PV String Sizing & 1500 V System Voltage Design

Short answer: string length is set by the coldest expected morning, not by the sunniest afternoon. Open-circuit voltage rises as temperature falls, so the maximum number of modules per string is the system voltage limit (1500 V DC for most utility and C&I equipment, 1000 V for many residential inverters) divided by the module's temperature-corrected Voc at the site's record low temperature. The minimum number of modules is then set by keeping the array's maximum-power voltage above the inverter's MPPT window on the hottest days. Everything between those two bounds is optimisation.

Getting this wrong is one of the few PV design errors that can destroy equipment on day one: a string that exceeds the inverter's maximum DC input voltage on a cold, bright morning can damage the inverter and voids both inverter and module warranties.

Why the cold morning sets the maximum string length

Silicon PV modules have a negative temperature coefficient of Voc: as cell temperature drops below the 25 °C standard test condition, open-circuit voltage climbs. The worst case is therefore a clear, freezing morning at first light — full irradiance on cold glass, with the array open-circuit or the inverter not yet started.

The correction is a single line of arithmetic:

Voc(T) = Voc(STC) × [1 + (β / 100) × (T − 25)]

where β is the module's temperature coefficient of Voc in %/°C (a negative number) and T is the design minimum cell temperature in °C. At first light, cell temperature is effectively ambient, so the site's record low ambient temperature is the standard design input.

Then:

Maximum modules per string = floor( system voltage limit ÷ Voc(T) )

Always round down. And use the highest power bin you might actually receive, because Voc rises slightly across a series' power bins — designing on the lowest bin and receiving the highest is a classic way to overshoot the limit.

Worked example — preliminary voltage screening only

Take the CSTB72 580–605W TOPCon module. Its linked source datasheet lists Voc = 53.10 V at the 605 W bin and temperature coefficient of Voc = −0.25 %/°C. For an illustrative design minimum of −10 °C and a 1500 V voltage ceiling:

• Temperature delta: −10 − 25 = −35 °C
• Correction factor: 1 + (−0.25 / 100 × −35) = 1.0875
• Voc(−10 °C) = 53.10 × 1.0875 = 57.75 V
• Arithmetic ceiling: 1500 ÷ 57.75 = 25.97 → 25 modules before additional design checks

This is not an approved string length. A qualified designer must apply the relevant design method, tolerances and safety factors, use the lowest voltage rating among all DC components, and check hot-condition operation. For a different format, use the separate CSHJ132 700–730W HJT electrical table; its voltage and current are not interchangeable with CSTB72.

At a colder −25 °C design temperature the same module reaches 59.74 V, and the limit is still 25 modules (1500 ÷ 59.74 = 25.11). That is typical: string limits move in whole-module steps, so a change in design temperature sometimes costs a module and sometimes costs nothing — which is why the calculation must be redone for each site rather than carried over from the last project.

Worst-case Voc across module formats

The table below applies the same method to several COTECH on-grid formats at their top power bin, using the Voc and Voc temperature coefficient printed on each datasheet, a −10 °C design minimum and a 1500 V DC system limit (the maximum system voltage stated for these series).

Two observations worth carrying into a design review. First, higher wattage does not mean fewer modules per string — the 210 mm formats carry more current at lower voltage, so they string longer than the 182 mm 144-cell formats. Second, HJT's flatter Voc coefficient partly offsets its higher Voc, which is why it is worth recomputing rather than assuming.

On a residential system with a 1000 V inverter, the same 108-half-cell module (43.83 V at −10 °C) allows 1000 ÷ 43.83 = 22.8 → 22 modules per string. Recheck this whenever the inverter changes; the limit belongs to the inverter, not to the module.

The hot-day check: staying inside the MPPT window

The maximum is a safety limit. The minimum is a performance limit. On a hot afternoon the array's maximum-power voltage falls, and if it drops below the inverter's minimum MPPT voltage the inverter derates or stops tracking properly.

Check the inverter's full MPPT operating window and start-up voltage, together with model-specific module behaviour across the design temperature and irradiance range. CSTB72-605 has Vmp = 44.60 V at STC, but its listed Voc temperature coefficient is not a documented Vmp coefficient. Do not substitute one for the other to approve a hot-day string length. Obtain manufacturer Vmp-versus-temperature data or a validated module model. PVsyst's model-parameter documentation describes the additional parameters used to model operating behaviour.

