PV system sizing
Start from your annual consumption or your desired power and find out, as a guideline, how many kWp you need, how many panels, which inverter and how to size the DC cable per I7-2011.
From consumption to kWp
The steps for sizing a residential PV system, in brief.
Required installed power (kWp)
Divide the target annual consumption by the specific yield of the location. In Romania the guide yield is ~1100–1200 kWh produced per year for each kWp installed, depending on orientation, tilt and shading.
kWp = annual consumption (kWh/year) ÷ specific yield (kWh/kWp/year)
Number of panels
The number of panels follows from the installed power relative to the power of one panel. It is rounded up, then the actual system power is recalculated for the whole number of panels.
number of panels = (kWp × 1000) ÷ panel power (Wp) → rounded up
Inverter power (inverter/panel ratio)
The inverter is chosen by a ratio to the panel power. Typically 0.8–1.0; usually ~0.9, i.e. a slight “oversize” of panels (DC oversize). A ratio below 1 clips the rare winter/summer peaks but increases the total annual energy; a ratio above 1 is rarely justified.
inverter power (kW) = panel power (kWp) × ratio (0.8…1.0)
The DC (direct current) cable
On the DC side the short-circuit current is very close to the working current, so an ordinary fuse cannot tell a fault from normal operation. That is why the string cable is sized with a current-carrying capacity of at least 1.25 × Isc (STC) at every point — the condition under which I7-2011 Art. 7.11.11 no longer requires overload protection on that cable. It is not an imposed minimum for sizing, but the threshold from which the exemption applies.
DC cable sizing current = 1.25 × Isc (STC)
Battery and self-consumption
The battery is sized for the average daily consumption and the desired number of days of autonomy. Self-consumption (the percentage of energy used directly in the house) affects the actual savings — the rest is fed into the grid. All values are guidelines; the final sizing is done with the equipment manufacturer.
What STC means and why you never see those values on a roof
Every figure on the panel label is measured under standard test conditions (STC): 1000 W/m², cell temperature 25 °C, air mass AM 1.5. In summer the cell reaches 60–70 °C, in winter −15 °C — and then the voltage is no longer the one on the label. There are also the NOCT conditions (800 W/m², air 20 °C, wind 1 m/s, open circuit), which are closer to reality.
Series or parallel — what adds up
Connected in series, the voltages add up and the current stays that of a single module; connected in parallel, the currents add up and the voltage stays that of one module. The condition, in both cases: identical modules. Within a string, the weakest module dictates the current of the whole string — which is why a single shaded cell drags the entire row down.
What your array plane actually produces
Coordinates, tilt, orientation — and the monthly production measured over your area, not an average picked up off the internet.
Specific yield differs by hundreds of kWh/kWp between a south-facing pitch and a west-facing one, between 20° and 40° of tilt, between lowland and mountain. We do not estimate here with rule-of-thumb formulas: we ask PVGIS for the figures, the tool of the European Commission’s Joint Research Centre, which works on satellite-measured climatologies spanning decades. Fill in the fields and press the button.
Cables, inverter, soiling, ageing. PVGIS uses 14% by default, a reasonable value for a new, clean system.
Guideline optimum tilt for this latitude: 46° (the rule from the course — around the angle of the latitude).
Below 45° the snow no longer slides off on its own. Where snowfall is heavy, the course asks for at least this much, even if you lose a few percent of annual production.
The figures come from PVGIS (European Commission, JRC) — multi-year climatology, non-commercial use. They are a good estimate of the average, not a guarantee for any particular year: a cloudy year, or a winter with snow sitting on the modules, brings the real production down.
String sizing
Three criteria, in this order: power, voltage, current.
The voltage on the panel label holds at 25 °C. At −15 °C the open-circuit voltage rises by about 10%, and if the string goes over the maximum inverter voltage, the inverter is destroyed. In summer, with the cell hot, the voltage falls and the string can drop below the MPPT window — the inverter starts late or does not start at all. So the check is made twice: cold and hot.
- 1. Power — the panel power is kept between 80% and 120% of the inverter power.
- 2. Voltage — Uoc at the minimum temperature × the number of modules must stay below the maximum DC voltage; Umpp with a hot cell must stay above the lower threshold of the MPPT window.
