PV Glossary

Voc (open-circuit voltage) is the highest voltage a PV module delivers when no current flows. At -10 °C it can be 10-12 % above the STC value. The maximum input voltage of the MPPT must be rated for this, otherwise the charge controller is damaged. Rule of thumb: (module Voc × number in series) × 1.12 ≤ MPPT vocMax.

Vmpp is the voltage at which the module delivers its maximum power. The MPPT charge controller actively regulates to this point. Important: the sum of the Vmpp of all modules in series must lie within the MPPT operating window, otherwise the tracker cannot hold the point.

Isc (short-circuit current) is the current that flows when the module terminals are connected directly. For cable and fuse sizing one typically calculates with Isc × 1.25, to keep a reserve for high irradiance (>1000 W/m²).

Imp is the current the module delivers at the MPP. Imp × Vmpp gives the maximum power (Pmpp = power in Wp). When sizing the MPPT, the tracker current limit (ipvMaxA) is compared with the Imp per string.

Wp (watt-peak) is the module power under standard test conditions (STC: 1000 W/m², 25 °C cell temperature, AM 1.5). In real operation the power is usually 10-20 % below Wp because of higher module temperature, soiling and losses. 1 kWp = 1000 Wp.

Standard test conditions are the standardised conditions under which Voc, Vmpp, Isc, Imp and Pmpp are stated in the module datasheet. In real operation the values are almost always worse (the module gets hotter than 25 °C). For realistic values there is NOCT.

NOCT is closer to everyday operation than STC. Module datasheets normally state both sets of values. For annual yield simulations NOCT is often more representative.

The temperature coefficient β states how much the Voc drops per degree of temperature rise (negative, because voltage falls as it gets warmer). Conversely, at -10 °C the Voc rises by about 10-12 %. This value is the basis of the cold correction in MPPT sizing.

Under partial shading the current would be limited by the weakest cell. Bypass diodes (typically 3 per module) route the current around the shaded sub-strings and prevent module damage. The price: the shaded module then only delivers 2/3 or 1/3 of its power.

Glass-glass modules last longer (often 30 years warranty), are heavier (about 25-28 kg vs. 20-22 kg) and are usually bifacial (they also use light reflected onto the rear side). Glass-foil is cheaper and lighter, but has a shorter service life (25 years). Relevant for the structural analysis and the mounting system.

The frame thickness matters for choosing end and mid clamps, which have to grip exactly this height. Glass-glass modules are often frameless (0 mm) — then special clamps or bonded systems are required.

On the current-voltage curve (I-V diagram) there is exactly one point at which current × voltage becomes maximum. This MPP shifts with irradiance and temperature. The MPPT tracker adjusts the load so that this point is always held.

MPPT (maximum power point tracker) is an electronic circuit that continuously searches for the optimum operating point. Modern MPPTs often have 2-4 independent trackers, so that shaded string groups or groups with different orientations can be optimised separately.

Outside this window the MPPT cannot work. If the voltage is too low it switches off; if it is too high the charge controller is damaged. Rule of thumb: the sum of the module Vmpp must stay inside the window at every temperature (summer as well as winter).

At low irradiance in the early morning a minimum Voc has to be reached before the MPPT switches on (for example 8 V above battery voltage on the Victron SmartSolar). Important in winter and in weak light.

A string is the basic electrical unit: several modules connected in series. The voltages add up, the current stays the same. Several strings in parallel have the same voltage but a higher current. Important: all modules in one string must have the same orientation and be free of shading.

With N modules in series: V_string = N × V_module, I_string = I_module. Commonly used to bring the voltage into the MPPT range (e.g. 8 modules at 40 Vmpp = 320 V for a 100/30 charge controller).

With N parallel strings: I_total = N × I_string, V_total = V_string. Used when the current limit of the MPPT is not yet reached. Important: all parallel strings must be electrically identical (same number of modules, same orientation).

Industry standard notation for describing strings. 'S' = series, 'P' = parallel. 4S2P means: 2 strings of 4 modules each, connected in parallel. Voc = 4 × Voc_module, Isc = 2 × Isc_module.

