String Sizing for Indian Temperatures: Installer Guide
Size PV strings for Indian conditions: design minimum temperatures by region, Voc and Vmp corrections, MPPT current, fuses, and worked 600 V examples.
Short answer: size each string so that its cold-morning Voc stays below the inverter's maximum DC input voltage and its hot-afternoon Vmp stays above the MPPT minimum. In India, use a design minimum of about 10°C in the south, 5°C on the Deccan and central plains, 0°C in the north, and −5°C in the hills, and a hot cell temperature of about 70°C. Then check MPPT current against modern high-current modules. Example: a typical 550 Wp PERC module allows up to 11 in series on a 600 V inverter.
The two voltage limits
1. Cold limit: maximum Voc
Voc rises as cells get colder. The worst case is sunrise on the coldest clear morning, when the cell is at or below ambient from night-sky cooling and irradiance is enough to bring up open-circuit voltage.
Voc(cold) = Voc(STC) × [1 + βVoc × (Tmin − 25)]
βVoc is negative (for example −0.27%/°C), so a temperature below 25°C raises Voc.
String Voc(cold) × modules ≤ inverter max DC voltage (and ≤ module max system voltage, usually 1,500 V).
2. Hot limit: minimum Vmp
Vmp falls as cells heat. On an Indian roof in April–May, cells reach 65–70°C.
Vmp(hot) = Vmp(STC) × [1 + γ × (Tcell − 25)]
Use the datasheet's Vmp coefficient if given; otherwise the Pmax coefficient is a conservative proxy.
String Vmp(hot) × modules ≥ inverter MPPT minimum (with margin above the start-up voltage).
Design minimum temperatures for India
| Region | Starting design minimum |
|---|---|
| South of 15°N (Chennai, Bengaluru, Kerala) | 10°C |
| 15–24°N (Pune, Nagpur, Hyderabad, Ahmedabad, Bhopal) | 5°C |
| Above 24°N (Jaipur, Lucknow, Delhi NCR) | 0°C |
| Hill states, altitude above 1,000 m, or above 28.5°N | −5°C |
Sunlectric Studio defaults (Oct 2026). Our engineering review raised the plains value after noting record lows of about 2–4°C in Pune and Nagpur and 2–5°C in Ahmedabad, Jaipur and Lucknow. Use local extreme-minimum data where you have it.
Common mistake: using the January mean (12–15°C in much of India). That understates cold Voc by several volts per module and can push a long string over 600 V on a winter morning.
Worked example: 550 Wp PERC on a 600 V inverter
Illustrative datasheet: Voc 49.9 V, Vmp 42.0 V, Isc 14.0 A, Imp 13.1 A, βVoc −0.27%/°C, γ −0.35%/°C.
| Design Tmin | Voc(cold) per module | Max in series at 600 V | at 1,000 V | at 1,100 V |
|---|---|---|---|---|
| 10°C | 51.9 V | 11 | 19 | 21 |
| 5°C | 52.6 V | 11 | 19 | 20 |
| 0°C | 53.3 V | 11 | 18 | 20 |
| −5°C | 53.9 V | 11 | 18 | 20 |
Hot Vmp at 70°C: 42.0 × (1 − 0.0035 × 45) ≈ 35.4 V per module.
For a 5 kW single-phase inverter (600 V max, MPPT about 80–550 V, illustrative): - 10 modules: Voc(cold, 5°C) ≈ 526 V ✓; Vmp(hot) ≈ 354 V ✓; Vmp(STC) 420 V within MPPT ✓ - Array 5.5 kWp → DC/AC 1.1 ✓
Worked example: 585 Wp TOPCon
Illustrative: Voc 52.3 V, Vmp 43.6 V, βVoc −0.25%/°C, γ −0.30%/°C.
| Design Tmin | Voc(cold) | Max at 600 V | at 1,000 V |
|---|---|---|---|
| 10°C | 54.3 V | 11 | 18 |
| 5°C | 54.9 V | 10 | 18 |
| 0°C | 55.6 V | 10 | 17 |
| −5°C | 56.2 V | 10 | 17 |
Note how the higher-voltage module drops to 10 per string at 5°C. The same "11 modules" habit from PERC would exceed 600 V on a cold Pune morning.
