Glossary

Definitions as they are used here. This field borrows vocabulary from power engineering, manufacturing and finance simultaneously, and the three do not always agree about what a word means. Where a term is genuinely used inconsistently, the entry says so rather than quietly picking a side.

Ancillary services

Services a system operator procures to keep the network stable, as distinct from the bulk energy itself: frequency response and reserve, voltage support, black start capability, and others. The names, the timescales and the procurement mechanisms differ between systems, so a service called by the same name in two markets may have different response requirements and be worth quite different amounts.

Balance of system

Everything in an installation other than the primary generating or storing equipment: mounting, foundations, cabling, combiners, protection, transformers, civil works, and often the installation labour itself. As the equipment has got cheaper, balance of system has become a larger share of delivered cost, which is why so much engineering effort now goes into things that are not the device.

Bankability

Whether a lender or insurer will accept a product or a project on ordinary terms. It is a function of certification, operating history, warranty quality and the financial standing of whoever issued the warranty — not of technical merit. A better product with no track record is routinely less bankable than a worse one with a long one, and this is rational rather than unfair.

Capacity

The most overloaded word in the field. In generation it usually means power — the maximum rate of output. In storage it may mean power or stored energy, and the two are unrelated ratings. In manufacturing it means how much a plant can produce in a period. Always establish which sense is meant and in what units before comparing two figures.

Capacity factor

Actual energy produced over a period divided by the energy that would have been produced running continuously at the nameplate rating. It compresses resource availability, losses, outages and curtailment into one figure, so a difference in capacity factor between two assets can be a difference in any of those.

Cell, module and array

Successive levels of assembly in a photovoltaic system: cells are the individual converting elements, modules are the packaged and interconnected assemblies sold as products, and an array is the installed collection of modules. Efficiency figures fall at each level — a module is less efficient than the cells inside it because it also contains frame, spacing and interconnection — so quoted efficiencies must be attributed to a level to mean anything.

Cleantech and climate tech

Two names for overlapping but differently drawn fields. “Cleantech” is the older term, associated with an earlier investment cycle and weighted towards clean generation equipment. “Climate tech” is the current preference and is broader, covering industrial process change, measurement and accounting, finance and adaptation alongside hardware. The shift in vocabulary reflects both a widening of scope and some distance-keeping from the earlier cycle.

Commissioning

The process of bringing installed equipment from constructed to operating: energising in sequence, testing protection, verifying performance against the contract, and clearing defects. It is routinely underestimated in schedules because it is the point at which every earlier optimism about interfaces is tested at once.

C-rate

The rate of charge or discharge of a storage system expressed relative to its stored energy. It is the compact way of stating duration from the other direction: a high C-rate means a system that empties quickly, which is a demand on the cells thermally and chemically as well as on the conversion equipment.

Curtailment

Output that could have been produced but was not, because the network or the market did not want it at that moment. Distinguish economic curtailment, where producing was not worth it, from technical curtailment, where the network physically could not accept it — they have different remedies and different contractual treatment.

Degradation rate

The proportional loss of output per year of an asset in service. Usually applied geometrically in models, though real behaviour includes a larger first-year loss and step changes as individual components fail. Warranty figures are floors that the manufacturer is prepared to guarantee, not expectations.

Depth of discharge

How much of a storage system’s usable energy is drawn in a cycle. Deeper cycles generally age cells faster per unit of energy delivered, which is why cycling strategy is an asset-life decision and not only a revenue decision.

Derate factor

A single multiplier applied to a nameplate rating to account for all real-world losses between the equipment rating and the meter. Convenient and crude: it collapses temperature, soiling, shading, mismatch, wiring, conversion, clipping and availability into one number, and several of those vary through the year and interact with each other.

Duration

Stored energy divided by rated power, expressed in hours. The most informative single figure about a storage installation, because it determines which services the asset can physically provide regardless of how it is contracted.

First-of-a-kind and nth-of-a-kind

Costs for the first built example of something versus a later one built by an organisation that has done it before. The gap is often large, and closing it requires that an nth actually gets built — a pipeline that stops disperses the workforce and supplier base, and the next attempt restarts from a worse position than the last one finished at.

