Solar String Sizing Calculator: Panels Per String for Any Inverter-Module Combo

A Denver installer wired 16 modules in series on a 600V residential inverter. At STC, Voc was 37.2V times 16, or 595V — close, but within limit. The installer never checked the cold correction. January hit -18°C, the ASHRAE design low for Denver. Voc climbed to 44.1V per module. The string reached 706V. The inverter shut down every morning for three weeks before the homeowner called. The warranty claim was denied. The permit was flagged. The redesign cost $4,200.

That failure is not rare. It is the most common voltage-related service call in residential solar. And it traces back to one skipped step in a calculation that takes four minutes to do by hand.

This guide is a complete solar string sizing calculator. It covers the full calculation chain for any module and inverter combination: Voc cold correction, Vmp hot correction, MPPT current limits, parallel string rules, and NEC 690.7 compliance. Every formula is shown step by step. Every number comes from a real datasheet. The worked example uses a 550W n-type TOPCon module and a SolarEdge SE10000H-US inverter — a common 2025–2026 pairing. Read this once and you will never need another string sizing reference. For everyday project work, solar design software handles string sizing automatically with live datasheet lookup and temperature correction.

In this guide:

• What solar string sizing is and why temperature governs every decision
• The four electrical parameters every datasheet hides in plain sight
• Inverter voltage input windows: min, max, and MPPT range
• Cold-side calculation: Voc at record low temperature with full worked example
• Hot-side calculation: Vmp at peak temperature with full worked example
• How to calculate panels per string: formula, table, and calculator logic
• Parallel strings and MPPT current limits
• Mismatch and shading: when strings fail silently
• String sizing for string inverters, central inverters, and hybrid inverters
• Reference tables for popular inverter-module combinations
• Common mistakes: overvoltage, undervoltage, and the wrong temperature input
• Contrarian view: why “rules of thumb” fail in extreme climates
• Extreme climate corrections for desert cold nights and tropical heat

What Solar String Sizing Is — and Why Temperature Rules It

A solar string is a series-connected chain of PV modules. In series, voltages add while current stays constant. String sizing is the process of choosing how many modules go in each chain so the string’s voltage stays within the inverter’s operating window at every temperature the site will see.

Three temperatures matter:

• The coldest morning — when Voc is at its highest and threatens to exceed the inverter’s maximum DC input
• The hottest afternoon — when Vmp is at its lowest and threatens to drop below the MPPT start voltage
• Standard test conditions (25°C) — where the datasheet numbers live, but where the array almost never operates

The gap between cold Voc and hot Vmp is the design envelope. Your string length must fit inside it. The width of that envelope depends on the module’s temperature coefficients, the site’s climate, and the inverter’s voltage range.

Most string sizing failures are not math errors. They are input errors — wrong temperature, wrong coefficient, or the right calculation applied to the wrong limit. The next sections cover each input in order.

The Four Datasheet Parameters That Govern String Sizing

Every module datasheet lists dozens of numbers. Only four matter for string sizing:

A fifth parameter, the Vmp temperature coefficient (gamma_Vmp), is also needed for the hot-side check. It typically runs -0.35 to -0.40 %/°C for crystalline silicon.

Where to Find These Values

On a typical 550W module datasheet, the electrical characteristics table looks like this:

These numbers are at STC: 1000 W/m² irradiance, 25°C cell temperature, AM1.5 spectrum. The array almost never operates at STC. That is why the temperature corrections matter.

Inverter Voltage Input Window: The Hard Limits

Every inverter has three voltage specifications. String sizing must satisfy all three simultaneously.

The maximum DC input voltage is an absolute hard limit. It is set by the input semiconductor ratings (MOSFETs or IGBTs) and the insulation class of the DC input circuitry. Exceeding it risks component failure and voids the warranty.

The MPPT voltage range is the window where the inverter’s maximum power point tracker can operate. Within this range, the inverter adjusts its input impedance to find the voltage at which the array delivers maximum power. Outside this range, the inverter either clips power (above max) or cannot track (below min).

The MPPT start voltage is the minimum voltage the inverter needs to begin operation. It is typically the same as the MPPT minimum, but some inverters specify a separate start voltage that is 10–20V higher. Always use the higher of the two for the hot-side check.

