Sizes an off-grid PV charge controller's required continuous current rating. PWM controllers track close to the array's short-circuit current, so sizing is based directly on Isc × parallel strings. MPPT controllers regulate power on the DC bus and draw current based on power balance, so sizing is based on array maximum power ÷ nominal battery voltage. Both paths apply the standard NEC 690.8(A) 125% continuous-current factor plus a user-settable design margin.
Standards followed
NEC 690.8(A) — 125% continuous-current factor for PV source/output circuits
What it can do
- PWM path: controller current from array Isc × parallel strings.
- MPPT path: controller current from array max power ÷ nominal battery voltage.
- 125% continuous factor plus an additional user-settable design margin.
- Rounds up to a commonly available charge controller current rating (practical reference, not an official standard series).
Example problem
MPPT controller, 4000W array, 48V nominal battery, 10% additional design margin.
- Base current = 4000/48 ≈ 83.33A
- Required rating = 83.33 × 1.25 × 1.10 ≈ 114.6A
≈114.6A required — 150A commonly available controller recommended.
MPPT controller current draw varies with array operating point and battery state of charge — this is a first-pass sizing estimate based on rated array power, not a full I-V curve tracking simulation. Always confirm against the specific controller manufacturer's maximum PV input voltage/power and battery-side current limits.