Cold-Weather String Sizing on a Mixed-Orientation Roof
A homeowner has 16 identical 410 W modules: 10 on a south roof plane and 6 on a west plane. The proposed 5.0 kW inverter has two MPPTs, a 600 V maximum DC input, a 90–550 V MPPT window, and a 15 A maximum input current per MPPT. Module data at STC are Voc 37.4 V, Isc 11.2 A, Imp 10.5 A, and βVoc −0.27%/°C. The record low design temperature is −18°C. The installer proposes one 16-module string to avoid a second home run, arguing that 16 × 37.4 V = 598.4 V, which is below 600 V. The roof has no shade and no room to add modules. The homeowner wants maximum annual production and is concerned about clipping. The AHJ requires string voltage calculations at record low temperature and compliance with inverter input limits.
1. Calculate the coldest-expected module Voc and the maximum number of modules allowed in one string at the inverter 600 V limit. Does the proposed 16-module string comply?
Read the worked answer
Use the temperature coefficient to correct Voc from 25°C to −18°C.
ΔT = −18°C − 25°C = −43°C
Voc,cold = 37.4 V × [1 + (−0.0027/°C)(−43°C)] Voc,cold = 37.4 V × [1 + 0.1161] = 41.74 V
Maximum modules at 600 V = floor(600 V ÷ 41.74 V) = floor(14.37) = 14 modules.
A 16-module string would reach 16 × 41.74 V = 667.8 V at record low temperature, well above the 600 V maximum. The installer's arithmetic used STC Voc and ignored cold-temperature voltage rise. The 16-module string does not comply. Also note that 14 modules would be 584.4 V, which is under 600 V but above the 550 V MPPT upper window; if the MPPT window is treated as an operating limit, the practical maximum is 13 modules, at 542.6 V cold.
2. Propose a compliant string and MPPT arrangement for all 16 modules, including a current check. Explain why paralleling strings on one MPPT is not acceptable.
Read the worked answer
Put the 10 south modules on MPPT 1 and the 6 west modules on MPPT 2.
MPPT 1 cold Voc = 10 × 41.74 V = 417.4 V. This is inside the 90–550 V window and below 600 V.
MPPT 2 cold Voc = 6 × 41.74 V = 250.4 V. This is also inside the window and below 600 V.
Current check: each MPPT has one string with Isc = 11.2 A, below the 15 A maximum input current per MPPT. If the two strings were paralleled on one MPPT, the combined Isc would be about 22.4 A, exceeding the 15 A input limit. That is not acceptable even though the voltage would be below 600 V.
Separate MPPTs also handle the orientation mismatch better: south and west planes produce at different times, and independent MPPT tracking reduces mismatch loss. The array is 6.56 kW DC on a 5.0 kW inverter, a DC/AC ratio of 1.31; the installer should verify the inverter's allowed DC/AC ratio and clipping behavior, but the electrical string limits are met.
3. The supply house substitutes a single-MPPT inverter with the same 600 V maximum and a 15 A input limit. What arrangement, if any, could use all 16 modules? What is the practical conclusion?
Read the worked answer
A single MPPT with a 15 A input limit cannot accept two parallel strings, because 2 × 11.2 A = 22.4 A exceeds 15 A. A single series string is limited by voltage: at 600 V the absolute maximum is 14 modules, and at the 550 V MPPT upper window the practical maximum is 13 modules. Thirteen modules would be 542.6 V cold, leaving 3 modules unused. That is not a full 16-module solution and would also mix south and west modules on one string, causing orientation mismatch.
Therefore, with a single-MPPT 15 A inverter, there is no compliant way to use all 16 modules at full production. The practical conclusion is to keep the two-MPPT inverter or change the array layout/module count. Do not simply parallel the strings and rely on the inverter to current-limit; the input current rating is a limit, not a control setpoint.