The rules
Series: Voc and Vmp add · Isc and Imp unchanged
Parallel: Isc and Imp add · Voc and Vmp unchanged
Power: module watts × total modules, either way
Total power does not depend on the arrangement. What depends on it is everything downstream: the conductor size, the controller you can use, and whether the array survives winter.
The trade-off, concretely
Twelve 400 W modules — Voc 49.5 V, Vmp 41.5 V, Isc 10.3 A — 100 ft from the equipment:
| Layout | String Voc | Array Isc | Consequence |
|---|---|---|---|
| 1 series × 12 parallel | 49.5 V | 124 A | Enormous conductors. Fuse per string mandatory. |
| 3 series × 4 parallel | 148.5 V | 41 A | Workable. Fits a 150 V controller only in a warm climate. |
| 4 series × 3 parallel | 198 V | 31 A | Comfortable current, needs a 250 V+ controller. |
| 6 series × 2 parallel | 297 V | 21 A | Cheapest wire. Needs a high-voltage controller and no string fuses. |
Same 4800 W in every row. The wire bill differs by a factor of six.
The cold-weather check
This is what turns the table above from an optimisation into a design decision.
Vmax = Voc(STC) × [1 + (β ÷ 100) × (Tmin − 25)]
β is the module’s temperature coefficient of Voc from the datasheet, negative for silicon. Because both β and (Tmin − 25) are negative below 25 °C, their product is positive and the voltage rises.
At −10 °C with β = −0.29 %/°C, the multiplier is 1.1015 — a 10 % rise. That 148.5 V three-panel string becomes 164 V, which is over a 150 V controller limit even though it looked fine on the datasheet.
NEC 690.7 also sets an absolute ceiling: 600 V for one- and two-family dwellings, 1000 V for other buildings. Those apply regardless of what your equipment accepts.
Practical rules that follow
Two strings need no string fuses. With only one other string able to back-feed a fault, the fault current cannot exceed what the conductors already carry. Three or more generally do, and that fuse must not exceed the module’s maximum series fuse rating.
Shading behaves differently in each layout. A shaded module in a series string drags the whole string’s current down, because series elements must pass the same current. Bypass diodes limit the damage but do not eliminate it. In parallel, a shaded module only affects its own string. If shading is unavoidable and moves across the array during the day, more parallel strings — or module-level electronics — is the mitigation.
MPPT wants voltage headroom. An MPPT controller can only convert downward, so the string’s operating voltage must stay comfortably above the battery voltage even on the hottest day, when Vmp falls. A string that just clears the battery voltage at 25 °C may drop below it at 60 °C cell temperature and stop charging in the middle of a hot afternoon.