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newSchoolWorks.com › VoTechWorks › Building & Construction › Solar / PV Installation › Inverters for PV Systems › Lesson 2
BLD-SOLA09-05 · Lesson 2 · 14 clock hours · demo

Inverter Specifications and Array Matching

Objective. Students will calculate whether a proposed string configuration falls within an inverter's acceptable input voltage and current window and revise string length when it does not.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Inverter Matching Matters

An inverter converts the DC power produced by your PV array into the AC power that your building or the grid can use. Every inverter has a defined input window — a range of acceptable DC voltage and current. If the array sends voltage or current outside that window, the inverter will either shut down for protection, operate inefficiently, or sustain permanent damage. String design is the process of choosing how many modules to connect in series (a string) and how many strings to connect in parallel so that the array always stays inside that window under all expected conditions. Getting this right is one of the most important technical skills in PV installation.

Key Module Datasheet Values

Module datasheets report performance at Standard Test Conditions (STC): 1000 W/m² irradiance, 25 °C cell temperature, and AM 1.5 spectrum. The four values you will use most are:

In a series string, voltages add and current stays the same. In parallel strings, currents add and voltage stays the same.

Temperature Correction — The Critical Step

Silicon modules are sensitive to temperature. As temperature rises, voltage drops. As temperature falls, voltage rises. The datasheet lists a temperature coefficient of Voc (expressed in %/°C or mV/°C) and a temperature coefficient of Vmp.

Cold temperature check (maximum voltage): Use the lowest expected ambient temperature for the site. The NEC and most inverter manufacturers require you to calculate the worst-case high voltage — which occurs on the coldest day. Add the temperature difference below 25 °C to the STC Voc, scaled by the coefficient.

> Corrected Voc = Voc(STC) × [1 + (coefficient_%/100) × (T_min − 25)]

Because T_min is below 25 °C, the factor (T_min − 25) is negative, and because the Voc coefficient is itself negative, the product is positive — voltage goes up.

Hot temperature check (minimum voltage / MPPT floor): High cell temperatures lower Vmp significantly. If Vmp at high temperature drops below the inverter's minimum MPPT voltage, the inverter cannot extract maximum power or may disconnect. Cell temperature at peak irradiance can run 25–30 °C above ambient; use the module's NOCT (Nominal Operating Cell Temperature) rating and your local design temperature to estimate this.

Inverter Input Specifications

Every string inverter datasheet lists at minimum:

Your corrected string Voc must stay below the inverter maximum DC input voltage. Your corrected string Vmp must stay within the MPPT range under both hot and cold conditions. String Isc must stay at or below the inverter's current limit for that input.

Calculating String Length

To find how many modules fit in one string:

1. Divide the inverter's maximum DC input voltage by the temperature-corrected module Voc → this gives the maximum number of modules per string.

2. Divide the inverter's minimum MPPT voltage by the temperature-corrected module Vmp at high temperature → this gives the minimum number of modules per string.

3. Choose a string length that falls between those two limits.

Always round down for the maximum (to avoid overvoltage) and up for the minimum (to ensure the MPPT range is reached).

Parallel Strings and Current Limits

Once string length is set, check whether the planned number of parallel strings keeps combined Isc within the inverter's current limit. Multiply single-string Isc by the number of parallel strings and compare to the inverter specification.

Safety Notes

PV systems are energized by sunlight and cannot be switched off at the source by disconnecting the inverter alone. Lockout/tagout (LOTO) procedures must be followed on both the AC and DC sides before working on wiring. Even a string sized within spec can produce lethal DC voltage — arc flash and electric shock hazards are present any time conductors are exposed. Wear appropriate PPE (arc-rated gloves and face shield when required by your site's arc flash analysis). Never assume a system is de-energized based on inverter display alone; use a properly rated DMM to verify zero voltage before touching conductors.

Revising a Flawed Design

When reviewing an existing single-line diagram, check the following in order:

1. Temperature-corrected string Voc vs. inverter maximum DC voltage — most common violation.

2. Temperature-corrected string Vmp (hot) vs. inverter minimum MPPT voltage.

3. String Isc × number of parallel strings vs. inverter input current limit.

If a violation exists, adjust string length or the number of parallel strings, recalculate, and document every step with the datasheet values and temperature correction math you used. A written justification protects both the installer and the system owner.