Because the string current is limited by the weakest module, every inequality causes a loss. Typical mismatch losses: 1-3 % with identical modules, up to 30 % with heavy partial shading (a single shaded module is enough). Bypass diodes soften the effect.

A single shaded cell can pull the current of the whole string down to the level of that cell. Bypass diodes limit the damage, but 1/3 or 2/3 of the module drops out. Remedy: separate MPPT trackers for shaded areas, or module optimisers.

The inverter is the central device of a grid-connected PV system. Variants: string inverter (one or more strings), hybrid inverter (with battery connection), off-grid inverter. Efficiency today is typically 96-98 %.

Hybrid inverters (e.g. Fronius GEN24 Plus, Solis Hybrid, Huawei SUN2000) have both PV inputs and a battery connection. Advantage: one device instead of two, simpler wiring. The backup-power function is often optional. Alternative: an AC-coupled solution with Victron MultiPlus and storage.

Off-grid systems need generous sizing (more than 3× the daily consumption as battery reserve), reliable components and usually a backup generator. Typical applications: mountain huts, boats, motorhomes. Victron Quattro and MultiPlus-II are the industry standard here.

Victron ESS is a software configuration of the MultiPlus/Quattro line that allows grid-parallel operation with battery buffering. The system maximises self-consumption, can switch to backup-power mode during a grid failure and is controlled via Cerbo GX or Venus OS.

The RS series (450/100 and 450/200) allows large string voltages up to 450 V Voc to be wired directly. Important: the RS 450/100 has 2 MPPT trackers, the RS 450/200 has 4. The classic SmartSolar units (100/30, 150/35, 250/100) are single-tracker devices for smaller systems.

MultiPlus and MultiPlus-II are the standard inverter/chargers of the Victron range for grid-parallel and off-grid applications. The Quattro additionally has a second AC input (e.g. for a generator) with uninterrupted transfer. EasySolar-II combines MultiPlus, SmartSolar and distribution in a single enclosure.

Phoenix Smart and Phoenix VE.Direct are pure inverters without a charging function. They are used when only DC energy (battery) has to be converted to AC, without grid-parallel operation. Typical in motorhomes or as a pure backup-power inverter.

Capacity is stated in kWh, often distinguishing between 'gross' (chemically available) and 'usable' (limited by DoD). With LiFePO4 90-95 % is usable, with lead only about 50 %. Sizing rule of thumb: 1-1.5 kWh of storage per 1 kWp of array.

The DoD determines the usable capacity. LiFePO4: 90-95 % DoD is safely possible. Lead-acid: typically 50 % DoD, so as not to halve the service life. AGM/gel: 50-60 %. The number of charge/discharge cycles depends strongly on the DoD.

With a 10 kWh battery, 0.5C means 5 kW of charge/discharge power. LiFePO4 typically tolerates 0.5-1C without loss of service life, lead only 0.1-0.2C. Important for inverter sizing: its charging power must not exceed the C-rate.

LFP (or LiFePO4) is the chemistry of most modern home storage systems (Pylontech, BYD, Victron Smart Lithium, Fronius Solar Battery). Advantages over NMC: higher thermal stability, longer service life, no cobalt. Disadvantage: lower energy density (more space for the same kWh).

Despite being technically inferior to lithium, lead is still common in off-grid systems. AGM (absorbent glass mat) and gel are the maintenance-free variants. Maximum DoD: 50 %, otherwise the cycle life is halved. Efficiency: 80-85 % (LiFePO4: 95 %+).

Stated in mm² (copper or aluminium cross-section). In PV systems typically 4 mm² or 6 mm² for PV strings and 25-95 mm² for battery-to-inverter connections. Selection criteria: current carrying capacity (DIN VDE 0298-4), voltage drop (max. 1-3 %) and fuse sizing.

Calculated as ΔU = 2 × I × L × ρ / A (for DC), for AC additionally with cos φ. On PV strings 1 % is the industry standard (more wastes energy); on 230 V AC 3 % is permitted (DIN VDE 0100-520). The voltage drop rises with current and length and falls with cross-section.

