How to size an MPPT controller
Two numbers matter. The amp rating is the charging current into the battery: array watts divided by battery voltage. The PV voltage rating must exceed the array's open-circuit voltage on the coldest morning, because panel voltage rises as temperature drops.
Example
Four 400 W panels (2 in series × 2 strings) = 1,600 W. On a 24 V battery: 1600 ÷ 24 = 66.7 A, so a 70 A controller. Cold Voc at 14°F (−10°C): 37 × 2 × 1.14 = 84.4 V, so a 100 V model (a 100/70 class controller).
NEC 690.7 temperature factors
| Lowest temperature | Factor |
|---|---|
| 32 to 39°F (0 to 4°C) | 1.10 |
| 14 to 23°F (−10 to −5°C) | 1.12–1.14 |
| −4 to 5°F (−20 to −15°C) | 1.16–1.18 |
| −22 to −13°F (−30 to −25°C) | 1.20–1.21 |
| −40 to −31°F (−40 to −35°C) | 1.23–1.25 |
Higher battery voltage means fewer amps for the same array: 1,600 W is 133 A at 12 V but only 33 A at 48 V.
Frequently asked questions
What size charge controller for 400 watts of solar?
About 33 A on a 12 V battery (a 35–40 A controller) or 17 A on 24 V (20 A controller).
Can I connect more solar than the controller rating?
Most MPPT controllers allow some over-paneling and clip the extra power, but the cold-weather Voc must never exceed the PV voltage limit.
MPPT or PWM?
MPPT harvests 15–30% more energy, especially in cold weather and when the panel voltage is much higher than the battery's.