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The maximum allowable DC voltage for the inverter or system.
Check the inverter's datasheet for its maximum input voltage rating, and any applicable system voltage limit set by code or the equipment installed.
The panel's rated open-circuit voltage at Standard Test Conditions.
Find this value on the panel's datasheet nameplate.
Accounts for Voc increasing as ambient temperature drops. NEC Table 690.7(A) gives 1.12 for a record low of -1 to -5 C (31 to 23 F), and 1.16 by -11 to -15 C (13 to 5 F).
Read the factor off NEC Table 690.7(A) against your site RECORD low ambient temperature, not its average winter minimum, or use the panel manufacturer own temperature coefficient, which is more accurate for a specific module. The table steps every 5 C: 1.10 at 4 to 0 C, 1.12 at -1 to -5, 1.14 at -6 to -10, 1.16 at -11 to -15, 1.18 at -16 to -20, rising to 1.25 at -36 to -40. Taking a factor one row too warm is how a string that measures fine in summer exceeds the inverter maximum voltage on the coldest morning of the year. The factor is a plain multiplier, so it carries no units and reads the same in either measurement system.
Maximum panels per string
13 panels
The correction factor depends on your site's record low ambient temperature and the panel manufacturer's temperature coefficient — use NEC Table 690.7(A) or the manufacturer's datasheet coefficient for your specific location, not an assumed value.
- Temperature-corrected Voc per panel
- 44.8 V
They open the calculator with your figures already in it
Solar PV String Sizing Calculator: 13 panels — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Maximum panels per string = floor(maximum system voltage ÷ (panel open-circuit voltage × low-temperature correction factor)), per NEC 690.7 which requires PV source circuit voltage calculations to account for Voc increasing in cold temperatures using either the manufacturer's temperature coefficient or NEC Table 690.7(A) correction factors
Inputs used
- Maximum System/Inverter Voltage (V)
- 600
- Panel Open-Circuit Voltage Voc (V, at STC)
- 40
- Low-Temperature Correction Factor
- 1.12
Intermediate steps
- Temperature-corrected Voc per panel
- 44.8 V
Confidence note: The correction factor depends on your site's record low ambient temperature and the panel manufacturer's temperature coefficient — use NEC Table 690.7(A) or the manufacturer's datasheet coefficient for your specific location, not an assumed value.
What this calculation does not cover
- This is the cold-weather upper voltage limit only. Nothing here checks the minimum string length: whether the string's operating voltage at the hottest expected cell temperature still sits above the inverter's MPPT window and start-up voltage. A string that passes this check can still be too short to make power on a hot afternoon.
- The calculation is voltage only. It does not look at short-circuit current, the module's maximum series fuse rating, the inverter's per-MPPT input current limit, conductor ampacity, or combiner fusing, so a string length that passes here has had only half of a string design checked.
- It applies whichever single voltage limit you type. It does not reconcile the inverter's maximum DC input against the module nameplate's own maximum system voltage rating or against the limit your code and occupancy impose - enter the lowest of the three, because the calculator cannot tell you which one governs. The Voc field also stops at 80 V, which covers crystalline silicon modules; thin-film modules run to several times that and will be clamped down to 80 V, returning a string far longer than the module can take.
- The calculator does no temperature arithmetic of its own. It never sees your site's record low ambient temperature or the module's Voc temperature coefficient - it multiplies by whatever factor you type, so a factor taken from the wrong row produces a confident, wrong answer. The factor field also stops at 1.25, the coldest row of NEC Table 690.7(A).
- This is a sizing check, not a design. It assumes every module in the string is identical and does not substitute for the inverter manufacturer's own string-sizing tool or for a design reviewed by the authority having jurisdiction.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Maximum panels per string = floor(maximum system voltage ÷ (panel open-circuit voltage × low-temperature correction factor)), per NEC 690.7 which requires PV source circuit voltage calculations to account for Voc increasing in cold temperatures using either the manufacturer's temperature coefficient or NEC Table 690.7(A) correction factors
Which documents these citations point at
- National Electrical Code (NFPA 70) — 690.7, Table 690.7(A) (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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