The current carrying capacity depends on the material (Cu vs. Al), the cross-section, the installation method (free air, buried, in walls) and the ambient temperature. Tables can be found in DIN VDE 0298-4. On PV strings the Iz is additionally multiplied by 1.25 as a reserve for high irradiance.

Standardised installation methods according to DIN VDE 0298-4: A1 (in a thermally insulated wall), B2 (in conduit in a wall), C (on a wall), E (free in air). The current carrying capacity can differ by a factor of 1.5-2 between A1 and E, which makes it important for sizing.

Solar cables are double-insulated, UV-resistant, halogen-free, temperature-resistant up to 120 °C and approved for 1500 V DC. Designation H1Z2Z2-K (formerly PV1-F). Standard cross-sections: 4 mm² and 6 mm². Never use ordinary NYY-J cable for DC runs.

The eaves are the lower horizontal edge of the roof. In the PV planner the 'eaves clearance' is the mandatory distance (typically 300 mm) between the lower edge of the module field and the roof edge, so that snow load does not press into the gutter.

On a gable roof the ridge runs lengthways in the middle between the two roof surfaces. In the PV planner the 'ridge clearance' is the mandatory distance (typically 300 mm) between the upper edge of the module field and the ridge, so that the ridge tile stays accessible.

The verge is the lateral edge of the roof towards the gable. In the PV planner the 'verge clearance' (typically 300 mm) is used as a safety distance between the side edge of the module field and the side edge of the roof.

Rafters are the structurally load-bearing timber beams (usually 8/16 cm or 10/20 cm). Typical rafter spacing is 70-90 cm. Important for fixing the mounting system: roof hooks and double hooks must be screwed INTO the rafter, not into the battens. Hence the rafter snap logic in the mounting-system view.

Purlins (eaves purlin at the bottom, middle purlin or ridge purlin at the top) carry the rafters in purlin roofs. On trapezoidal sheet and sandwich-panel roofs, hanger bolts are set directly into the purlin.

The roof batten (about 30/50 mm) lies crossways on the rafters and carries the tiles. Important: PV fasteners must never sit in the battens alone — they always have to reach the rafter. Roof hooks often have a shaped geometry that clears the tile profile AND reaches the rafter.

The roof covering determines the fixing logic of the mounting system: tiles → roof hook (with or without a recess for the tile profile), plain (beaver-tail) tiles → double hook, trapezoidal sheet → hanger bolt with sealing washer, standing seam → seam clamp (clamps without drilling), corrugated fibre cement → hanger bolt through the crest of the corrugation.

The standard fixing for tiled and slate roofs. The hook is screwed onto the rafter (with two or three 6×80 or 8×80 screws); the tile is partly notched, or special hook shapes clear the tile profile. The rail is then clamped onto the top of the hook.

The double hook is used for higher pull-out loads (wind suction) or with two-layer systems. It spreads the load over two screw points and can take two or more screws per fixing. In the plan it appears as a plate with two black dots (versus a single dot for the normal roof hook).

Hanger bolts are the standard fixing for trapezoidal sheet, corrugated fibre cement and sandwich-panel roofs (M8 or M10, lengths 150-300 mm). One end is a wood thread (into the rafter or purlin), the other a metric thread with an EPDM sealing washer and a nut to take the rail. On trapezoidal sheet it always sits on the crest, never in the trough.

C47 is the most widely used PV rail in the German-speaking region (Solarplanit, K2, IBC). Cross-section: 47 mm wide, 40 mm high, with a T-slot for the clamps. Stock lengths: 3,600 mm and 5,900 mm. For longer rail runs, several pieces are coupled together with rail splices.

Required when the total rail length exceeds the 5.9 m stock length. The splice is pushed into the inner section of both rails and clamped tight. Shown in the plan as a yellow symbol at the joint. Important: joints should if possible not lie in the area of a clamp.

End clamps sit at the edge of a module row (one side presses the module, the other side ends free). Mid clamps sit between two modules and hold both at once. Clamps must match the module frame thickness exactly (30, 35, 40 mm), otherwise there is no secure hold. Typical tightening torque: 10-15 Nm.