Inverter Voltage Tiers by Application

The 600V residential limit is non-negotiable. It applies to the entire DC conductor system, not just the inverter input. A 600V inverter fed by strings reaching 620V on a cold morning is a code violation regardless of what the inverter’s protection circuit does.

Cold-Side Calculation: Voc at Record Low Temperature

The cold-side check determines the maximum number of modules per string. Crystalline silicon modules have a negative voltage temperature coefficient. Voc rises as temperature drops. The coldest expected condition produces the highest voltage — and that voltage must stay below the inverter’s maximum DC input.

The Formula

Voc_cold = Voc_STC × [1 + (beta_Voc / 100) × (T_min - 25)]

Max modules per string = floor(V_inverter_max / Voc_cold)

Where:

• Voc_STC = open-circuit voltage at standard test conditions (from datasheet)
• beta_Voc = Voc temperature coefficient in %/°C (from datasheet, negative value)
• T_min = ASHRAE 99.6% design low temperature in °C
• V_inverter_max = inverter maximum DC input voltage

Worked Example — Cold Side

Module: 550W n-type TOPCon Inverter: SolarEdge SE10000H-US (1000V max DC, 250–1000V MPPT) Site: Denver, Colorado ASHRAE 99.6% T_min: -18°C

Step 1 — Calculate cold-corrected Voc per module:

Voc_cold = 49.8 × [1 + (-0.25 / 100) × (-18 - 25)]
         = 49.8 × [1 + (-0.0025) × (-43)]
         = 49.8 × [1 + 0.1075]
         = 49.8 × 1.1075
         = 55.15V per module

Step 2 — Calculate maximum modules per string:

Max modules = floor(1000 / 55.15)
            = floor(18.13)
            = 18 modules

At -18°C, each module produces 55.15V. Eighteen modules in series reach 992.7V — 7.3V under the 1000V limit. Nineteen modules would reach 1,047.9V, exceeding the inverter maximum. Good solar software flags this automatically and suggests the optimal string length for your module-inverter pairing.

Maximum string length: 18 modules

Hot-Side Calculation: Vmp at Peak Temperature

The hot-side check determines the minimum number of modules per string. Hot cell temperatures depress Vmp. If the string’s Vmp at peak cell temperature falls below the inverter’s MPPT minimum start voltage, the inverter cannot track the string and produces zero output.

The Formula

T_cell_max = T_ambient_max + 25°C  (rack-mounted modules)

Vmp_hot = Vmp_STC × [1 + (gamma_Vmp / 100) × (T_cell_max - 25)]

Min modules per string = ceil(V_mppt_min / Vmp_hot)

Where:

• Vmp_STC = maximum power voltage at standard test conditions
• gamma_Vmp = Vmp temperature coefficient in %/°C (negative value)
• T_ambient_max = highest expected ambient temperature
• T_cell_max = peak cell temperature (ambient + 25°C for rack-mounted)
• V_mppt_min = inverter MPPT minimum start voltage

The +25°C adder is a standard approximation from IEC 61215 for rack-mounted modules with adequate rear ventilation. Flush-mounted roof installations with less than 3 inches of air gap run 5–8°C hotter — use +30°C in those cases.

Worked Example — Hot Side

Same module and inverter, Denver site:

Step 1 — Calculate peak cell temperature:

T_cell_max = 38 + 25 = 63°C

Step 2 — Calculate hot-corrected Vmp per module:

Vmp_hot = 42.1 × [1 + (-0.35 / 100) × (63 - 25)]
        = 42.1 × [1 + (-0.0035) × 38]
        = 42.1 × [1 - 0.133]
        = 42.1 × 0.867
        = 36.50V per module

Step 3 — Calculate minimum modules per string:

Min modules = ceil(250 / 36.50)
            = ceil(6.85)
            = 7 modules

At 63°C cell temperature, each module produces 36.50V at its power point. Seven modules deliver 255.5V — 5.5V above the 250V MPPT minimum. Six modules would deliver only 219.0V, below the start threshold.

Minimum string length: 7 modules

Valid String Length Range

From the cold-side and hot-side calculations:

• Maximum: 18 modules (cold Voc limit)
• Minimum: 7 modules (hot Vmp limit)
• Valid range: 7 to 18 modules

The optimal target is 16–17 modules per string. At 17 modules, string Vmp at STC is 42.1 × 17 = 715.7V — 71.6% of the 1000V MPPT top. At 16 modules, it is 673.6V — 67.4%. Both sit comfortably within the MPPT range and leave margin for both temperature extremes.

