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Solar calculator

Solar string sizing calculator: how many panels per string?

Checks a PV string against the inverter’s maximum DC voltage, MPPT window, start-up voltage and current limits at your record cold and hottest conditions, using the NEC 690.7 and IEC methods.

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Results

12 modules per string passes every check: 515 V cold Voc vs the 1,100 V limit, and a 335–440 V operating range inside the 160–1,000 V MPPT window. Allowed range: 6–25.
Allowed modules per string
6 – 25
limited by max DC voltage
String Voc at record cold
515V
42.94 V/module at -20 °C (×1.113)
String Vmp, hot cells
335V
cells at 70 °C
String Vmp, cold
440V
MPPT window 160–1,000 V
Max modules (voltage)
25
1,100 V ÷ 42.94 V
Min modules (MPPT)
6
MPPT 6, start-up 5
Max circuit current
17.6A
1 string(s) × Isc × 1.25 vs 22 A
Array DC power
10.32kWp
12 × 1 × 2 MPPT = 24 modules
DC/AC ratio
1.03
healthy 0.9–1.5
Checks passed
6 / 6
all good
String voltage vs. cell temperature (12 modules)

Design checks

CheckValueResult
Voc at record cold ≤ max DC voltage515 V ≤ 1,100 VOK
Vmp at hot cells ≥ MPPT low335 V ≥ 160 VOK
Vmp at cold ≤ MPPT high440 V ≤ 1,000 VOK
Voc on hot morning ≥ start-up446 V ≥ 180 VOK
Isc × 1.25 ≤ inverter max Isc17.6 A ≤ 22 AOK
Imp ≤ max input current13.6 A ≤ 16 AOK

String length options

ModulesVoc cold (V)Vmp hot (V)Vmp cold (V)kWp/stringStatus
41721121471.72✗ below MPPT
52151401832.15✗ below MPPT
62581682202.58✓
73011962573.01✓
83442242933.44✓
93862513303.87✓
104292793674.3✓
114723074034.73✓
12 ◀5153354405.16✓
135583634775.59✓
146013915136.02✓
156444195506.45✓
166874475876.88✓
177304756237.31✓
187735036607.74✓
198165316978.17✓
208595597338.6✓
219025877709.03✓
229456158079.46✓
239886438439.89✓
241,03167188010.32✓
251,07469891710.75✓
261,11772695311.18✗ over max V
271,15975499011.61✗ over max V
  • Cold Voc = Voc × [1 + β × (T_min − 25)] with β = -0.25 %/°C. Cells are assumed to be at air temperature at dawn on the coldest day (no solar heating).
  • Vmp drift uses the Pmax coefficient as a proxy for the Vmp coefficient (datasheets rarely list it; the true value is usually 0.02–0.05 %/°C more negative). Keep a few percent margin below the max DC voltage.
  • Module tolerance, irradiance above 1,000 W/m² (cloud-edge effects) and bifacial gain raise current. Inverters limit current safely, but Isc above the input rating is not allowed.

How this string sizing calculator works

A PV string is a set of modules wired in series, so their voltages add up. The inverter (or charge controller) on the other end has three voltage limits: an absolute maximum DC input voltage, an MPPT window in which it can track maximum power, and a start-up voltage. It also limits input current. Module voltage changes with temperature, so a string has to respect those limits at both climatic extremes:

  • Coldest dawn: the highest open-circuit voltage (Voc). This sets the hard maximum number of modules.
  • Hottest afternoon: the lowest maximum-power voltage (Vmp), which sets the minimum number of modules for the MPPT window.
  • Cold, sunny noon: the highest operating Vmp. It should stay below the upper MPPT limit for full tracking.

For each case the calculator corrects the datasheet voltages and compares them with the inverter’s limits. It also checks the short-circuit and operating currents per MPPT input and lists every possible string length with its status.

The formulas

Voc,cold = Voc × [1 + β_Voc × (T_min − 25)] (or × NEC Table 690.7(A) factor) Vmp,hot = Vmp × [1 + γ × (T_air,max + ΔT_cell − 25)] N_max = ⌊ V_max,DC ÷ Voc,cold ⌋ N_min = max( ⌈ V_MPPT,low ÷ Vmp,hot ⌉ , ⌈ V_start ÷ Voc,hot ⌉ ) Full tracking: N × Vmp,cold ≤ V_MPPT,high Current per MPPT: N_parallel × Isc × 1.25 ≤ I_sc,max,inverter

β is the Voc temperature coefficient and γ the power temperature coefficient (a close proxy for Vmp’s coefficient), both in %/°C and negative. ΔT_cell is how much hotter the cells run than the air at full sun: about 25 °C on open racks, 30–35 °C on pitched roofs and 40 °C or more when flush-mounted.

Worked example: 430 W TOPCon on a 10 kW three-phase inverter

The defaults use a 430 W N-type module (Voc 38.6 V, Vmp 32.3 V, β = −0.25 %/°C, γ = −0.30 %/°C) and a typical 10 kW inverter: 1,100 V maximum, 160–1,000 V MPPT window, 180 V start-up, 16 A per MPPT (22 A short-circuit). The site’s record low is −20 °C and its hottest days reach 40 °C.

At −20 °C the module’s Voc rises by 11.3% to 42.9 V, so at most 1,100 ÷ 42.9 = 25 modules fit in a string. On a 40 °C day the cells reach 70 °C and Vmp falls to 27.9 V, so the string needs at least 160 ÷ 27.9 → 6 modules. The chosen 12-module string has a cold Voc of 515 V and operates between 335 and 440 V, comfortably inside every limit. Two such strings, one per MPPT, give 24 modules or 10.32 kWp, a DC/AC ratio of 1.03. The string current (13.6 A hot, 17.6 A with the NEC factor) fits the inputs.