The CSTB72 and CSHJ132 model pages flag inconsistencies in their supplied NMOT electrical tables. Obtain corrected technical sheets before relying on those table values in a design or yield model.

To estimate cell temperature, use the module's NMOT figure — nominal module operating temperature, measured at 800 W/m², 20 °C ambient and 1 m/s wind. COTECH on-grid series publish NMOT of 43 ± 2 °C, and the datasheets state an operating range of −40 °C to +85 °C. In a hot climate with restricted rear ventilation, a design cell temperature of 65–75 °C is a reasonable worst case; a well-ventilated elevated ground mount runs cooler than a flush rooftop.

Design tip: keep the low end of the window comfortable rather than marginal. A string that only just clears the MPPT minimum will spend hot afternoons — the highest-value hours in many tariffs — tracking badly.

Current, fuses and conductors

Voltage sets string length; current sets everything downstream.

• Short-circuit current (Isc) rises slightly with temperature — typically around +0.04 to +0.05 %/°C (COTECH on-grid series publish +0.044 to +0.045 %/°C). The increase is small but must be applied on top of the code factor.
• Code sizing factors. In IEC-based practice the maximum circuit current is derived from Isc with a safety multiplier, and in NEC jurisdictions the maximum circuit current is 125% of Isc, with the continuous-duty factor applied again when sizing conductors and overcurrent devices. Use the rule for the jurisdiction, not the more convenient one.
• Maximum series fuse rating is printed on the module datasheet and caps what the string can be protected with. COTECH publishes 25 A for the 108- and 120-half-cell TOPCon formats and for the back-contact series, and 30 A for the 144-cell, rectangular-wafer and 210 mm TOPCon and HJT formats — always check the specific model rather than the family.
• Inverter and combiner input limits. Each MPPT input has a maximum current and, on many inverters, a maximum short-circuit current. High-current 210 mm modules can hit an input limit before they hit a voltage limit, which changes the strings-per-MPPT layout.

Bifacial modules require a separate electrical check

Rear-side irradiance adds effective irradiance and makes current headroom particularly important. It is not a reason to waive the maximum-voltage check. Apply the module manufacturer's bifacial design method and applicable requirements to both voltage and current. Use a bifacial-adjusted Isc for fuses, conductors, combiner boxes and inverter inputs, based on the installation rather than the bifaciality factor alone. The mounting factors that drive rear gain are covered in bifacial solar modules explained.

1000 V versus 1500 V systems

Moving from 1000 V to 1500 V DC does not change the modules; it changes how many of them share a set of cables and a fuse. Longer strings mean:

• Fewer strings for the same capacity, so fewer combiner inputs, fewer fuses and less DC labour.
• Lower current per kW transmitted, so lower resistive loss for the same conductor, or thinner conductors for the same loss.
• Higher clearance, insulation and arc-flash requirements, and equipment that is rated for 1500 V throughout — modules, connectors, cable, combiners, switchgear and inverter.

Both COTECH on-grid module families are rated 1500 V DC (IEC/UL) maximum system voltage, so the constraint in practice is usually the inverter and the balance-of-system components. Never mix a 1500 V string design with a component rated to 1000 V anywhere in the DC path.

DC/AC ratio, clipping and layout

Once string length is settled, the remaining decisions are about how much DC capacity to put behind each inverter and how to lay the strings out.

• DC/AC ratio. Oversizing the array relative to inverter AC capacity raises annual yield and improves the shape of the generation curve, at the cost of clipping the peak. Commonly specified ratios fall in the 1.1 to 1.3 range, higher where energy prices are flat and lower where a peak-price window rewards the midday maximum.
• Keep strings electrically identical. Same model, same power bin, same orientation, same tilt and the same number of modules per MPPT input. Mixed strings on one MPPT input cost energy every day, and the loss is invisible without module-level monitoring.
• Split by shading, not by convenience. Rows that shade differently at different times of day belong on different MPPT inputs.
• Match the structure. Tracker rows have a maximum module count per row — COTECH single-axis trackers are specified for rows of up to 90 modules across a ±60° range — so string length and row length should be chosen together to avoid stranded positions. Structure options are on the Mounting System page.
• Cable runs and voltage drop. Aim for a low DC-side voltage drop at maximum current; long home runs on high-current 210 mm strings are where DC losses accumulate.