Hot Vmp at 70°C ≈ 37.7 V per module.
Current: the new bottleneck
Modern modules have high currents. A 182 mm-cell module has Isc around 14 A; 210 mm-cell modules can reach 18 A or more. Check:
- MPPT max input current (operating) ≥ string Imp × strings on that MPPT
- MPPT max short-circuit current ≥ string Isc × strings (with any bifacial gain)
- If the inverter's MPPT current is lower than the module Imp, the inverter clips current; you lose energy in strong sun. Many older 12.5 A inputs are a poor match for 14 A modules.
Parallel strings and fuses
- One or two strings per MPPT: fuses usually not required, because reverse current cannot exceed the module's series fuse rating.
- Three or more strings in parallel: fit gPV fuses on both poles of each string. A common rule is a fuse rating of at least about 1.5 × Isc and not above the module's maximum series fuse rating (check IEC 62548 and the datasheet).
- Parallel strings on one MPPT must have the same length, orientation and module type.
Orientation and MPPTs
- Different directions (east and west roof faces) → separate MPPTs.
- Different string lengths → separate MPPTs.
- Partial shade on one area → put it on its own MPPT.
Quick string-sizing checklist
- Get datasheet Voc, Vmp, Isc, Imp, βVoc, γ.
- Pick Tmin for the site (table above or local records) and Tcell(hot) about 70°C.
- Max modules = floor(Vmax_inverter ÷ Voc(cold)).
- Min modules = ceil(MPPT_min ÷ Vmp(hot)) with margin.
- Check MPPT current and Isc limits.
- Decide fuses for three or more parallel strings.
- Confirm DC/AC ratio (see inverter selection).
Try it with your modules and inverter:
Bifacial modules
Bifacial modules can deliver extra current from the rear side. Use the datasheet's bifacial-gain table when checking MPPT current and fuse ratings, especially on elevated structures over light roofs.
Talk to Sunlectric
Need datasheets or a second check on a string design? WhatsApp +91 89567 89307 (wa.me/918956789307) with the module and inverter models. We stock inverters with high-current MPPTs and modules across brands at our Pune hub. Sunlectric Studio checks strings against site temperature automatically, and the Academy electrical module covers string design in depth. Related: cable sizing and voltage drop.
Sources
- Design temperatures and string rules: Sunlectric Studio bom_rules and engineering review, Oct 2026
- String fusing and array design: IEC 62548 (adopted in India as IS/IEC 62548; check current edition)
- Module values are illustrative; use the manufacturer datasheet for the model you install
Questions people ask
What minimum temperature should I use for Voc in India?
Use the extreme annual minimum ambient for the site, not the January average. As a starting point: about 10°C south of 15°N, 5°C between 15°N and 24°N, 0°C above 24°N, and −5°C in hill states, above 1,000 m or above 28.5°N. Check local records.
How many 550 Wp modules can I put in a string on a 600 V inverter?
With a typical 550 Wp PERC module (Voc about 49.9 V, −0.27%/°C), the cold Voc is about 52.6 V at 5°C, so 11 modules is the maximum. Always use your actual datasheet and inverter limits.
When do I need string fuses?
When three or more strings are paralleled on one input, reverse current into a faulty string can exceed the module's series fuse rating. With one or two strings per MPPT, fuses are usually not needed. Check IEC 62548 and the module datasheet.
Why does my string voltage drop so much in summer?
Module voltage falls as cells heat up. At about 70°C cell temperature, Vmp can be 15% or more below its STC value, so a short string may fall below the inverter's MPPT minimum on hot afternoons.