- 3. Current — the Impp and the Isc of the strings must fit within what the MPPT input accepts.
Panel — figures from the datasheet (STC)
Datasheets print the coefficient in any of the three forms. Pick the one on your own datasheet — the calculator does the conversion, referring each coefficient to its own reference voltage.
The sign does not matter: with silicon the voltage always falls as the temperature rises, so a coefficient written without a sign is still used as a negative one.
Inverter — figures from the datasheet
The maximum panel power the inverter datasheet allows on one string. If the datasheet gives it per MPPT input and you put two strings on the same input, divide it by two first. Leave it empty if the datasheet does not state it — then the criterion simply does not apply.
Site and configuration
Result
Compatible, but with caveats
Checks
String Uoc at the minimum temperature
473.2 V below the 950 V limit — the inverter holds up even on the coldest morning.
String Umpp with a hot cell
267.2 V above the 200 V threshold — the string stays inside the MPPT window in summer as well.
String Umpp at the minimum temperature
395.3 V below 800 V — the string works inside the MPPT window all year round.
Working current at the MPPT input
10.4 A, below the 11 A limit.
Short-circuit current at the input
11.7 A below the 16 A allowed on short circuit.
Panel power / inverter power ratio
1.17 — within the usual 0.8…1.2 range.
120 V DC threshold (SELV/PELV)
String Uoc at STC = 401 V, above 120 V. Where protection is by SELV/PELV, Uoc(STC) must not go over 120 V DC.
Class II modules
String Uoc above 120 V: class II construction modules, or modules with equivalent insulation, are recommended.
Maximum module voltage
473.2 V below the maximum system voltage declared by the module manufacturer (1000 V).
1.Correct the open-circuit voltage for the coldest morning
formula:Uoc(T) = Uoc(STC) × (1 + β/100 × (T − 25))
with your numbers:Uoc(-15°C) = 40.10 × (1 + (-0.45/100) × (-15 − 25))
gives: 47.32 V per module
The coefficient is negative and the temperature is below 25 °C, so the product comes out positive: the voltage rises. This is the case that destroys the inverter.
2.Find how many modules fit within the maximum voltage
formula:n_max = U_DC,max / Uoc(T_min)
with your numbers:n_max = 950 / 47.32 = 20.08
gives: 20 modules
Round DOWN — one module more means going over the maximum voltage, and there is no tolerance to negotiate there.
3.Correct the working voltage for a hot cell
formula:Umpp(T) = Umpp(STC) × (1 + β/100 × (T − 25))
with your numbers:Umpp(70°C) = 33.50 × (1 + (-0.45/100) × (70 − 25))
gives: 26.72 V per module
4.Find the minimum number of modules
formula:n_min = U_MPPT,min / Umpp(T_max)
with your numbers:n_min = 200 / 26.72 = 7.49
gives: 8 modules
Round UP. If the manufacturer also declares a start-up voltage, the minimum is the more severe of the two thresholds — otherwise the inverter sits “inside the window”, but it never starts.
5.Check the chosen string against voltage
formula:Uoc,str(T_min) = n × Uoc(T_min) ≤ U_DC,max
with your numbers:10 × 47.32 = 473.2 V vs 950 V
gives: within the limit
6.Check the current at the MPPT input
formula:I_MPPT = Impp × n_str ≤ I_DC,max
with your numbers:10.42 × 1 = 10.42 A vs 11.0 A
gives: within the limit
7.Check the panel power / inverter power ratio
formula:DC/AC = P_PV / P_AC
with your numbers:(10 × 1 × 350 / 1000) / 3.00 = 3.50 / 3.00
gives: 1.17
The usual range in the course is 0.8…1.2. Above it you clip the production peaks, below it you pay for an inverter you do not use.
8.Calculate the sizing current of the DC cable
formula:I_b = 1.25 × Isc(STC)
with your numbers:1.25 × 11.15
gives: 13.94 A
Art. 7.11.11: if the Iz of the cable is at least 1.25 × Isc(STC) at every point, overload protection is no longer needed on the string cable.