How to Calculate Panels Per String: The Complete Formula

The full string sizing calculation combines both temperature checks into a single decision framework. Here is the complete method, reproducible for any module-inverter-site combination.

Step 1 — Gather Inputs

Step 2 — Cold-Side Maximum

Voc_cold = Voc_STC × [1 + (beta_Voc / 100) × (T_min - 25)]
N_max = floor(V_inverter_max / Voc_cold)

Step 3 — Hot-Side Minimum

T_cell_max = T_ambient_max + 25
Vmp_hot = Vmp_STC × [1 + (gamma_Vmp / 100) × (T_cell_max - 25)]
N_min = ceil(V_mppt_min / Vmp_hot)

Step 4 — Current Check (Parallel Strings)

N_strings_max = floor(I_mppt_max / Isc_STC)

For bifacial modules, apply NEC 690.8(A)(3): multiply Isc by 1.25 before dividing.

Step 5 — Select String Length

Choose N such that:

N_min ≤ N ≤ N_max

Target 70–85% of the MPPT voltage range at STC for optimal efficiency:

Target Vmp_STC = 0.70 to 0.85 × V_mppt_max
Target N = Target Vmp_STC / Vmp_STC

Quick Reference Table: Panels Per String by Voltage Tier

The table below shows typical string lengths for common module voltages and inverter tiers. Values assume beta_Voc = -0.25 %/°C, gamma_Vmp = -0.35 %/°C, T_min = -10°C, T_ambient_max = 35°C.

Maximum values are cold-corrected. Minimum values are hot-corrected. Actual values depend on site-specific temperatures and module coefficients.

Parallel Strings and MPPT Current Limits

The voltage checks determine how many modules go in each string. The current check determines how many strings can connect to each MPPT input.

Every MPPT input has a maximum current rating. If the combined short-circuit current of all paralleled strings exceeds this rating, the inverter cannot safely handle the input.

The Formula

Max strings per MPPT = floor(I_mppt_max / Isc_module)

Worked Example — Current Check

Inverter: SolarEdge SE10000H-US Module Isc: 14.12A MPPT max current: 26A (per input)

Max strings per MPPT = floor(26 / 14.12)
                     = floor(1.84)
                     = 1 string per MPPT

This inverter supports only one string per MPPT input with this module. Two strings in parallel would draw 28.24A, exceeding the 26A limit.

Bifacial Current Correction

For bifacial modules, NEC 690.8(A)(3) requires a 1.25× multiplier on Isc for conductor and overcurrent protection sizing:

Isc_corrected = Isc × 1.25
Max strings per MPPT = floor(I_mppt_max / Isc_corrected)

With the same module as bifacial:

Isc_corrected = 14.12 × 1.25 = 17.65A
Max strings per MPPT = floor(26 / 17.65) = 1 string

In this case, the bifacial correction does not change the result. But on inverters with higher MPPT current limits, it can reduce the allowable parallel strings from 3 to 2.

Mismatch and Shading: When Strings Fail Silently

String sizing assumes every module in the string is identical and operating under the same conditions. Reality is messier. Mismatch and shading create voltage imbalances that the MPPT tracker cannot resolve optimally.

Mismatch Loss Data

Fronius published field data on mismatch losses from unequal string lengths on the same MPPT:

A 0.82% annual loss on a 100 kW commercial system producing 150,000 kWh/year equals 1,230 kWh — about $148 per year at $0.12/kWh, every year for 25 years. Proper string assignment costs nothing and prevents this.

Shading Impact on String Voltage

Partial shading on one module in a string drops that module’s current. The bypass diode activates, shunting current around the shaded substring. The string voltage drops by the substring voltage — typically one-third of the module voltage for a three-diode module.

A string with one partially shaded module loses approximately 30–35% of its voltage, not 30–35% of its power. The MPPT tracker sees a lower voltage point and may settle on a suboptimal operating point for the entire array.

String Sizing for Different Inverter Types

String Inverters

String inverters are the most common type for residential and small commercial systems. Each MPPT input accepts one or more strings in parallel. The string sizing rules in this guide apply directly.

Typical residential string inverter: 3–4 kW, 1–2 MPPT inputs, 600V max DC. Typical commercial string inverter: 10–50 kW, 2–6 MPPT inputs, 1000V max DC.