With the NEC table method the cold factor rises to 1.18 and the maximum drops to 24 modules.

How to read the results

  • Allowed modules per string is the safe range. Prefer the middle-upper part: longer strings run at higher voltage, lose less in cables and keep tracking on hot days.
  • Design checks separate hard failures (over-voltage or over-current, never acceptable) from performance warnings (below MPPT on hot days, above MPPT on cold days, late start-up).
  • The voltage vs temperature chart shows how far the string stays from each limit across the full temperature range.

Typical values

Module class Voc (STC) Voc at −20 °C Max per string, 600 V 1,000 V 1,100 V 1,500 V
300 W poly, 60-cell 39.8 V 45.4 V 13 22 24 33
430 W TOPCon, 108 half-cell 38.6 V 42.9 V 13 23 25 34
550 W PERC, 144 half-cell 49.9 V 55.9 V 10 17 19 26
620 W TOPCon, 210R 132-cell 48.2 V 53.6 V 11 18 20 27

Voc at −20 °C uses each class’s typical coefficient (−0.25 to −0.31 %/°C).

Tips

  • Use the real record low, not the average winter low. ASHRAE “extreme annual minimum” values or national weather records are the accepted source; at mountain sites, add a few degrees of margin.
  • Leave 2–3% headroom under the maximum DC voltage for module tolerance and Vmp/Voc coefficient differences.
  • Check the full-power MPPT range, not just the operating range, if the datasheet lists both; below it the inverter derates.
  • Keep strings on one MPPT identical in length, tilt and orientation; put east and west roof faces on separate MPPT inputs.
  • Then size the cables and the inverter. DC losses at 300–600 V are small, but check them with the cable size calculator, and size the inverter with the inverter sizing calculator.

Limitations

The calculator uses linear temperature coefficients, which are accurate for crystalline silicon over normal ranges. Vmp is corrected with the power coefficient because Vmp coefficients are rarely published; real Vmp shifts slightly more. The model leaves out irradiance-dependent voltage changes, module degradation (which lowers Vmp by a few percent over 25 years), optimisers and microinverters. Always confirm the final design against the inverter manufacturer’s own string-design tool and your local electrical code.

Method & assumptions

Maximum string length = ⌊ V_max,inverter ÷ (Voc × [1 + β_Voc × (T_min − 25 °C)]) ⌋, because the open-circuit voltage peaks at sunrise on the coldest day. Minimum length = ⌈ V_MPPT,low ÷ (Vmp × [1 + γ × (T_air,max + ΔT_cell − 25 °C)]) ⌉, also checked against the start-up voltage. The string's cold Vmp should stay below the upper MPPT limit for full tracking.

Currents: per MPPT, I_sc × strings × 1.25 (NEC 690.8) must not exceed the inverter's maximum short-circuit current; I_mp × strings above the maximum input current leads to clipping.

Frequently asked questions

How many solar panels can I put in one string?
Divide the inverter's maximum DC input voltage by the module's open-circuit voltage at the coldest temperature expected on site, and round down. For a 1,100 V inverter, a module with Voc 38.6 V and a −0.25 %/°C coefficient at −20 °C (42.9 V per module), the limit is 25 modules. On a 600 V single-phase inverter it would be 13.
Why does cold weather increase solar panel voltage?
A silicon cell's open-circuit voltage falls as temperature rises, by about 0.25–0.31% per °C for modern modules, and below the 25 °C test temperature it climbs at the same rate. At sunrise on a frosty morning the cells are as cold as the air but already lit, so Voc can be 10–15% above the datasheet value. For the inverter's maximum input voltage, that is the worst case.
What happens if string voltage exceeds the inverter's maximum?
Exceeding the maximum DC input voltage can permanently damage the inverter's input stage and voids the warranty. In the US and many other countries it also breaks the electrical code. The inverter simply limits excess current, but it has no way to control excess voltage.
What is the minimum number of panels per string?
The string's MPP voltage on the hottest day must stay above the inverter's lower MPPT limit (and its open-circuit voltage above the start-up voltage). Hot cells at 65–75 °C lower Vmp by 12–15%, so a string that looks fine at 25 °C can drop out of the tracking window in July. The result is lost energy, not damage.
Should I use the NEC table or the temperature coefficient?
NEC 690.7(A)(1) requires using the manufacturer's temperature coefficient when it is provided, which is almost always. Table 690.7(A) is a fallback for crystalline modules without data and is more conservative (for example a factor of 1.18 at −20 °C instead of about 1.11 for a −0.25 %/°C module), which can cost you a module per string.
Can I connect two strings to one MPPT input?
Only if the strings are identical in length, orientation and module type and the combined current fits the input. Two strings of modules with Isc 14.1 A give 28.2 A, or 35.3 A with the NEC 1.25 factor. That is too much for a 16 A/22 A input, so large modern modules usually need one string per MPPT on residential inverters.

Sources & references

  1. NFPA 70 National Electrical Code — Article 690.7 (maximum voltage) and 690.8 (circuit sizing)
  2. IEC 62548-1:2023 — Photovoltaic (PV) arrays, Design requirements
  3. ASHRAE Climatic Design Conditions (extreme minimum temperatures)
  4. Sandia National Laboratories — PV performance modeling collaborative: module temperature and voltage models

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