Key terms10

Voc (Open-Circuit Voltage)The maximum voltage a PV module or string produces when no load is connected; used to check against the inverter's maximum DC input voltage limit.
Vmp (Voltage at Maximum Power)The module or string voltage at the point of greatest power output; must fall within the inverter's MPPT voltage range during normal operation.
Isc (Short-Circuit Current)The maximum current a PV module produces under a direct short at STC; used to verify the inverter's input current rating is not exceeded.
Imp (Current at Maximum Power)The module or string current at the point of greatest power output; the operating current during peak production.
Temperature Coefficient of VocA datasheet value, expressed as %/°C or mV/°C, that describes how much Voc changes per degree Celsius; negative for silicon modules, meaning colder temperatures raise voltage.
MPPT (Maximum Power Point Tracking) RangeThe inverter's DC input voltage window within which its tracking algorithm can extract maximum power from the array; string Vmp must remain inside this range under real operating temperatures.
Standard Test Conditions (STC)The reference conditions (1000 W/m² irradiance, 25 °C cell temperature, AM 1.5 spectrum) at which module datasheet ratings are measured.
String LengthThe number of PV modules connected in series in a single string; determines string voltage and must be chosen so corrected Voc and Vmp stay within inverter limits.
Maximum DC Input VoltageThe absolute upper voltage limit of the inverter's DC input; exceeding it can permanently damage the inverter and creates a serious electric shock hazard.
NOCT (Nominal Operating Cell Temperature)A datasheet rating that estimates cell temperature under defined irradiance and ambient conditions; used to estimate operating cell temperature for the hot-temperature voltage correction.

Worked examples4

1. A module has Voc = 45.2 V and a Voc temperature coefficient of −0.30%/°C at STC (25 °C). The site's record low ambient temperature is −10 °C. The inverter's maximum DC input voltage is 600 V. What is the maximum number of modules allowed in one string?
  1. Step 1 — Find the temperature-corrected Voc at −10 °C. Temperature difference = −10 − 25 = −35 °C. Correction factor = 1 + (−0.30/100) × (−35) = 1 + 0.105 = 1.105.
  2. Step 2 — Corrected Voc = 45.2 V × 1.105 = 49.95 V (round to 49.9 V).
  3. Step 3 — Maximum string length = inverter max voltage ÷ corrected Voc = 600 V ÷ 49.9 V = 12.02.
  4. Step 4 — Round down to the nearest whole module: maximum string length = 12 modules.
Answer: Maximum 12 modules per string. At 12 modules, corrected string Voc = 12 × 49.9 V = 598.8 V, which is below the 600 V inverter limit.
2. Using the same module (Vmp = 37.0 V, Vmp temperature coefficient = −0.40%/°C at STC). The site's peak summer design cell temperature is 70 °C. The inverter's minimum MPPT voltage is 200 V. What is the minimum number of modules required in the string to keep Vmp above the MPPT floor?
  1. Step 1 — Find the temperature-corrected Vmp at 70 °C. Temperature difference = 70 − 25 = +45 °C. Correction factor = 1 + (−0.40/100) × (45) = 1 + (−0.18) = 0.82.
  2. Step 2 — Corrected Vmp = 37.0 V × 0.82 = 30.34 V.
  3. Step 3 — Minimum string length = inverter minimum MPPT voltage ÷ corrected Vmp = 200 V ÷ 30.34 V = 6.59.
  4. Step 4 — Round up to the nearest whole module: minimum string length = 7 modules.
Answer: Minimum 7 modules per string. At 7 modules, corrected string Vmp at 70 °C = 7 × 30.34 V = 212.4 V, which is above the 200 V MPPT floor.
3. Combining both constraints from Examples 1 and 2, what is the valid string length range? Then check whether 3 parallel strings of 10 modules each keeps total Isc within the inverter's 30 A maximum input current limit, given module Isc = 9.5 A.
  1. Step 1 — From Example 1, maximum string length = 12 modules. From Example 2, minimum string length = 7 modules. Valid range: 7 to 12 modules per string.
  2. Step 2 — The proposed design uses 10 modules per string, which falls within the 7–12 range. ✓
  3. Step 3 — Check current: 3 parallel strings × 9.5 A per string = 28.5 A total Isc.
  4. Step 4 — Compare to inverter limit: 28.5 A < 30 A. ✓
Answer: A string length of 10 modules is valid. Three parallel strings produce a combined Isc of 28.5 A, which is within the 30 A inverter input current limit. The design is acceptable.
4. A flawed single-line diagram shows 14 modules in series using the same module from Examples 1 and 2 (corrected Voc at −10 °C = 49.9 V). The inverter maximum DC input voltage is 600 V. What is the corrected string Voc, is there a violation, and what is one corrected string length that resolves it?
  1. Step 1 — Calculate flawed string Voc: 14 modules × 49.9 V = 698.6 V.
  2. Step 2 — Compare to inverter limit: 698.6 V > 600 V. This is an overvoltage violation that could destroy the inverter and create a shock hazard.
  3. Step 3 — From Example 1, the maximum allowable string length is 12 modules. Set string length to 12.
  4. Step 4 — Verify corrected design: 12 × 49.9 V = 598.8 V < 600 V. Violation is resolved.
Answer: The 14-module string exceeds the inverter's 600 V maximum (698.6 V). Reducing to 12 modules yields a corrected Voc of 598.8 V, which is within limits. Written justification must cite the −10 °C corrected Voc of 49.9 V and the inverter datasheet maximum of 600 V.

