Engineering methodologyLast updated: March 2026

How Solar Stack calculates string compatibility

Solar Stack verifies that your solar panel string configuration is electrically safe and compatible with your inverter. The calculations follow IEC 62548, NEC 690.7, and the same physics professional design tools use.

What we check

Each calculation runs 11 validation checks. A failed check means the configuration is unsafe or non-functional. A warning means reduced performance but no damage.

FAILInverter max DC voltage

String open-circuit voltage at coldest temperature must not exceed the inverter's absolute hardware limit. Exceeding this destroys the inverter.

FAILModule insulation rating

String voltage must stay below the panel's rated system voltage (1000V or 1500V class). Exceeding this can cause insulation breakdown and arcing.

WARNINGMPPT upper range

Cold morning Voc above MPPT max delays startup until panels warm up. Not dangerous, but wastes morning energy.

FAILMPPT lower bound

In hot weather, panel voltage drops. If the string falls below MPPT minimum, the inverter can't track power and shuts down.

WARNINGMPPT cold operating voltage

Cold weather Vmpp above MPPT max forces the inverter away from the optimal power point. Reduced efficiency, no damage.

WARNINGInput current limit

Total current from parallel strings above the inverter limit causes power clipping. The inverter limits current safely, but you lose energy. On inverters with unequal MPPT inputs, each tracker is checked against its own limit.

FAILShort-circuit current

Short-circuit current flows even when the inverter is off. Exceeding the rated Isc damages protection circuits and creates fire risk. On inverters with unequal MPPT inputs, each tracker is checked against its own limit. A second, stricter test follows the installation standard (IEC 60364-7-712): the input should withstand 1.25 × the panels' Isc at STC, for sun brighter than 1000 W/m². Without that margin the check warns instead of passing.

WARNINGDC/AC ratio

Compares total DC panel power to inverter AC output. Optimal range is 1.0–1.3. Above 1.5, the inverter clips significant energy during peak hours — you lose what your panels produce.

WARNINGString inputs

Each MPPT tracker has a rated number of connectors for parallel strings. Planning more strings than that still builds, but they must be joined with Y-connectors or a combiner box, and each then needs its own fuse. When the inverter's own manual allows two strings on one input (a 36 A tracker with a single connector pair, for example), the check passes and only names the Y-connector.

WARNINGMax PV input power

The inverter's own rating for how much panel power it accepts. The DC/AC ratio uses one threshold per equipment type and cannot see a unit whose datasheet allows less — a hybrid rated 1.6× its AC sits well inside a 2.0 threshold. Where the datasheet also prints a larger “max PV access power” — how much may be connected, not converted — the check uses that figure.

WARNINGString fuses

Strings joined onto one MPPT input feed each other, so a fault in one is fed by all the others. Past the panel's own maximum series fuse rating, every parallel string needs its own fuse.

Temperature model

Temperature has the strongest effect on string voltage. Cold weather increases voltage (safety risk), hot weather decreases it (performance risk). Our model correctly distinguishes between ambient temperature and cell temperature.

Cold checks: ambient temperature

For maximum voltage calculations, we use ambient air temperature directly. On a cold clear morning, panels are at ambient temperature when sunlight first hits them — this is the worst case for overvoltage, before cells heat up.

T_cell_cold = T_ambient_min (panels not yet heated)

Hot checks: cell temperature

For minimum voltage calculations, we need cell temperature — which is significantly higher than ambient air. We support two methods, automatically choosing the more accurate one.

T_cell_hot = T_ambient_max + (NOCT − 20) × 1.25

Method 1: NOCT formula (preferred)

When the panel's NOCT (Nominal Operating Cell Temperature) value is available from the datasheet, we use the IEC 61215 formula. This is the standard method used by professional design tools. For a typical NOCT of 45°C, the cell temperature offset is 31.25°C above ambient.

T_cell = T_ambient + (NOCT − 20) / 800 × 1000

Method 2: Mounting offset (fallback)

When NOCT is not available, we use a simplified offset based on mounting type.

