Solar String Sizing Calculator — Modules per String & MPPT

Quick start

PV module

Module

Module power (STC)

Open-circuit voltage Voc

MPP voltage Vmp

Short-circuit current Isc

MPP current Imp

Temp. coefficient of Vocβ, from datasheet

%/°C

Temp. coefficient of Pmax (≈ Vmp)γ, from datasheet

%/°C

PV module advanced

Bifacial current gainadds to Isc/Imp for current checks (5–10% typical)

Inverter

Inverter / controller

Max DC input voltage

MPPT range (low)use the full-power MPPT range if the datasheet gives one

MPPT range (high)

Start-up voltage

Max input current per MPPT

Max short-circuit current per MPPT

MPPT inputs used

Inverter AC rating

Site

Record low temperatureASHRAE extreme minimum (Voc peaks at dawn on the coldest day)

°C

Record high air temperaturee.g. ASHRAE 2% design high

°C

Cell temperature above airopen rack ≈ 25 · roof ≈ 30–35 · flush ≈ 40

°C

Cold-voltage method

Your design

Modules per string

Parallel strings per MPPT

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)

1,100 V ÷ 42.94 V

Min modules (MPPT)

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

Check	Value	Result
Voc at record cold ≤ max DC voltage	515 V ≤ 1,100 V	OK
Vmp at hot cells ≥ MPPT low	335 V ≥ 160 V	OK
Vmp at cold ≤ MPPT high	440 V ≤ 1,000 V	OK
Voc on hot morning ≥ start-up	446 V ≥ 180 V	OK
Isc × 1.25 ≤ inverter max Isc	17.6 A ≤ 22 A	OK
Imp ≤ max input current	13.6 A ≤ 16 A	OK

String length options

Modules	Voc cold (V)	Vmp hot (V)	Vmp cold (V)	kWp/string	Status
4	172	112	147	1.72	✗ below MPPT
5	215	140	183	2.15	✗ below MPPT
6	258	168	220	2.58	✓
7	301	196	257	3.01	✓
8	344	224	293	3.44	✓
9	386	251	330	3.87	✓
10	429	279	367	4.3	✓
11	472	307	403	4.73	✓
12 ◀	515	335	440	5.16	✓
13	558	363	477	5.59	✓
14	601	391	513	6.02	✓
15	644	419	550	6.45	✓
16	687	447	587	6.88	✓
17	730	475	623	7.31	✓
18	773	503	660	7.74	✓
19	816	531	697	8.17	✓
20	859	559	733	8.6	✓
21	902	587	770	9.03	✓
22	945	615	807	9.46	✓
23	988	643	843	9.89	✓
24	1,031	671	880	10.32	✓
25	1,074	698	917	10.75	✓
26	1,117	726	953	11.18	✗ over max V
27	1,159	754	990	11.61	✗ 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.

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