9.Find how many strings fit on the inverter
formula:n_str = min( 1.2 × P_AC / P_str,max ; I_DC,max / Impp )
with your numbers:by input current: 1
gives: 1
Two independent criteria: the power budget of the inverter divided by the power allowed per string, and the maximum input current divided by the current of one string. The more severe one wins. It is a budget estimate — the actual number of MPPT inputs is what the datasheet states.
Art. 7.11.17: „Modulele PV pot fi conectate în serie până la tensiunea de funcționare maximum permisă a modulelor PV (U a lanțurilor PV) și invertorului PV, dar cea mai mică dintre cele două valori. OC STC Specificațiile pentru acest echipament trebuie obținute de la fabricantul echipamentului (fig. 7.11.1 și 7.11.2).” Art. 7.11.17: “PV modules may be connected in series up to the maximum permitted operating voltage of the PV modules (U OC STC of the PV strings) and of the PV inverter, but the lower of the two values.” The letter of the standard refers to Uoc under standard conditions; the calculator checks the string at Uoc corrected to the minimum design temperature — a more conservative criterion, because in winter, on open circuit, the actual voltage is higher than the one on the label.
Art. 7.11.18: „Dacă se utilizează diode de blocare, tensiunea lor nominală inversă trebuie să fie de 2U a OC STC lanțului PV. Diodele de blocare trebuie conectate în serie cu lanțurile PV.” The figure above is 2 × Uoc (STC) of the string: the Uoc on the panel label multiplied by the number of modules, at 25 °C — the article refers to standard conditions, not to the minimum design temperature. The diodes are not mandatory; the condition applies only if you choose to fit them.
In the extract from the Monitorul Oficial the subscripts sit on a line of their own, so here they appear inline: “2U a OC STC” reads as 2 × Uoc(STC), and “U a OC STC” reads as Uoc(STC).
The calculation uses the linear correction with temperature: X(T) = X_STC × (1 + coef/100 × (T − 25)). The default coefficients (−0.45%/°C on voltage) are the ones taught in the course for crystalline silicon modules; replace them with the values from the datasheet of your own panel. The minimum design temperature is taken from the weather data for the area, not by eye.
Cable, string fuse and losses
On the DC side you work in copper only, in two cross-sections: 4 and 6 mm².
A PV string has one dangerous peculiarity: the short-circuit current is only slightly above the normal working current. An ordinary fuse never “sees” the fault. That is why I7 does not require overload protection if you meet the current-carrying capacity condition, and the string fuse is chosen squeezed between two thresholds.
DC cable run
The current-carrying capacity is taken from the solar cable datasheet; the default value (31 A for 6 mm² copper) is the one in the table used by the rest of the application, for embedded installation — a solar cable in free air carries more.
Result
The course generally brings string fuses in only above four strings in the whole system — if the inverter has several MPPT inputs, count the strings on all of them, not only the ones on this input. Art. 7.11.11 drops overload protection anyway if Iz ≥ 1.25 × Isc(STC).
The fuse has to operate at twice the short-circuit current at STC, so the rating is chosen below 2 × Isc. On low-current strings there is no rating that fits between the two thresholds — and that is a correct result, not a calculation error.
On the alternating current side the cross-section calculation starts from a 3% drop against the nominal voltage of the supply.
Working rules for the DC side
- Copper only — aluminium is not used on the direct current side in photovoltaic systems.
- The accepted cross-sections between the panels and up to the inverter are 4 mm² and 6 mm² — the limitation comes from the connectors.
- Solar cable with double insulation, rated for 1000 V or 1500 V DC; there is no such thing as a 500 V DC cable.
- The current used to calculate the losses is the one at maximum power (Impp), not the short-circuit current.
- The loss calculation also takes in the factory-fitted leads of the modules (usually 2 × 0.3 m per panel, 4 mm²), not just the main cable.
- Put the inverter as close to the panels as you can: on the DC side you are limited to 4 and 6 mm², on the AC side you can go to any cross-section.
- There is no maximum distance laid down in the standard — only the obligation that the cross-section be matched to the current and the length.