Mounting type	Cell temp offset
Ground / open rack	+25°C
Roof rack (>15 cm gap)	+30°C
Flush roof mount	+35°C

Advanced corrections

When the datasheet provides additional parameters, we apply corrections for engineering accuracy.

Bifacial current gain

Bifacial panels receive additional irradiance from ground reflection (albedo). This increases short-circuit current, which matters for overcurrent checks. The gain depends on the ground surface: grass ≈ 20%, sand ≈ 30%, snow ≈ 60%. A view factor of 0.7 is applied to account for real-world conditions (non-uniform illumination, structural shading, mounting height). This is the industry standard referenced in AS/NZS 5033:2021.

Isc_effective = Isc_hot × (1 + bifaciality × albedo × 0.7)

Microinverter mode (per-input checks)

When the selected inverter is a microinverter, we switch to per-input verification: each input carries the panels the unit wires onto it — one on almost every model, two in series on a few — so the voltage checks use that group while the current checks use a single module, because panels in series share one current. We verify Voc(cold) vs the input hardware limit, Vmpp(hot) vs MPPT lower bound, Isc(hot) vs the per-input short-circuit rating, and Impp vs the per-input current rating. The DC/AC ratio compares panel Pmax × inputs-per-inverter against the microinverter's rated AC output — microinverter count cancels out because each unit is sized the same way. When the datasheet gives a DC input rating, the ratio only warns past that rating — the manufacturer sizes it for the unit's own AC.

Multiple identical inverters

A system with several identical inverters (say, 78 panels on two 15 kW units) is checked one inverter at a time: an inverter only sees the chains wired into its own MPPT inputs, so a neighbouring inverter never changes the result. The “Several identical inverters” toggle multiplies the whole-system totals — panels, DC and AC power — by the count, while every safety check stays per-inverter. If your arrays differ, run a separate calculation per inverter with its actual chains.

Inverters with power optimizers (SolarEdge)

An inverter that runs only with power optimizers holds its string at a fixed DC voltage — 380 V single-phase, 750 V three-phase — whatever the panel count, so the string-voltage and current checks above describe a circuit that does not exist. For this type we check the optimizer datasheet's design rules instead: the number of optimizers per string against the minimum and maximum for the inverter class, and the string's nominal power against the per-string limit (an inverter rated at or below that limit may carry one string up to its own maximum DC power). Name your optimizer and we also check the single panel against it: its cold-weather Voc against the optimizer's maximum input voltage, its hot short-circuit current against the module current the optimizer accepts, and its rated power against the optimizer's input rating. Temperatures and power output are modelled exactly as for a string inverter.

The count and power-per-string rules are fail-level: below the minimum the inverter never reaches its operating voltage, and above the limits the design is outside the manufacturer's rules. The panel-power rule is a warning — the optimizer limits its own output, so the cost is clipped watts, not damage.

Orientation-aware current analysis

When parallel strings on the same MPPT face different directions (e.g., east and west on a dual-slope roof), they can never produce maximum current simultaneously. Solar Stack calculates the realistic worst-case combined current using solar geometry.

How it works

For a given installation location, we compute the sun's position every 15 minutes on the 21st of every month — both solstices and both equinoxes among them, because a steep or vertical face (a balcony, a wall) is lit best by a lower sun. For each moment, we calculate the irradiance hitting each string based on its azimuth and tilt, then sum the currents of the strings on one input. The highest combined current of the year is the realistic worst case. In simple mode the calculator does not know which strings you will put together, so it sums the strongest strings at every moment — the worst pairing, never the kindest.

I_combined(t) = Σ Isc_hot × (POA_irradiance_string_i(t) / 1000)

Safety and standards

Protection equipment (cables, fuses) must always be sized using worst-case current — all strings at full Isc simultaneously, per IEC 62548 and NEC 690 — so the short-circuit check never uses the orientation-aware value. The input-current check is different: above that rating the inverter only holds the extra current back, which costs energy, not hardware. When the worst case exceeds the input rating but the realistic peak stays within it, that check passes and says why; our simulation of such layouts over five years of hourly weather found no measurable energy loss.

