solar PV system typically qualifies as plant and machinery. The Annual Investment Allowance lets that full qualifying cost be deducted in the year of expenditure up to an annual cap (currently £1 million), with writing-down allowances at the relevant pool rate applying thereafter. Read more
Business rates on solar: Non-domestic property rates can apply to solar generating equipment. In England, eligible on-site rooftop generation used mainly to power the occupier's own premises is exempt from business rates between 1 April 2023 and 31 March 2035; arrangements differ for export-led or third-party-owned systems, so the rating treatment should be confirmed for the specific project.
Capital expenditure (capex): The upfront cost of buying and installing the solar PV system, including modules, inverters, mounting, cabling, the grid connection and commissioning. It is the figure against which payback, ROI and capital allowances are assessed, and contrasts with ongoing operating expenditure such as maintenance and monitoring. Read more
Feed-in Tariff (FiT): The earlier UK subsidy that paid both a generation tariff and an export tariff for renewable installations. It closed to new applicants in March 2019 and has been succeeded by the Smart Export Guarantee; only systems accredited before closure continue to receive FiT payments.
Internal rate of return (IRR): The discount rate at which the net present value of a system's cash flows (capital cost, savings, export income, maintenance) equals zero. It allows a solar investment to be compared on a like-for-like basis with other uses of capital, accounting for the timing of every cash flow.
Levelised cost of energy (LCOE): The average cost per unit of electricity (typically pence per kWh) generated by a system across its full operating life, found by dividing total lifetime costs by total lifetime generation, both discounted. Comparing the LCOE of on-site solar against the grid import price shows the underlying unit-cost saving.
Payback period: The time taken for the cumulative savings and export income from a system to equal its net capital cost. A simple payback ignores the time value of money and energy-price inflation; a discounted payback accounts for them and so gives a longer, more conservative figure. Read more
Power Purchase Agreement (PPA): A long-term contract under which a third party funds, installs and owns the solar system, and the site occupier buys the electricity it generates at an agreed unit rate, usually below grid price. The occupier avoids the upfront capital cost but does not own the asset or claim its capital allowances. Read more
Return on investment (ROI): A profitability measure expressing the total net financial benefit of a system as a percentage of its capital cost, usually stated per year or over the asset's life. It is straightforward to communicate but, unlike IRR, does not weight cash flows by when they occur. Read more
Smart Export Guarantee (SEG): The current UK obligation requiring larger licensed electricity suppliers (those with at least 150,000 domestic customers) to pay generators for surplus power exported to the grid, with each supplier setting its own tariff. It replaced the closed Feed-in Tariff and pays only for measured export, not for total generation. Read more

Equipment & installation

Ballasted mounting: A flat-roof mounting method that holds the array down with weighted ballast (usually concrete blocks) instead of penetrating the roof membrane, preserving the waterproofing and any roof warranty. It requires a structural assessment of the roof's load capacity and a wind-uplift calculation, since the ballast mass is what resists wind.
Battery energy storage system (BESS): A storage installation, typically lithium iron phosphate, that stores energy in kWh and is rated separately for power (kW) and capacity (kWh). On commercial sites it shifts solar generation to match demand, lowers peak-time grid import and can provide backup; it is a distinct G99-relevant generating asset and must be sized to a genuine load and tariff profile. Read more
Bifacial module: A module that generates from both faces, capturing light reflected onto the rear cells as well as direct front-side irradiance. The rear-side gain depends on mounting height, row spacing and ground reflectivity, so it is most useful on ground-mounts and high-albedo flat roofs; the extra yield should be modelled, not assumed. Read more
Energy management system (EMS): The control layer that coordinates generation, storage and site loads, deciding in real time when to charge or discharge the battery, when to export, and how to respect any grid connection limit. A well-configured EMS is what turns a BESS into a financial benefit; it is software and control logic, not the battery itself.
Ground-mount and solar carport: Two off-roof structures. A ground-mount is a steel or aluminium frame on screw piles or concrete foundations on open land, allowing optimal tilt and orientation. A solar carport is a raised canopy over parking that generates while sheltering vehicles and often hosts EV charging; both may need planning permission and a structural and ground assessment that a roof array does not.
Photovoltaic module (PV module): A factory-assembled panel of series-wired solar cells laminated under glass that converts sunlight into direct-current electricity, rated in watts-peak under standard test conditions. It is the basic generating unit of an array; commercial roofs typically use 60 to 72 half-cell modules wired into strings.
Power optimiser: A module-level DC-DC device fitted behind each panel that performs per-module maximum power point tracking, reducing the yield penalty from shading, soiling or mismatch and enabling module-level monitoring and rapid shutdown. It adds component count and cost, so it is specified where shading or roof complexity justifies it rather than by default.
Solar cell technology (PERC, TOPCon, HJT): The three mainstream silicon cell architectures on the UK market. PERC (Passivated Emitter and Rear Cell) is the established baseline; TOPCon (Tunnel Oxide Passivated Contact) and HJT (Heterojunction) are higher-efficiency successors with better low-light and temperature performance, usually at a higher cost per module. Specify the cell type and warranted degradation rate rather than headline efficiency alone.
String: A set of modules wired in series so their voltages add, forming one DC circuit into an inverter input or combiner. String length is set by the inverter's voltage window and is verified against IEC 62446-1 commissioning tests; shading or a faulty module affects its whole string, which is why string layout follows the roof's shade map.
String inverter versus central inverter: Two ways to convert the array's DC to grid-synchronised AC. String inverters are distributed units each serving a few module strings, giving granular monitoring and partial-failure resilience; a central inverter is one large unit handling the whole array, with lower cost per kW but a single point of failure. Most commercial rooftops use multiple string inverters; central units suit large uniform ground-mounts.
Trapezoidal versus standing-seam roof: The two common profiled-metal commercial roof types, which dictate the fixing system. Trapezoidal sheets are usually fixed with clamps to the raised ribs or short rails; standing-seam roofs accept non-penetrative seam clamps that grip the upstand, avoiding any holes. Identifying the profile early sets the mounting cost and whether penetration is needed.

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