- If you fit string fuses outside the inverter, check them against the ones inside it: many inverters already have fuses on the inputs, and if yours are larger, the inverter’s own fuses blow first — exactly the ones that are not easy to replace.
- On the generation side the diversity factor is always 1. With loads you are used to reckoning that not everything runs at once; panels do run all at once, by definition — so the protective device upstream of a junction box is sized on the actual sum of the circuits in it, not on a fraction of it.
1.Choose the rating of the string fuse
formula:1.25 × Isc ≤ In < min( Iz ; 2 × Isc )
with your numbers:1.25 × 11.15 = 13.94 A ≤ In < min(31 ; 22.30) = 22.30 A
gives: In = 16 A
It has to be above 1.25 × Isc, so that it does not operate at the normal working current, but below the current-carrying capacity of the cable, so that it protects it.
2.Add up the voltage drop over the sections
formula:ΔU = Σ 2 × L × I / (γ × S)
with your numbers:2×6×10.42/(56×4) + 2×20×10.42/(56×6)
gives: 1.80 V (0.54 %)
The factor 2 is for the out-and-back run, and γ = 56 is the conductivity of copper. The course recommends staying below 1% on the DC side.
3.Calculate the power lost in the cables
formula:P = ΔU × I
with your numbers:1.80 × 10.42
gives: 18.7 W
The formula used: ΔU = Σ 2·L·I / (γ·S), with γ = 56 m/(Ω·mm²) for copper — exactly the constant “56” from the formula taught in the course, and the same value the voltage drop engine of the application uses.
Size your system
Enter the data and see the power, panels, inverter and DC cable instantly.
System parameters
Isc (STC) is the short-circuit current of a PV string, from the panel datasheet; it is used to size the DC cable (1.25 × Isc).
Guideline result
Caution — the DC side
PV equipment on the DC side remains live even after disconnection on the AC side (I7-2011 Art. 7.11.4). Work is carried out only by authorised personnel, with the DC switch-disconnector open.
The results are guidelines (standard STC conditions) and do not replace the technical design. The specific yield, self-consumption and actual production depend on location, orientation, tilt, shading and equipment. The final sizing of the inverter, cables and protective devices is done by the registered electrician, using the datasheet data.
Requirements for PV installations (ch. 7.11)
The special provisions from I7-2011, consistent with SR HD 60364-7-712.
The DC side is always live
PV equipment on the DC side must be considered live even when the system is disconnected on the AC side (Art. 7.11.4) — the panels produce for as long as there is light, and there is no switch that stops them. Earthing a live conductive part in the DC area is permitted only if there is at least simple separation between the DC side and the AC side (Art. 7.11.3); that is a condition for earthing, not a general obligation to provide separation — installations without separation are dealt with below.
Type B RCD where there is no separation
When the installation contains a PV system without at least simple separation between the AC and DC sides, the residual current device (RCD) provided for fault protection by automatic disconnection must be of type B (Art. 7.11.7). If the inverter does not allow DC fault current to pass, the RCD is no longer required (Art. 7.11.8).
The DC cable and equipotential bonding
On the cables of PV strings and groups, overload protection is not provided if the cable’s current-carrying capacity is at least 1.25 × Isc (STC) at every point (Art. 7.11.11). Protective equipotential bonding conductors are run parallel to, and in as close contact as possible with, the DC cables, the AC cables and their accessories (Art. 7.11.25) — not “somewhere in the vicinity”, but stuck to the route, so that the loop stays small.
Switch-disconnector and isolation for maintenance
On the DC side of the inverter a switch-disconnector is provided (Art. 7.11.23). For maintenance of the inverter, means of isolation must be provided from both the DC side and the AC side (Art. 7.11.21).
Warning labels and DC cables
Every junction box — generator boxes and group boxes alike — carries a warning label showing that the live parts inside may remain live even after the inverter has been isolated (Art. 7.11.24). It is the label that saves whoever opens that box two years from now, when you are no longer around. The DC cables of the strings, the DC cables of the groups and the main cable are selected and installed so that the risk of an earth fault and of a short circuit is reduced to a minimum — in practice, by using single-core sheathed cables (Art. 7.11.20).
