A string is a set of modules wired in series. Their voltages add up and the current stays the same. String design means choosing how many modules to put in series so that the inverter is safe on the coldest morning and still working well on the hottest afternoon. Get it wrong one way and you can burn the inverter's input stage. Get it wrong the other way and the inverter drops out of MPPT every summer afternoon, which is exactly when the customer is watching the app.
Why temperature changes voltage
Module voltage falls as the cell gets hotter and rises as it gets colder. The datasheet gives this as a temperature coefficient, usually written as β(Voc) in %/°C. For mono PERC and TOPCon modules it is typically around −0.25 to −0.30 %/°C. The datasheet values (Voc, Vmp) are at STC: 25 °C cell temperature and 1,000 W/m².
Two cases matter:
- Cold dawn, sun just up: the cells are as cold as the air, the module is open circuit because the inverter has not started yet, and Voc is at its highest. This sets the maximum number of modules.
- Hot afternoon at full load: the cells may be 30 °C above air temperature. Vmp is at its lowest. This sets the minimum number of modules.
The formula is the same for both:
V(T) = V(STC) × [1 + β × (T − 25)]
where β is written as a fraction (−0.27 %/°C = −0.0027).
Choosing design temperatures in India
Use the lowest ambient temperature the site sees in a typical year for the cold case. Cell temperature at first light is close to air temperature.
| Region | Typical design minimum | Typical hot-day cell temperature |
|---|---|---|
| Kerala, coastal Tamil Nadu | 15 °C | 65 °C |
| Mumbai, Hyderabad, Bengaluru | 8–10 °C | 65–70 °C |
| Pune, Nashik, Nagpur, Indore, Bhopal | 3–5 °C | 70 °C |
| Delhi, UP, Rajasthan plains | 2–5 °C | 70–75 °C |
| Hill towns: Shimla, Darjeeling | −5 °C | 55–60 °C |
| Ladakh, Spiti | −20 °C or lower | 55 °C |
These are design values for teaching. For a real project, use the local weather station record or the design software's climate file, and stay on the safe side for the cold figure. A flush-mounted module on a tin shed runs hotter than one on an elevated RCC structure with air underneath, so add 5 °C to the hot case for flush mounts.
Worked example: 5 kW single-phase inverter in Delhi
Module (example datasheet, 540 Wp mono PERC): Voc 49.5 V, Vmp 41.6 V, Isc 13.85 A, Imp 12.98 A, β(Voc) −0.27 %/°C, Pmax coefficient −0.35 %/°C.
Inverter (example): maximum DC input 600 V, MPPT range 80–550 V, full-power MPPT range about 200–500 V, start-up voltage 100 V, two MPPTs, maximum input current 15 A per MPPT.
Design temperatures: 5 °C cold, 70 °C hot.
Step 1: maximum modules (cold Voc)
Voc at 5 °C = 49.5 × [1 + (−0.0027) × (5 − 25)] = 49.5 × 1.054 = 52.17 V
Maximum modules = 600 ÷ 52.17 = 11.5, so round down to 11 modules. Check: 11 × 52.17 = 573.9 V, below 600 V.
Step 2: minimum modules (hot Vmp)
Many datasheets do not give a Vmp coefficient. The Pmax coefficient is a reasonable stand-in because most of the power loss with heat comes from voltage.
Vmp at 70 °C = 41.6 × [1 + (−0.0035) × (70 − 25)] = 41.6 × 0.8425 = 35.05 V
To stay in the full-power MPPT range: 200 ÷ 35.05 = 5.7, so round up to 6 modules. Below six, the inverter can still track power (its MPPT goes down to 80 V), but at full sun it may not reach rated output because the current limit is hit first.
Step 3: current check
Isc is 13.85 A against a 15 A input limit. One string per MPPT works. Bifacial modules on a white roof can add 5–10% to current, so 13.85 × 1.10 = 15.2 A would slightly exceed the limit. Most inverters simply clip the extra current, but check the inverter manual: some quote a separate "maximum short-circuit current" that must not be exceeded.
Step 4: choose the layout
The customer wants about 5.4 kWp, which is 10 modules. Options:
- One string of 10 on MPPT 1: cold Voc 521.7 V, hot Vmp 350.5 V, STC Vmp 416 V. All inside the windows. Good if all modules face the same way.
- Two strings of 5, one per MPPT: hot Vmp 175 V, which is below the full-power range. Acceptable only if the roof has two orientations or different shading and you need the separate MPPTs.
The single string of 10 is the better choice on a clean, south-facing roof.
Try it yourself
Change the city and watch the cold Voc move. In Leh, the same 11-module string that is fine in Delhi goes over 600 V.
Common mistakes
- Using 25 °C for the cold case. Voc at STC hides the 5–10% rise on a winter dawn. Inverters that see over-voltage may survive once and fail the next winter, and the warranty will not cover it.
- Rounding up the maximum. 11.5 means 11, never 12.
- Forgetting module and cable voltage ratings. A 1,500 V inverter does not help if the modules, MC4 connectors or DC isolator are rated 1,000 V.
- Mixing strings of different lengths on one MPPT. Two strings in parallel on the same MPPT must have the same number of identical modules facing the same way. Otherwise the lower-voltage string drags the other one down.
- Ignoring DC/AC ratio. A 5.4 kWp array on a 5 kW inverter (1.08) is normal. Going to 1.3 or more on a south-facing Indian roof will clip power at noon in March and April. Check what the inverter maker allows.
Key points
- Cold mornings set the maximum string length: correct Voc to the site's lowest temperature and round down.
- Hot afternoons set the minimum: correct Vmp to a realistic cell temperature (ambient + 25–30 °C) and round up.
- Check string Isc against the inverter's maximum input current per MPPT, with headroom for bifacial gain.
- Every DC component must be rated for the cold-corrected Voc, not just the inverter.
- Parallel strings on one MPPT must be identical in length, model and orientation.
Check yourself
- A module has Voc 50.0 V and β(Voc) −0.28 %/°C. What is Voc at 0 °C?
- With that module, how many can go in series on an inverter with a 1,000 V maximum, for a site whose design minimum is 0 °C?
- Why does the hot-case check use Vmp and not Voc?
Answers
- 50.0 × [1 + 0.0028 × 25] = 50.0 × 1.07 = 53.5 V.
- 1,000 ÷ 53.5 = 18.7, so 18 modules.
- In the afternoon the inverter is running and holding the string at its maximum power point, so Vmp is the voltage it actually sees. If hot Vmp falls below the MPPT range, the inverter loses power or stops.