In a two-layer mounting system (DP_2L) there is a continuous lower bearer rail and upper module rails that cross it at right angles. A cross connector sits at every crossing point. This configuration carries more load than a single-layer system, but is more expensive and heavier.

On flat-roof systems the mounting structure is not drilled through the roof but weighted with ballast against wind suction. Calculated according to ÖNORM B 1991-1-4 / EN 1991-1-4. Typically 100-300 kg per module, depending on wind zone, building height and module tilt. The calculation takes into account the friction coefficient between the roof membrane and the structure.

On flat roofs the modules are not laid flat (too little yield) but elevated at 10-15°. Variants: south elevation (highest yield per module) or east/west elevation (more modules on the same area, a more even daily curve). Standard products: K2 D-Dome, IBC TopFix 200.

Calculated according to ÖNORM B 1991-1-3 or DIN EN 1991-1-3. Snow load zones 1-4 in Austria, plus special values for high alpine areas. A typical base load of 0.75 kN/m² is modified by an altitude factor and a shape coefficient (roof pitch). PV modules must carry the full snow load without deformation — hence the structural calculation in the planner.

Calculated according to ÖNORM B 1991-1-4. Key inputs: wind zone (1-4), building height, terrain category (urban vs. rural) and roof shape. At the roof edges (corner and edge zones) the suction can be 2-3 times higher than in the middle — additional fasteners are then required there.

For every roof-hook fixing point the calculated pull-out load (kN) must stay below the permissible load of the wood screw. Rule of thumb: an 8×80 screw in spruce holds about 2.5-3 kN of pull-out force. For higher loads: double hooks or thicker screws.

In the structural result: a utilisation of 24 % means the system uses only a quarter of its load capacity (plenty of reserve). 95 % would be critical (close to the limit); over 100 % means calculated failure — then more fasteners or stronger components are needed.

The specific yield allows systems to be compared independently of their size. Influencing factors: location (southern Germany 1000-1100, northern Germany 850-950), orientation (south 100 %, east/west 85-90 %), tilt (20-35° optimal), shading, module quality and inverter efficiency.

Without storage typically 25-35 %, with a 10 kWh battery 60-80 %, with a heat pump, an electric car and storage up to 90 %. A higher self-consumption rate means better economics, because electricity used on site (about 25-30 ct/kWh) is worth more than electricity fed in (5-12 ct/kWh in Austria in 2025).

While self-consumption says how much of the PV electricity is used, self-sufficiency describes the share of the household demand that comes from PV. With 10 kWp plus a 10 kWh battery, 60-75 % self-sufficiency is typical. 100 % would mean off-grid operation (realistically not achievable in winter without a huge battery).

In Austria, OeMAG pays a subsidised tariff for 13 years (between 5.5 and 12 ct/kWh in 2024/25, depending on the investment grant received). The alternative is free market participation via an electricity trader (often slightly more, but variable). Important: high self-consumption is usually more economical than high feed-in.

Defines snow load zones 1-4 in Austria, the altitude adjustments from 200 m above sea level and the shape coefficients for each roof pitch. German counterpart: DIN EN 1991-1-3 with the national annex. Eurocode basis: EN 1991-1-3.

Defines wind zones 1-4 in Austria (map), the height categories, the pressure coefficients for each area of a component (corner, edge, middle) and the friction coefficients. German counterpart: DIN EN 1991-1-4.

OVE E 8101 is the Austrian erection standard for low-voltage installations. The cable calculator uses the current carrying capacity tables from this standard.

Defines, for every installation method (A1, B2, C, E, D) and every cross-section, the maximum continuous current carrying capacity. It is the basis of the capacity column in the cable calculator.

Erection of PV power supply systems: requirements for DC wiring, protective measures, PV fuses and isolating devices. The corresponding international base standard is IEC 60364-7-712.

Defines the requirements for PV special cables: 1500 V DC, double insulation, UV resistance, halogen-free, temperature range -40 to +120 °C, service life 25 years at 90 °C. All PV string wiring should comply with this standard.