What is NOT allowed on the DC side
The protective measures using non-conducting locations and local equipotential bonding are not permitted in the DC area (Art. 7.11.10); for fault protection, class II or equivalent insulation is preferred (Art. 7.11.9). And the modules are installed so that the heat can dissipate under conditions of maximum local solar radiation (Art. 7.11.19) — not laid flat against the roof covering, however tidy that may look.
The AC cable of the inverter is not connected just anywhere
On the AC side, the PV supply cable — the one between the AC terminals of the inverter and the building installation — is connected to the protective device that automatically disconnects the circuits supplying the equipment in use (Art. 7.11.6), and against short circuit it is protected by a device placed in the main AC circuit (Art. 7.11.13). Translated onto the board: the inverter gets its own breaker, not a pair of lugs bolted straight onto the incoming busbars because “it feeds in anyway”. With lugs on the busbar, a short circuit on the AC cable trips nothing until the fuse in the service cut-out — and that one is no longer yours.
For isolation from the grid, the grid is the source
“When selecting and commissioning the isolation and switching devices to be installed between the PV installation and the public distribution network, the public distribution network must be considered the source and the PV installation must be considered the load” (Art. 7.11.22). That reversal of perspective catches plenty of people out, because the PV installation generates and the reflex is to treat it as the source. The standard asks for the opposite, and it has practical consequences: the prospective short-circuit current the device has to withstand comes from the grid, not from the panels (the panels give little more than their working current), and if the device has its supply direction marked on the case, the supply is taken from the grid side.
Switchgear made for direct current
“The electrical equipment on the DC side must be suitable for the DC working voltage and current” (Art. 7.11.16). It is the article broken every time somebody saves money on the switch-disconnector. On alternating current the arc puts itself out, because the voltage passes through zero a hundred times a second and the arc is left with nothing to restart on. On direct current it never passes through zero: a 230 V AC disconnector put on an 800 V DC string opens its contacts, and the arc stays hooked between them and burns until something melts. That is why DC devices have longer arc chutes, sometimes a blow-out magnet, and a marked connection direction — which you respect. And the voltage you choose them for is the Uoc of the string at the minimum temperature, not the one on the panel label.
Blocking diodes, if you choose to fit them
Blocking diodes are not imposed by the standard, but if you use them they come with two conditions: their rated reverse voltage must be 2 × Uoc (STC) of the PV string, and the diodes are connected in series with the strings (Art. 7.11.18). The factor of 2 is a generous margin precisely because, under a fault, the diode ends up taking the voltage of the group connected in parallel on top of the voltage of its own string. The string calculator above gives you the figure directly, for the configuration you have chosen.
The quotations above are from the text of I7-2011 ch. 7.11. The list covers what counts on a residential installation, but it is not the whole chapter: there remain the definitions (Art. 7.11.2), the 120 V DC threshold for protection by SELV and PELV (Art. 7.11.5), the exemption from overload protection for the PV main cable (Art. 7.11.12) and Art. 7.11.15, the only article in the chapter that mixes a requirement with a recommendation: the modules must comply with the equipment standards, but class II above 120 V DC is only recommended. Always check the edition of the standard in force.
What is an obligation and what is only a recommendation
The same three subjects, two standards, two different legal regimes.
Ever since P 118/1-2025 appeared (Ordinul MDLPA nr. 267 din 28 februarie 2025, published in Monitorul Oficial al României, Partea I, nr. 204 și 204 bis din 10 martie 2025), in every other discussion somebody asks for an arc fault detector “because that is what the fire standard says”. It does not. Art. 2.4.18.6 opens with the verb “se recomandă” (it is recommended), and letters a)–e) inherit the verb from that first sentence. The obligations for the electrical installation come from somewhere else — from I7-2011, chapter 7.11. The confusion costs money for nothing and, worse, it covers up what really is mandatory.
The verb that decides everything — Art. 2.4.18.6, first sentence
„La construcţiile pe care se amplasează sisteme/ dispozitive fotovoltaice, se recomandă îndeplinirea condiţiilor de siguranţă ale echipelor de intervenţie care să asigure:”
Every letter that follows, from a) to e), hangs on this sentence. The text is reproduced from the verified extract of the standard, diacritics and all.