The math in this lesson2

Learners compute temperature-corrected Voc and Vmp to bound valid string length.
A-SSE.1A-REI.3
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates step-by-step datasheet matching: reading module Voc, Vmp, Isc, and Imp; applying temperature correction factors for the local climate; comparing results against inverter minimum and maximum input voltage and current limits on the board while narrating each decision. · Guided practice: students work paired problems on three pre-built scenario worksheets (residential rooftop, high-temperature desert climate, partial-shade site) using a scientific calculator; instructor circulates and corrects errors in real time before students move to the next scenario. · Design review exercise: students receive a flawed single-line diagram where string voltage exceeds inverter Voc max at low temperature and must write a corrected string configuration with a written justification citing module datasheet values and temperature correction math.
Materials: Scientific calculators, module and inverter datasheets (minimum three module models, two inverter models), temperature correction factor tables, scenario worksheet packets, Photovoltaic Systems by Dunlop (array sizing chapters)
Check: Graded worksheet set; students must correctly solve all three scenarios with no uncorrected voltage or current violations and show full calculation work; 70% overall score required.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

PV System Design — Array Sizing
1. A PV module has a Voc of 40.0 V at STC and a Voc temperature coefficient of −0.35%/°C. The site's lowest expected ambient temperature is −5 °C. What is the temperature-corrected Voc?
PV System Design — Array Sizing
2. When calculating the maximum number of modules allowed in a series string, the installer must use which module voltage value corrected to which temperature extreme?
PV System Design — Array Sizing
3. An inverter datasheet lists an MPPT voltage range of 200–480 V and a maximum DC input voltage of 600 V. A string's temperature-corrected Voc at minimum site temperature is 510 V and its temperature-corrected Vmp at maximum cell temperature is 190 V. Which violation exists?
PV System Design — Array Sizing
4. If three identical strings are wired in parallel and each string has an Isc of 8.8 A, what is the combined short-circuit current presented to the inverter input?
PV System Design — Array Sizing
5. A designer is checking a string's minimum length to ensure the inverter stays in MPPT. Which combination of values is used for this check?
Safety and Codes
6. Before a technician touches exposed DC conductors on a string that has been disconnected at the inverter, what is the required first action?
PV System Design — Array Sizing
7. A module datasheet shows Voc = 38.5 V and a Voc temperature coefficient of −0.28%/°C. For a site with a design minimum temperature of −15 °C, what correction factor should be applied to Voc?
PV System Design — Array Sizing
8. A single-line diagram shows 15 modules in series. The temperature-corrected Voc per module at the site's minimum temperature is 46.0 V. The inverter's maximum DC input voltage is 600 V. What corrective action is needed?
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