Gigafactory

A loose marketing term for a very large manufacturing plant, with no agreed threshold and no standard basis for the capacity claimed. Announced capacity may refer to a final phase not yet built, to nameplate rather than achieved output, or to a measure that is not comparable with another site’s. Treat as an indication of ambition rather than as a specification.

Interconnection queue

The ordered set of requests to connect to a transmission or distribution network, and the study process that works through them. Positions matter because each project’s studies assume everything ahead of it is built, so queue order affects both timing and the network upgrades a project is asked to fund.

Inverter loading ratio

The ratio of installed DC generating capacity to the AC rating of the inverters, sometimes called the DC-to-AC ratio. Deliberately exceeding one is normal practice: it raises utilisation of the conversion equipment and the grid connection at the cost of discarding some output at the peaks, which is called clipping.

Learning rate

The proportional fall in unit cost associated with each doubling of cumulative production. It is an observed regularity, not a law, and it varies widely between technologies. It is also conditional on production actually continuing — the independent variable is cumulative output, so a technology that is not being built is not descending any curve.

Levelised cost of energy

Lifetime costs divided by lifetime energy, discounted, expressed per unit of energy. Used inconsistently to the point where two published figures are rarely comparable: they may differ on discount rate, on whether grid connection and network upgrades are included, on assumed asset life, on financing structure, and on whether the value of when the energy is produced is considered at all. It is a useful internal comparison tool and a poor public argument.

Nameplate capacity

The rating on the equipment. For solar installations it is ambiguous by default, because a site can be described by its DC array rating or by its AC export rating, and the two differ by the inverter loading ratio. A figure quoted without saying which is not interpretable.

Network upgrade

Reinforcement of the existing network required to accommodate a new connection — new or uprated lines, transformers, switchgear, protection changes. Whether the triggering project pays for it, or the cost is spread across users, is a policy choice usually described as deep versus shallow connection charging, and it can decide whether a given site is viable.

Peak sun hours

Daily solar irradiation on the plane of an array expressed as the equivalent number of hours at standard test conditions. It is a property of the array’s orientation and surroundings, not only of the location, and an annual average of it conceals the seasonal range.

Power electronics

The switching, conversion and control equipment that sits between a source and a network: inverters, converters, rectifiers, and their control systems. It sets the response characteristics of an asset, is where most efficiency losses in conversion occur, and is frequently the component that determines the asset’s availability.

Round-trip efficiency

Energy out divided by energy in for a storage cycle. Almost always quoted without stating the measurement boundary, which changes the answer substantially: a figure taken at the cell terminals excludes conversion, transformation and the auxiliary loads that run continuously, while a figure taken at the grid connection includes all of them. Only the latter corresponds to what is bought and sold.

Specific yield

Annual energy per unit of installed capacity, typically kilowatt-hours per kilowatt peak per year. A convenient way to compare sites and designs because it removes the size of the installation, and one that carries the same DC-versus-AC ambiguity as the capacity it is divided by.

State of charge

How full a storage system currently is. It is a single shared resource that every service contracted from the asset draws on, which is why a revenue stack presented as the sum of independent services usually overstates what the asset can deliver.

Takt time

The rate at which a production line must complete a unit to meet demand, and therefore the pace every station has to keep. It is the discipline that turns a collection of processes into a line: any station slower than takt paces the whole line, and any station faster than takt is idle capital.

Type certification

Testing and certification of a product design against a recognised standard, usually by an independent body. Distinct from factory quality assurance, which confirms that units conform to the certified design. Both are usually required before a product can be insured or financed, and both take time that cannot be compressed by spending more.

Yield

Two unrelated meanings that regularly collide. In manufacturing it is the proportion of started material that emerges as sellable product; because stage yields multiply, a line with many high-yielding steps can still have poor overall throughput. In project development it is the energy an installation produces. Context usually disambiguates, but not always, and the confusion is worth pre-empting in writing.