Example: East–West strings at 50°N

Two strings at 20° tilt, one facing East (90°), one facing West (270°). The sun can never be perpendicular to both at the same time. Around noon, both get moderate irradiance — this is when the combined current peaks. At 50°N the realistic maximum is about 87% of the naive sum at this tilt and about 81% on a 30° roof: both faces still see most of the noon sun.

Wiring topologies

Solar panels can be wired to an inverter in three different ways. Solar Stack auto-detects the topology from inverter specs and shows contextual guidance in the calculator.

Series (String)

All panels wired in series form a single string. Voltage adds up, current stays the same. This is the standard topology for grid-tied inverters with moderate MPPT input current (10–30A). Example: 15 panels × 40V = 600V string.

Series-Parallel (nSnP)

Multiple identical strings wired in parallel before the inverter input. Voltage is the same as one string, but current multiplies. Common on low-voltage hybrid inverters (48V/96V battery systems) that accept 50–100A per MPPT. Written as nSnP — for example, 3S3P means 3 panels in series × 3 strings in parallel = 9 panels total. Requires MC4 branch connectors or a combiner box.

Multi-MPPT

The inverter has multiple independent MPPT trackers, each handling separate string groups. Each tracker is checked independently. This allows mixing different orientations or panel types. Solar Stack's advanced mode configures each tracker separately.

How topology is detected

The calculator determines topology automatically: if the inverter has more than one MPPT tracker, it's multi-MPPT. If it has a single MPPT with high current capacity (≥45A) or supports 3+ strings per input, it's series-parallel. Otherwise, it's a simple series topology. No user input needed — the banner appears automatically when relevant.

Multi-MPPT mode

Modern inverters often have multiple MPPT trackers, each handling independent string groups. Solar Stack supports per-tracker configuration for precise compatibility analysis.

Per-tracker checks

In multi-MPPT mode, you configure each tracker separately — number of panels per string, strings per tracker, and optional orientation data. Each tracker runs the full set of checks independently, because each MPPT operates as an electrically separate input.

Result aggregation

The overall system result takes the worst status across all trackers. If tracker 1 passes all checks but tracker 2 has a warning, the system result is "warning". Power analysis sums across all trackers for the total system output.

Power output estimation

Solar Stack estimates DC power output at operating conditions. The rated STC power drops in hot weather due to the temperature coefficient of Pmax (typically −0.30 to −0.40%/°C). When per-string orientations are provided, realistic peak power accounts for the fact that differently-oriented strings cannot all produce maximum power simultaneously.

P_dc = N_modules × N_strings × Pmax × (1 + TC_Pmax/100 × (T_cell − 25)) × (G_poa / 1000)

The orientation-adjusted power uses the irradiance ratio at the peak combined moment — the same time step used for current analysis. This is a conservative estimate for system sizing.

Best-case peak (cold + bifacial uplift)

On cold sunny days panels actually exceed their STC rating: Pmax rises as cell temperature drops below 25°C. Solar Stack projects this peak by mirroring the NOCT formula used for hot-temp derate, but evaluated at the user's coldest design ambient under full sun. Bifacial panels add a rear-side albedo gain on top.

T_cell_peak = T_min + (NOCT − 20) × 1.25;   P_peak = P_stc × (1 + TC_Pmax/100 × (T_cell_peak − 25)) × (1 + bifaciality × albedo × 0.7)

In warm climates the cell temperature stays above 25°C even on the coldest day, so the temperature uplift collapses and only the bifacial gain remains. The displayed +X% combines both effects.

Core formulas

All calculations use STC (25°C) datasheet values adjusted by temperature coefficients. TC_Voc is used for both Voc and Vmpp corrections — standard practice since TC_Vmpp is rarely on datasheets.