Central Inverters

Central inverters aggregate many strings through DC combiner boxes before the inverter. String sizing still follows the same voltage rules, but the combiner box adds a layer of overcurrent protection and monitoring. At 1500V, central inverters allow strings of 28–32 modules, reducing conductor count dramatically on utility-scale projects.

Hybrid Inverters

Hybrid inverters combine solar MPPT inputs with battery charge controllers. The MPPT voltage range is often narrower than standalone string inverters to accommodate battery voltage requirements. Check both the solar MPPT range and the battery voltage window when sizing strings for hybrid systems.

String Sizing Tables for Popular Inverter-Module Combinations

The tables below show pre-calculated string length ranges for common 2025–2026 module and inverter pairings. All values assume ASHRAE 99.6% T_min = -10°C and T_ambient_max = 35°C unless noted.

Residential (600V) — Popular Pairings

Commercial (1000V) — Popular Pairings

Utility (1500V) — Popular Pairings

Optimal range targets 70–85% of MPPT maximum at STC. Actual optimal depends on site-specific DC/AC ratio targets.

Common Mistakes in String Sizing

The mistakes below come from permit rejections, warranty disputes, and production underperformance investigations. Each has a real cost.

Mistake 1: STC Voc Without Temperature Correction

This is the Denver scenario from the opening. An installer calculates 49.8V × 16 = 796.8V at STC, sees it is under 1000V, and ships the job. At -18°C, Voc climbs to 55.15V. Sixteen modules reach 882.4V — still under 1000V, so this particular example passes. But with a 600V residential inverter and 12 modules: 49.8V × 12 = 597.6V at STC looks safe. At -18°C, it reaches 661.8V — 61.8V over the 600V limit. Permit rejected.

Mistake 3: Ignoring Hot-Side MPPT Minimum

A designer in Miami sizes a string for a 250V MPPT minimum using 6 modules with 42.1V Vmp. At STC, 6 × 42.1 = 252.6V — just over the limit. But Miami’s peak cell temperature reaches 70°C. Vmp drops to 35.6V. Six modules deliver only 213.6V — 36.4V below the MPPT start. The string produces nothing during the hottest afternoons of the year.

Why “Rules of Thumb” Fail in Extreme Climates

Most string sizing guides offer rules of thumb: “14 modules per string for 600V,” “20 modules for 1000V,” “30 modules for 1500V.” These numbers work for temperate climates with moderate temperature swings. They fail in extreme climates where the temperature envelope is wider than the rule assumes.

The Problem with Rules of Thumb

A rule of thumb assumes a typical temperature range. It does not know your site. Consider three cities using the same 550W module (Voc 49.8V, Vmp 42.1V) on a 1000V inverter:

The “20 modules for 1000V” rule of thumb fails in Denver (max 18) and Fairbanks (max 15). It works in San Diego and Phoenix. A designer using the rule without site-specific temperature correction risks overvoltage in cold climates and undervoltage in hot climates.

Opinionated take: Rules of thumb should not exist in permit documentation. They are fine for preliminary estimates during sales conversations. They are not fine for designs submitted to an AHJ. Every permit package should show the temperature input, the correction formula, and the resulting string length. Anything less is a liability.

Extreme Climate Corrections

Desert Cold Nights

Desert climates have the widest temperature envelopes. Daytime highs reach 45–50°C. Nighttime lows in winter drop to -10°C or lower. The same array sees both extremes.

Phoenix example: T_min = 5°C, T_ambient_max = 47°C.

Voc_cold = 49.8 × [1 + (-0.0025) × (5 - 25)] = 54.3V
Max modules = floor(1000 / 54.3) = 18

T_cell_max = 47 + 25 = 72°C
Vmp_hot = 42.1 × [1 + (-0.0035) × (72 - 25)] = 35.2V
Min modules = ceil(250 / 35.2) = 8

Valid range: 8–18 modules. The hot side is the tighter constraint. A designer who only checks the cold side and strings 18 modules might not notice that 8 is the real minimum for reliable summer operation.

Tropical Heat

Tropical climates have high ambient temperatures year-round with narrow seasonal variation. The hot-side check dominates.

Singapore example: T_min = 20°C, T_ambient_max = 34°C.