Commissioning checks worth writing into the specification

• Measure string open-circuit voltage before connecting to the inverter, at a recorded ambient temperature, and compare against the temperature-corrected prediction. A string reading materially low usually means a reversed or missing module; a high reading means an extra module.
• Measure string current under stable irradiance and compare strings against each other. Outliers point to connector faults, shading or a damaged module.
• Record module serial numbers per string. Without a string map, a later performance-warranty claim becomes an argument.
• Confirm connector compatibility. Mating different connector brands is a recognised cause of hot-spot failures at the connector, and is usually excluded from warranty cover.
• Log torque values and clamp positions from the mounting instructions — the mechanical side of the same document set that keeps the module warranty valid.

Common mistakes

• Sizing on the average winter temperature instead of the record low.
• Using the lowest power bin's Voc and receiving a higher bin.
• Designing a 1500 V string and installing a 1000 V-rated connector, cable or switch somewhere in the path.
• Ignoring the bifacial current adder on fuses and combiners.
• Mixing module models or orientations on the same MPPT input.
• Forgetting that the inverter's maximum DC input voltage — not the module's 1500 V rating — is often the binding limit.

Full electrical tables (Voc, Isc, Vmp, Imp per power bin, temperature coefficients, maximum system voltage and fuse ratings) for every series are published in the module datasheets, and the platform overview is on the Solar Modules page. If you want a string layout checked against a specific inverter and site temperature range, send the project details.

FAQ

How do you calculate the maximum number of solar modules in a string?

For preliminary screening, divide the lowest applicable DC voltage limit by the module's temperature-corrected Voc and round down. Use Voc(T) = Voc(STC) × [1 + (β/100) × (T − 25)], with β in %/°C. The CSTB72 example gives an arithmetic ceiling of 25 modules at −10°C and 1500 V before additional design factors. A qualified designer must confirm the final string length against the applicable requirements, tolerances, all component ratings and hot-condition MPPT operation.

Why does cold weather limit string length instead of hot weather?

Open-circuit voltage rises as cell temperature falls, so the highest voltage the array will ever produce occurs on a cold, bright morning — typically at first light, before the inverter starts. That peak must stay below the maximum DC input voltage of the inverter and below the module's maximum system voltage rating. Hot weather creates the opposite problem: it lowers the maximum-power voltage, which sets the minimum string length needed to stay inside the inverter's MPPT window.

What design temperature should be used for string sizing?

Use the site's record or statistically extreme minimum ambient temperature, because at first light the cell temperature is effectively ambient. Local codes and standards often specify which extreme-low dataset to use, and project specifications sometimes add margin on top. For the hot-day MPPT check, estimate cell temperature from the module's NMOT rating plus the site conditions; 65 to 75 °C is a common worst-case assumption in hot climates with limited rear ventilation.

Do bifacial modules change string sizing?

Bifacial contribution requires a model-specific electrical check. Current headroom in fuses, conductors, combiner boxes and inverter inputs is particularly important, but the maximum-voltage check must not be skipped. Use the manufacturer's bifacial design method and the project's rear-irradiance assumptions, not the bifaciality percentage as a current or energy multiplier.

What is the advantage of a 1500 V system over 1000 V?

At 1500 V DC each string can contain roughly half again as many modules, so a plant of the same capacity needs fewer strings, fewer combiner inputs and fewer fuses, and carries less current per kilowatt — which reduces resistive loss or allows thinner conductors. The trade-off is that every component in the DC path must be rated for 1500 V, and clearance, insulation and arc-flash requirements are stricter. Mixing a 1000 V-rated component into a 1500 V design is a serious safety fault.

What happens if a string exceeds the inverter's maximum DC voltage?

The inverter can be damaged, and both the inverter and module warranties typically exclude damage caused by exceeding rated input limits. Because the peak occurs on cold, clear mornings, an over-length string can pass a mild-weather commissioning test and fail months later in the first hard frost. This is why the calculation should use record-low temperatures and the highest power bin that could be delivered, and why measured string open-circuit voltages should be recorded at commissioning and compared against the prediction.