The switch-disconnector on the DC side
“On the DC side of the PV inverter a switch-disconnector shall be provided” (I7-2011, Art. 7.11.23). Shall be provided — not is recommended. The obligation sits in the electrical installations standard, not in the fire safety one.
Letter d), verbatim:
„prevederea unui întreruptor de sarcină, uşor acţionabil;”
P 118 recommends “prevederea unui întreruptor de sarcină, uşor acţionabil” — the provision of a load-break switch, easily operated — so that the fire crew can get to it without hunting for it. The fire standard uses a term of its own, întreruptor de sarcină where I7 says separator de sarcină, and it does not say on which side the device goes — letter c) speaks of the main switch itself — so do not read it as a mere quality added to the DC switch-disconnector. What is clear: through I7 the device on the direct current side is mandatory, and everything P 118 adds here is a recommendation. You meet both with the same effort, if you put the switch-disconnector where it can be reached, not behind the inverter, 20 cm off the wall.
The label on the junction boxes
“All junction boxes (PV generator and PV groups) must have a warning label indicating that the live parts inside the boxes may remain live after the PV inverter has been isolated” (I7-2011, Art. 7.11.24; “aibe” is the form used in the published text). An obligation, with no shades to it — and it is a label that costs one leu.
Letter c), verbatim:
„marcarea şi etichetarea distinctă şi vizibilă a întregului sistem fotovoltaic; (întreruptorul general - care trebuie amplasat într-o zonă accesibilă a clădirii, toate conductele, cablurile, invertoarele, tablourile și cutiile circuitului de curent alternativ, precum şi bateriile și acumulatorii), cu materiale reflectorizante, rezistente la intemperii şi reciclabile;”
Letter c) asks for something else, wider: marking and labelling the whole system, with reflective, weather-resistant and recyclable materials. Useful and cheap, but a recommendation. The difference shows at handover — the warning label on the boxes can be demanded of you, the reflective marking of the whole system cannot.
The arc fault detector
There is no obligation. Chapter 7.11 of I7-2011 does not mention the electric arc in any of its 25 articles. The AFDD brought in by the Actualizări 2023 (pct. 4.1.5.8) is an alternating current device — the standard gives it a rated voltage of 230 V AC and a rated frequency of 50 Hz — intended for the final circuits of the fixed installation. It is not the device that sees an arc on a direct current string.
Letter e), verbatim:
„echiparea sistemului fotovoltaic cu dispozitiv de detectare a arcelor electrice.”
Letter e) recommends equipping the system with an arc fault detection device and stops there: P 118 names no product standard for it, neither IEC 63027 nor any other. So “complies with P 118” is not a checkable claim on the datasheet of a DC arc fault detector. If you fit one, you fit it because you judge it useful, and you choose it on the standard declared by the manufacturer.
The quotations from P 118/1-2025 are reproduced from the verified extract of the standard, not retyped. The status of each provision — obligation or recommendation — is read off the verb of the article, not assumed.
What to do with panels on a house that has a lightning protection system
Cable loops and the distance from the lightning protection system.
Keep the cable loops tight
“In order to minimise the voltages induced by lightning, the area of all cable loops must be as small as possible” (I7-2011, Art. 7.11.14). In practice: the plus and the minus conductor of the same string run together, not along separate detours.
The distance from the lightning protection system
If the generator falls inside the protection zone of the existing lightning protection installation, the separation distance from its components still has to be kept. The course gives a practical figure — at least 0.5 m has proved sufficient on the small systems fitted to houses — but the number that counts is the distance s calculated per Anexa 6.8, which grows with the length of the route and with the class chosen.
When the distance cannot be kept
If you cannot keep those 0.5 m, the PV generator and the lightning protection system are bonded to each other with a copper connection of at least 16 mm²; the mounting frame is bonded to the equipotential bonding of the house in the same way (design rule from the course). Checking the separation distance s against the standard is done on the lightning risk page.
Practise the sizing
Three levels, with data taken from the examples worked through in the course.
Choose the correct option
Design decisions, no calculation.
At which temperature you check the maximum voltage
You have a string of 21 modules on an inverter with a maximum DC voltage of 950 V. You want to be sure you do not destroy it.