Voc_cold = 49.8 × [1 + (-0.0025) × (20 - 25)] = 50.4V
Max modules = floor(1000 / 50.4) = 19

T_cell_max = 34 + 25 = 59°C
Vmp_hot = 42.1 × [1 + (-0.0035) × (59 - 25)] = 37.1V
Min modules = ceil(250 / 37.1) = 7

Valid range: 7–19 modules. The cold side is barely a constraint. Tropical designers should focus their attention on the hot-side margin — especially for flush-mounted rooftop systems where T_cell_max may reach 65°C.

High-Altitude Cold

High-altitude sites have cold temperatures and high irradiance. The cold-side check is critical.

La Paz, Bolivia (3,640m): T_min = -12°C, T_ambient_max = 25°C.

Voc_cold = 49.8 × [1 + (-0.0025) × (-12 - 25)] = 54.4V
Max modules = floor(1000 / 54.4) = 18

T_cell_max = 25 + 25 = 50°C
Vmp_hot = 42.1 × [1 + (-0.0035) × (50 - 25)] = 38.4V
Min modules = ceil(250 / 38.4) = 7

Valid range: 7–18 modules. The high altitude also means lower air density and higher UV exposure, but those factors affect degradation, not string sizing.

The Tradeoff No One Talks About

There is a tension in string sizing that most guides gloss over. Longer strings are more efficient — fewer conductors, lower I²R losses, less combiner box hardware. But longer strings push closer to the cold-side voltage limit, leaving less margin for temperature uncertainty.

The tradeoff: Every module you add to a string increases annual energy yield by reducing conductor losses and hardware cost. But it also reduces the voltage margin between operating Voc and the inverter maximum. In climates with uncertain temperature data — remote sites without nearby ASHRAE weather stations — that margin is your safety buffer.

My recommendation: in well-documented climates with reliable ASHRAE data, design to 90–95% of the maximum string length. In remote or poorly documented climates, design to 85–90%. The lost module or two per string is cheap insurance against a warranty dispute.

What Most Designers Get Wrong About Temperature Coefficients

The temperature coefficient on the datasheet is a single number. But it is not constant across all temperatures. At very low temperatures (-40°C and below), the linear approximation starts to break down. Voc rises slightly less than the coefficient predicts. At very high temperatures (80°C+), Vmp drops slightly more.

For 99% of installations, the linear approximation is fine. The error is under 1%. But for extreme climate projects — Arctic installations, desert deployments, high-altitude sites — the nonlinearity matters. IEC 61215 testing validates the coefficient from -20°C to +60°C. Outside that range, the datasheet number is an extrapolation.

What most get wrong: They assume the coefficient is exact at all temperatures. It is not. For a -40°C installation, the actual Voc may be 1–2% lower than the linear calculation. That is enough to add one module per string on a 1500V system. For critical projects in extreme climates, request extended temperature testing data from the module manufacturer or use the conservative end of the coefficient range.

String Sizing Checklist for Every Project

Use this checklist before finalizing any string design:

• Module datasheet Voc, Vmp, Isc, beta_Voc, gamma_Vmp recorded
• Inverter spec sheet V_max, V_mppt_min, V_mppt_max, I_mppt_max recorded
• ASHRAE 99.6% design low temperature identified for site
• Peak ambient temperature identified for site
• Cold-side Voc calculated at T_min
• Maximum modules per string calculated and floored
• Hot-side Vmp calculated at T_cell_max
• Minimum modules per string calculated and ceilinged
• MPPT current check run for parallel strings
• Bifacial correction applied if using bifacial modules
• String length selected within valid range
• Target Vmp at STC checked against 70–85% of MPPT max
• Unequal strings assigned to separate MPPT inputs
• Shade analysis completed before string assignment
• All calculations documented in permit package

Conclusion

Solar string sizing is not a single formula. It is a sequence of checks that must all pass for the design to be valid. Skip the cold-side check and the inverter trips on winter mornings. Skip the hot-side check and the string produces nothing on summer afternoons. Skip the current check and the MPPT input overloads.

The calculation is simple arithmetic. The discipline is running every check, every time, with the correct temperature inputs. ASHRAE 99.6% for the cold side. Ambient max + 25°C for the hot side. Datasheet coefficients, not generic defaults.

