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Charge controller sizing and getting the array and bank to agree

Controller current from array size and battery voltage, and what happens when the array is oversized for the bank.

The engineering question

The charge controller clips the output. Is it undersized, or is the array too large for the batteries?

Worked example

The figures below were produced by running the Charge Controller Sizing Calculator with the same inputs — during the build, from its authoritative engine.

Results
Recommended controller rating 60 A
Calculated array current 52.0833 A
Controller type MPPT
Assessment MPPT controller rated ≥ 60 A (array current at battery voltage plus a 25% margin).
  1. Array current at the battery voltage is array watts divided by bank voltage.
  2. Add 25 % headroom and round up to the next standard rating.
  3. A PWM controller harvests less energy but still needs the same controller current.

Sanity check

If output is being clipped on a sunny day, the array is larger than the controller — the fix is fewer modules or a larger controller, not a bigger bank.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

References & standards

  • NREL PVWatts — inverter and system losses
  • Manufacturer controller data sheets

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