Step 1: Which temperature do you use for the maximum voltage check?
A panel too “strong” for the inverter
The panel has Isc = 18 A and Impp = 17 A. The inverter accepts 11 A on the MPPT and 15 A short-circuit at the input.
Step 1: Is the configuration acceptable?
How much panel you put on a 3 kW inverter
A 3 kW rated inverter, 350 Wp panels.
Step 1: Between which limits do you keep the panel power relative to the inverter?
Why you do not rely on a fuse in a PV string
The panel in the example has Impp = 10.42 A and Isc = 11.15 A.
Step 1: Why can an ordinary fuse not disconnect a fault on a string?
Calculate step by step
You get hints if you get stuck.
The maximum number of modules in series
A panel with Uoc(STC) = 40.1 V and a voltage temperature coefficient of −0.27%/°C. Minimum design temperature: −15 °C. Inverter with a maximum DC voltage of 950 V.
Step 1: What is the Uoc of one module at −15 °C?
The sizing current for the string cable
The panel has Isc(STC) = 11.15 A.
Step 1: What current-carrying capacity must the string cable have, per Art. 7.11.11?
The losses along the direct current route
A string of 10 modules, working current 10 A, voltage 400 V. Route: 6 m of 4 mm² cable (the factory-fitted leads of the modules) + 20 m of 6 mm² cable up to the inverter. Copper, γ = 56.
Step 1: What is the total voltage drop, in volts?
The string fuse
A string with Isc = 10 A, on a 6 mm² cable that carries 30 A.
Step 1: What is the minimum threshold for the fuse, in amperes?
Solve it on your own
A single final answer.
A string on a 1100 V inverter
A panel with Uoc(STC) = 41.5 V and a coefficient of −0.28%/°C, minimum design temperature −20 °C, inverter with a maximum DC voltage of 1100 V. How many modules can you connect in series?
Choose the string fuse
String Isc = 12 A, 6 mm² cable with a current-carrying capacity of 31 A. Which DC fuse rating do you choose, from the 4-6-8-10-16-20-25 A series?
The panel power / inverter power ratio
You have 14 panels of 450 Wp on a 5 kW inverter. What is the ratio between the panel power and the inverter power?
Data sources
Where you get the figures when you need them
What you see above comes from two services the application queries directly. The rest are tools you open yourself — for radiation data, comparisons between sites, or equipment datasheets.
PVGIS — European Commission (JRC)
Production and irradiation for any location in Europe, with tilt and azimuth of your choosing. The application calls it directly in the section above; the web tool also gives hourly graphs and simulations for tracking systems.
Global Solar Atlas — World Bank
A worldwide map of radiation and photovoltaic potential, with downloadable reports per location. Good for an overview or for quick comparisons between areas.
NASA POWER
Weather and radiation series for any point on the globe, temperature and wind included. It has an open API; useful when you need data outside the European coverage.
Open-Meteo — weather archive
Global radiation and daily temperatures from the archive, with no access key. The application uses it in the autonomy calculator to replace the guideline profile with real data.
ENF Solar — catalogue of modules and inverters
A database of manufacturers’ datasheets: Uoc, Isc, temperature coefficients, MPPT windows. That is where you take the figures you put into the calculators above.
ANRE — regulations and grid connection
The energy regulator: connection conditions for prosumers, orders and technical approvals. The administrative side of a project, the one that most often delays commissioning.
We have no contract with any of these services; the ones marked INTEGRATED are queried through public, free requests, for non-commercial use. If a service does not respond, the calculators keep working on the values entered by hand.
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Where the figures come from
The sources for this page
The normative requirements quoted (Art. 7.11.x) come from the text of I7-2011, chapter 7.11 “Electrical installations for photovoltaic power supply systems”, in line with SR HD 60364-7-712. Since 2023 the standard applies explicitly to “photovoltaic installations belonging to buildings” as well (Actualizări 2023, pct. 1.1 lit. i). The rules marked “design rule” are not the letter of the standard: they are the practice taught in the accredited solar photovoltaic systems installer course — useful, but to be checked against the equipment datasheet and with the designer.
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