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Solar Charge Controller Sizing Calculator

Work out solar charge controller sizing instantly with clear inputs, formula shown and shareable results.

About Solar Charge Controller Sizing Calculator

An MPPT controller's current rating is array power divided by battery voltage, with 25% added for irradiance above standard test conditions. Equally important is the input voltage limit: open-circuit voltage rises in cold weather, so the string Voc must be multiplied by about 1.25 before comparing with the controller's maximum.

Formula

Charge controller

I = array W / battery V, x1.25 margin; check string Voc x 1.25 against the controller input limit

Worked example

With the default values below, Solar Charge Controller Sizing Calculator returns Controller current rating of 37.50. Change any input above and every figure here is recalculated in your browser.

Inputs used

Array power
1,800 W
Battery voltage
48 V
Panel open-circuit voltage
49.5 V
Panels in series
3 panels

Results

Controller current rating
37.50 A
String open-circuit voltage
148.50 V
Cold-weather Voc
185.63 V
Recommended controller size
50 A

Step by step

  1. 1Values used
  2. 2Charge controller
  3. 3Controller current rating= 37.50 A
  4. 4String open-circuit voltage= 148.50 V
  5. 5Cold-weather Voc= 185.63 V
  6. 6Recommended controller size= 50 A

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why multiply Voc by 1.25?

Panel voltage rises as temperature falls, roughly 0.3% per degree below 25 C. On a cold clear morning Voc can exceed the rating and destroy the controller.

MPPT or PWM?

MPPT harvests 15-30% more energy and allows higher string voltages. PWM is only sensible for very small systems where cost dominates.

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