SolarMathKit

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Controller sizing

Solar charge controller calculator

Enter panel-datasheet values and the proposed series-parallel layout. The result keeps cold PV voltage, PV input current, and battery-side output current separate.

Size the controller limits

Use the electrical values and temperature coefficient from the panel datasheet.

Planning headroomEditable planning values
Check a controllerOptional documented limits

Your planning estimate

Local calculation

Enter your values, then calculate. Results will show the formula inputs and rounding used.

Formula used

cold array Voc = panel Voc × series count × [1 + Voc coefficient × (cold °C − 25°C)]; output A = array W ÷ battery charging V

Voltage headroom is applied after the panel's documented Voc temperature correction. Output-current and PV short-circuit-current limits are calculated independently because they refer to different sides of an MPPT controller.

See the full assumptions and rounding policy.

Maintenance and verification

Maintained by: SolarMathKit Formula version: 1.3.0 Last reviewed:

The calculator calls the shared, tested TypeScript calculation modules documented in the methodology. Automated tests cover published examples, invalid inputs, rounding, and result invariants.

SolarMathKit maintains this implementation. The review is an internal technical check, not an independent engineering certification.

Worked example: four 400W panels as 2S × 2P

Each panel has 49.5V Voc, 41.5V Vmp, 10.4A Isc, 9.65A Imp, and a −0.27%/°C Voc coefficient. The cold design temperature is −10°C and the battery charges at 28.8V.

  1. Array power: 4 × 400W = 1,600W.
  2. Cold panel Voc: 49.5 × [1 + (−0.0027 × (−10 − 25))] ≈ 54.18V.
  3. Cold string Voc: 54.18V × 2 in series ≈ 108.36V.
  4. With 10% voltage headroom, the planning minimum is about 119.2V.
  5. With 25% current headroom, output-current planning minimum is 1,600W ÷ 28.8V × 1.25 ≈ 69.4A.
Result: Look for documented ratings above every calculated minimum, then verify the complete operating window in the current controller manual.

Common mistakes to avoid

  • Sizing only from array watts and nominal battery voltage.
  • Adding current for panels in series or adding voltage for parallel strings.
  • Ignoring the increase in Voc at low temperature.
  • Treating a model-name voltage/current pair as the controller's complete specification.

Questions people ask

Why use battery charging voltage instead of nominal voltage?

MPPT output current is related to the actual power-conversion voltage. Using only 12V, 24V, or 48V can distort the battery-side current estimate.

Does this choose a controller model?

No. It calculates minimum planning limits and optionally checks the limits you enter. Product databases change and cannot replace the current manufacturer datasheet.

Is the coldest air temperature the same as cell temperature?

Not always. Use the equipment manufacturer's required design method and a defensible site temperature. This calculator applies the coefficient you enter to the stated planning temperature.

Does this size fuses or cables?

No. Conductor ampacity, voltage drop, overcurrent protection, disconnects, grounding, and local rules remain outside this tool.

Before you build: Check voltage, current, temperature, protection, cable sizing and installation rules with the equipment manuals and a qualified professional.