Three decisions make the difference between a reliable design and a service call:

• Use site-specific temperatures. The ASHRAE 99.6% value is not a conservative estimate — it is the code-required input. In warm climates, it often adds one module per string compared to record-low approaches.
• Check both voltage limits, not just the cold side. The hot-side failure is less dramatic than an inverter trip, but it costs more production because it happens every hot afternoon, not just the coldest mornings.
• Document every calculation in the permit package. AHJs running 2017+ NEC expect to see the temperature input, the coefficient source, and the corrected voltage. A string diagram without calculations is a permit rejection waiting to happen.

For installers and designers working at scale, manual string sizing on every project is not sustainable. Solar design software that integrates ASHRAE data, inverter specs, and NEC 690.7 rules catches violations during layout — before the permit package leaves the office — and exports the full calculation set as a byproduct of the design process.

Frequently Asked Questions

How do you calculate solar string size?

Calculate the maximum panels per string by dividing the inverter’s maximum DC input voltage by the cold-corrected open-circuit voltage (Voc at the site’s ASHRAE 99.6% design low temperature). Calculate the minimum panels per string by dividing the inverter’s MPPT minimum start voltage by the hot-corrected maximum power voltage (Vmp at the site’s peak cell temperature). The valid string length sits between these two bounds.

What is the maximum voltage per solar string?

The maximum voltage per solar string is set by the inverter’s maximum DC input voltage — typically 600V for residential systems per NEC 690.7, 1000V for commercial buildings, and 1500V for utility-scale ground mounts. The cold-corrected Voc of the entire string must stay at or below this limit at the site’s design low temperature.

How many solar panels can you put in one string?

The number of solar panels per string depends on the module Voc, the inverter maximum input voltage, and the site’s temperature range. A typical 1000V commercial system with 49.5V Voc modules fits 18–19 panels per string in temperate climates. A 600V residential system with the same modules fits 11 panels. A 1500V utility system fits 28–30 panels.

What happens if a string voltage is too high for the inverter?

If string voltage exceeds the inverter’s maximum DC input, the inverter’s input protection circuit shuts the unit down. On cold mornings, this causes repeated startup failures until the array warms. Repeated over-voltage events can damage input MOSFETs or IGBTs and void the inverter warranty. NEC 690.7 makes exceeding the rated voltage a code violation.

What temperature coefficient is used for string sizing?

Use the Voc temperature coefficient (beta_Voc) from the module datasheet, typically -0.25% to -0.30% per degree Celsius for PERC modules and -0.22% to -0.25% for n-type TOPCon or HJT modules. For the hot-side Vmp calculation, use the Vmp temperature coefficient (gamma_Vmp), typically -0.35% to -0.40% per degree Celsius. Never use generic defaults.

Can you mix different solar panels in the same string?

No. Mixing different panel models in the same string creates current mismatch — the string current is limited by the lowest-current module. Voltage mismatch also disrupts the MPPT algorithm. NEC 690 does not explicitly ban mixed strings, but the resulting underperformance and potential hot-spot failure make it bad practice. Keep identical modules in every string.

How does temperature affect solar panel voltage?

Solar panel voltage decreases as temperature rises and increases as temperature falls. For every degree Celsius below 25°C STC, Voc rises by approximately 0.25–0.30%. For every degree above 25°C, Vmp drops by approximately 0.35–0.40%. A module with 49.5V Voc at 25°C reaches 58.8V at -40°C and drops to 34.7V Vmp at 70°C cell temperature.

What is the difference between Voc and Vmp in string sizing?

Voc (open-circuit voltage) is the voltage with no load connected. It is the highest voltage the module produces, so it governs the cold-side maximum string length. Vmp (maximum power voltage) is the voltage at the module’s peak power point under load. It governs the hot-side minimum string length — if hot Vmp falls below the inverter’s MPPT start voltage, the string produces no power.

How many strings can connect to one MPPT input?

The number of strings per MPPT input is limited by the inverter’s maximum input current per MPPT. Divide the MPPT maximum current by the module short-circuit current (Isc) and take the floor. For bifacial modules, multiply Isc by 1.25 per NEC 690.8(A)(3) before dividing. Typical string inverters allow 1–3 strings per MPPT.

What is ASHRAE 99.6% temperature and why does it matter?

ASHRAE 99.6% is the design low temperature that hourly readings fall below only 0.4% of the year — about 35 hours. NEC 690.7(A)(3) requires this value for cold-side Voc correction. It is warmer than all-time record lows, which means designers using the correct ASHRAE value can often fit one more module per string than those using conservative record lows.