Open-circuit voltage at cold

Voc_cold = Voc_stc × (1 + TC_Voc/100 × (T_cold − 25))

Operating voltage at hot (NOCT cell temp)

Vmpp_hot = Vmpp_stc × (1 + TC_Voc/100 × (T_cell_hot − 25))

Short-circuit current at hot (with bifacial gain)

Isc_hot = Isc_stc × (1 + TC_Isc/100 × (T_cell_hot − 25)) × (1 + bif × albedo × 0.7)

Design short-circuit current (IEC 60364-7-712)

I_design = 1.25 × Isc_stc × N_strings × (1 + bif × albedo × 0.7)

String voltage (series)

String_Voc = N_modules × Voc_cold

Current on the busiest MPPT (worst-case)

Isc_per_MPPT = ceil(N_strings / N_MPPT) × Isc_hot

Total current per MPPT (orientation-aware)

Total_Isc_realistic = Σ (Isc_hot × POA_ratio_i) where POA_ratio = G_poa / 1000

DC/AC ratio

DC_AC_ratio = (N_modules × N_strings × Pmax_stc) / Inverter_AC_power

Worked example

A bifacial LONGi Hi-MO 9 system with NOCT data available.

Configuration

Panel: LONGi 660W — Voc = 49.92V, Vmpp = 41.38V, Isc = 18.35A, TC_Voc = −0.20%/°C, TC_Isc = +0.048%/°C, NOCT = 45°C, bifaciality = 75%, system voltage 1500V

Inverter: Huawei SUN2000-100KTL — Max DC = 1100V, MPPT = 200–1000V, max input current = 30A/MPPT, max Isc = 40A/MPPT

Configuration: 16 panels per string, 2 strings on 1 MPPT tracker

Site: Ukraine, T_min = −20°C, T_max = 35°C, ground mount, grass albedo (0.2)

Cell temp hot = 35 + (45−20) × 1.25 = 66.25°C (NOCT formula)Voc_cold = 49.92 × (1 + (−0.20/100) × (−20−25)) = 49.92 × 1.09 = 54.41V → max 20 panelsVmpp_hot = 41.38 × (1 + (−0.20/100) × (66.25−25)) = 41.38 × 0.917 = 37.95V → min 6 panelsVmpp_cold = 41.38 × 1.09 = 45.10V → String Vmpp_cold = 16 × 45.10 = 721.6VIsc_hot = 18.35 × 1.020 × 1.105 = 20.67A (temperature + bifacial gain)Total Isc = 2 strings × 20.67A = 41.34A

All 7 check results

✓ Max DC voltage: 870.6V ≤ 1100V — safe, 21% margin

✓ Module insulation: 870.6V ≤ 1500V — safe

✓ MPPT upper: 870.6V ≤ 1000V — within range

✓ MPPT lower: 607.5V ≥ 200V — within range

✓ MPPT cold Vmpp: 721.6V ≤ 1000V — within range

⚠ Input current: 41.34A > 30A — clipping, energy loss

✗ Short-circuit current: 41.34A > 40A — unsafe, reduce strings or use separate MPPTs

Result: Incompatible — voltages are safe, but 2 parallel strings exceed current limits. Solution: connect each string to a separate MPPT tracker.

What this calculator does not cover

Solar Stack focuses on electrical string compatibility. The following factors are outside the current scope:

Shading analysis — partial shading reduces string output unevenly. Use site-specific tools like Google Project Sunroof for shading studies.
Cable voltage drop — DC cables lose 1–3% of voltage depending on length and cross-section. For long cable runs (>30m), verify voltage at the inverter input.
AC-side compatibility — grid voltage, transformer capacity, and export limits are not checked. Consult your local grid operator.
Battery storage — hybrid inverter battery compatibility, charge/discharge rates, and DoD are not analyzed.
Soiling and degradation — dust, bird droppings, and age-related degradation (0.4–0.5%/year) reduce output over time but are not modeled.
Economic analysis — ROI, payback period, feed-in tariffs, and electricity price projections are not calculated.

