Two checks, and the second one is the important one
Current decides whether the controller can carry what the array delivers. Voltage decides whether it survives at all. Sizing tools overwhelmingly do the first and skip the second, which is backwards — an undersized controller on current runs hot or limits output, while an overvoltage event is instant and terminal.
Current
MPPT: I ≥ 1.25 × array watts ÷ battery voltage
PWM: I ≥ 1.25 × array Isc
The two differ because the controllers work differently. A PWM controller switches the array directly onto the battery, so the array operates near battery voltage and the current through the controller is essentially array Isc. An MPPT controller converts array power down to battery voltage, so its output current is power divided by battery voltage — higher than the array’s own current, which surprises people.
The 1.25 comes from NEC 690.8(A)(1) and is not padding: modules produce above their nameplate in cold, bright conditions, which is exactly when the array peaks.
Voltage — NEC 690.7(A)(1)
Vmax = Voc(STC) × [1 + (β ÷ 100) × (Tmin − 25)]
β is the module’s temperature coefficient of Voc in %/°C, printed on the datasheet and negative for silicon — typically −0.25 to −0.35. Tmin is the lowest ambient temperature your site sees.
A module with β = −0.29 %/°C at −10 °C gains 0.29 × 35 = 10.15 % over its datasheet Voc. At −25 °C it gains 14.5 %.
NEC 690.7 also caps maximum system voltage at 600 V for one- and two-family dwellings and 1000 V for other buildings, independently of what any controller accepts.
The bank voltage decision
Same 1200 W array, three different worlds:
| Bank | Charge current needed | Typical controller |
|---|---|---|
| 12 V | 125 A | Beyond most single units |
| 24 V | 63 A | 70 A |
| 48 V | 31 A | 40 A |
Controllers are priced roughly by current rating, so this is a direct cost consequence — and it compounds with the cable savings a higher bank voltage brings.
String layout is a trade-off, not an optimisation
More modules in series raises string voltage, which lowers current, which makes the run from the array dramatically cheaper in copper. It also raises the cold-weather voltage, which is the thing that kills controllers.
More strings in parallel does the reverse: safe on voltage, expensive on wire, and past two strings you generally need a fuse per string because the others can back-feed a fault in one.
The right answer is the highest series count that stays comfortably under the controller’s input limit at your coldest temperature. The calculator reports that number directly, so you do not have to iterate.
What the controller rating does not tell you
Maximum PV input power is a separate specification from output current on most MPPT units, and some controllers also specify different maxima per battery voltage. Read both.
Overpaneling — connecting more array than the controller can pass — is permitted by many manufacturers, with output simply clipped at the rating. It can be a sensible choice for a fixed array that rarely reaches full output. It is manufacturer-specific, so check the datasheet, and note that it does nothing to relax the voltage limit.