"How many modules per string?" is the most consequential arithmetic in PV design: get it wrong on the high side and cold-morning voltage destroys inverters (and voids warranties); get it wrong on the low side and strings drop out of the MPPT window on hot afternoons. The answer comes from three constraints checked at your site’s temperature extremes — never at STC.
The three constraints
| # | Constraint | Condition | Checked at |
|---|---|---|---|
| 1 | Maximum system voltage | String Voc(cold) ≤ 1000 V or 1500 V | Coldest expected morning |
| 2 | MPPT window | String Vmp(hot) ≥ inverter MPPT minimum | Hottest operating afternoon |
| 3 | Current limit | String current ≤ MPPT max input | Peak irradiance + bifacial gain |
Constraint 1: cold-day Voc (the hard ceiling)
Voc rises as temperature falls, per the module's Voc temperature coefficient (typically −0.24 to −0.28%/°C). Correction formula:
Voc(cold) = Voc(STC) × [1 + βvoc × (Tmin − 25)]
Worked example — 620 W TOPCon module, Voc 55.4 V, βvoc = −0.25%/°C, site minimum −10°C:
- Voc(cold) = 55.4 × [1 + 0.0025 × 35] = 55.4 × 1.0875 = 60.25 V
- 1000 V system: 1000 ÷ 60.25 = 16.6 → max 16 modules
- 1500 V system: 1500 ÷ 60.25 = 24.9 → max 24 modules
Always round down. Exceeding maximum system voltage even briefly voids module and inverter warranties and violates electrical codes. Use the site's record low, not the average winter morning.
Constraint 2: hot-day Vmp (the soft floor)
Vmp falls with heat. With cell temperatures reaching 60–70°C (see our temperature guide), a string that is too short slides below the inverter's minimum MPPT voltage and the tracker loses the operating point during peak production.
Same module, Vmp 46.1 V, βvmp ≈ −0.30%/°C, cell temperature 65°C:
- Vmp(hot) = 46.1 × [1 − 0.0030 × 40] = 46.1 × 0.88 = 40.6 V per module
- Inverter MPPT minimum 500 V (typical C&I): 500 ÷ 40.6 = 12.3 → minimum 13 modules
Result for this example: 13–16 modules per string on 1000 V, 13–24 on 1500 V. Within that band, longer is generally better — fewer strings, less cabling, lower BoS cost (the same logic as the 1500 V transition itself).
Constraint 3: current
Modern high-power modules deliver 14–18 A Isc. Check two limits: the MPPT's maximum operational input current, and its maximum short-circuit rating. For bifacial modules add the rear-side boost — a 620 W bifacial at 15% rear gain effectively presents ~16–17 A where the datasheet says 14.5. Some older inverters cap at 12.5 A per input and force one string per MPPT or Y-branch restrictions.
Practical rules that prevent rework
- Use ASHRAE extreme-low or local meteorological record minimum for Tmin — not the January average.
- Keep all strings on one MPPT identical in length and orientation; mismatch wastes the tracker.
- Re-run the check for the actual module bin you procure — Voc shifts between bins of the same model family.
- Document the calculation; grid operators, inspectors and insurers increasingly ask for it.
- In design software, verify with hourly simulation — the formulas above bound the answer; simulation confirms energy inside the window across the whole year.
Econo Solar checks string design compatibility against Sungrow inverter limits on every BOM we quote — send your module choice and site temperatures and we'll return the string table with the pricing.