Common mistakes in online calculators

Not applying temperature correction at all — using STC voltage directly for string sizing.
Using ambient temperature for hot voltage checks instead of cell temperature. This underestimates voltage drop by 30–40%.
Confusing the inverter's absolute max voltage with the MPPT tracking range upper limit. These are different constraints.
Ignoring module insulation class (1000V vs 1500V) as a separate voltage constraint.
Not accounting for NOCT or mounting type. Actual cell temperature during operation can be 25–35°C above ambient.
Not accounting for bifacial current gain. On reflective surfaces (snow, sand), bifacial panels produce significantly more current than the STC rating.
Assuming all parallel strings produce full current simultaneously — or the opposite, that east and west strings never add up. On east–west roofs the realistic peak current is only 10–25% below the naive sum, depending on pitch and latitude.
Ignoring production tolerance of panels (typically ±3% for Voc). A panel rated at 49.92V may actually produce 51.42V — this 3% can push a borderline string over the voltage limit.

Standards and references

Our methodology aligns with international PV design standards:

IEC 62548 — Photovoltaic array design requirements (voltage correction factors)
NEC 690.7 — Maximum PV system voltage accounting for temperature
IEC 61730 — Module safety and maximum system voltage ratings
AS/NZS 5033 — PV array installation (current safety factors)
IEC 61215 — PV module design qualification and type approval (source of NOCT testing methodology)
EN 50530 — Overall efficiency of PV inverters (MPPT tracking efficiency testing)
VDE 0100-712 — Erection of low-voltage installations: Solar PV power supply systems
DIN EN 62548 (VDE 0126-14) — PV array design requirements (German adoption of IEC 62548)
EEG 2023 §9 — German Renewable Energy Sources Act (feed-in limitation for systems ≤25 kWp)

German standards (VDE) →

German standards (VDE)

For installations in Germany, national VDE standards apply in addition to international IEC norms. Solar Stack covers all relevant DC-side checks required by German regulations.

VDE 0100-712

Installation of low-voltage systems: Solar PV supply systems. Governs maximum string voltage and overcurrent protection. Solar Stack verifies this via the 'Max DC voltage' and 'Short-circuit current' checks.

DIN EN 62548 (VDE 0126-14)

PV array design requirements. Defines temperature correction factors and production tolerances (+3% Voc, +5% Isc). Solar Stack applies these when 'Production tolerance (IEC 62548)' is enabled.

VDE-AR-N 4105

Generators connected to the low-voltage grid. Defines grid connection requirements up to 135 kW. AC-side compatibility is outside Solar Stack's scope, but DC string sizing follows the same design principles.

Solar Stack implements the core checks from VDE 0100-712 and DIN EN 62548: worst-case voltage at minimum temperature, MPPT range verification, short-circuit current with production tolerance, and NOCT-based cell temperature calculation. The short-circuit check also warns when the input lacks the 1.25 × Isc margin of VDE 0100-712 Annex B.

Worked example: System in Munich

Module: LONGi Hi-MO X6 580W — Voc = 51.90V, Vmpp = 43.50V, TC_Voc = −0.26%/°C, NOCT = 43°C

Location: Munich, T_min = −16°C, T_max = 32°C, roof rack mount, production tolerance active (+3% Voc)

Voc with tolerance = 51.90 × 1.03 = 53.46VVoc_cold = 53.46 × (1 + (−0.26/100) × (−16 − 25)) = 53.46 × 1.107 = 59.18VCell temp hot = 32 + (43 − 20) × 1.25 = 60.75°C (NOCT formula)Vmpp_hot = 43.50 × 1.03 × (1 + (−0.26/100) × (60.75 − 25)) = 44.81 × 0.907 = 40.64V

With production tolerance, the maximum string length drops from 20 to 18 modules — this 3% margin can make the critical difference between a safe and an unsafe design.

Try the calculator Find compatible panels Find a matching inverter

Already know your inverter? Find all compatible solar panels automatically.

Mounting type

Cell temp offset

Ground / open rack

+25°C

Roof rack (>15 cm gap)

+30°C

Flush roof mount

+35°C