Initial symmetrical short-circuit current at the fault point (IEC 60909-0): the rms current the network drives into a bolted three-phase fault, with the voltage source replaced by the c·Un/√3 equivalent.

Why it matters: It is the number every device rating is compared with — Icw, breaking capacity and the cable thermal check all read against Ik. Under-estimate Ik and the panel you quote will be destroyed on the first fault; over-estimate it and the client pays for switchgear he does not need.

Short-circuit impedance voltage of a transformer: the primary voltage, in percent of rated voltage, that drives rated current through the short-circuited secondary winding (IEC 60076-1).

Why it matters: It fixes the LV fault level (approximately I″k ≈ In / uk) and at the same time the voltage drop under load. This is a real quotation trade-off: a larger uk lowers the fault level (cheaper switchgear downstream) but increases voltage drop and losses. Always quote uk together with the transformer rating.

Point of common coupling (PCC): the interface in the network where the customer installation and the utility network are coupled, and therefore where the connection conditions apply.

Why it matters: It defines what the utility may complain about and what you must prove: voltage band, harmonic distortion, flicker and voltage-dip behaviour are all specified at the PCC, not at the equipment terminals. Studies or measurements made somewhere else do not demonstrate compliance.

Voltage total harmonic distortion THDu = √(Σ Uh²) / U1 × 100 %, measured as required by IEC 61000-4-7.

Why it matters: It is the quantity a grid code or a utility limits at the point of common coupling (for reference: IEEE 519 sets 5 % THDv at the PCC, IEC 61000-2-2 gives 8 % for LV class 2). Exceeding it means filters, detuned banks or a larger transformer — a cost that appears only if the study is done before the order.

Current total harmonic distortion THDi = √(Σ Ih²) / I1 × 100 %: the harmonic content of the load current relative to its fundamental.

Why it matters: It is what sizes harmonic filters and detuning reactors and what drives cable and transformer derating. IEEE 519 limits it as a function of the ratio between the short-circuit current and the load current — so THDi is meaningless for a quotation unless the fault level is quoted with it.

Power factor cos φ: the ratio of active power to apparent power (displacement factor between voltage and current), with the distortion factor included when the current is distorted.

Why it matters: It sets the kVA that the transformer, generator or supply contract must provide for a given kW, and it is contractual: a low power factor triggers penalty charges or a mandatory compensation bank. It is therefore both a design input and a line in the price.

Reactive compensation rating Qc: the output of the capacitor bank, sized as Qc = P × (tan φ1 − tan φ2) to move the installation from its present to its target power factor.

Why it matters: It is the item being quoted, and its usable output drops in a harmonic-rich network unless a detuned (reactor-protected) bank is used. The target power factor is a customer input, not a default — quoting a bank without stating the target pf is not a complete offer.

Capacity factor: the energy actually produced over a period divided by the energy the plant would have produced running at its rated power for the whole period (for PV this is dominated by irradiance and temperature, not by the inverter).

Why it matters: It is the number that converts a nameplate kWp into annual kWh, and therefore into revenue and payback. A quotation that gives only the kWp figure does not say what the plant will actually deliver — ask for the assumed capacity factor.

Low-voltage ride-through: the duty (and the capability) of a generator or inverter to remain connected while the voltage at the PCC dips along a specified voltage-versus-time profile, instead of tripping off.

Why it matters: It is a grid-connection condition in most markets: a plant that cannot ride through the required curve cannot be connected or must be retrofitted. Confirm the required LVRT curve before sizing the inverters, because it decides the hardware class.

State of charge of a battery or BESS: the remaining energy as a percentage of the rated (or usable) capacity.

Why it matters: It bounds what the storage can deliver at any instant, so it drives the protection settings, the dispatch strategy and the delivered-energy guarantee. A quoted availability figure is meaningless unless the SOC window it assumes is stated.

Depth of discharge: the complement of the state of charge — how much of the capacity is cycled out, normally quoted as a maximum in cycle-life specifications.

Why it matters: It is the trade between usable capacity and battery life: the same nominal kWh quoted at 80 % DOD delivers fewer cycles than one quoted at 60 %. Because it changes both the usable kWh and the replacement date, the DOD must be on the datasheet next to the price.

Round-trip efficiency of a storage system: the energy returned during discharge divided by the energy absorbed during charge over a complete cycle.

Why it matters: It decides how much of the energy you pay for actually comes back out, and therefore the real cost per delivered kWh. Watch the boundary of the figure (DC-DC looks much better than AC-AC) — comparing two offers on different boundaries is a common source of wrong quotations.

Simple payback period: the capital cost divided by the annual net saving, without discounting or price escalation (a screening figure, not a financial model).

Why it matters: It is the number that decides whether the customer buys now. Because it is a simple ratio, the two inputs that drive it (energy price and unit capex) must be shown next to it — otherwise nobody can tell which assumption the figure stands on.