Solar Charge Controller Sizing
MPPT or PWM amps for your panel array and battery bank.
About
Required controller amps = array wattage ÷ battery voltage × safety margin. The margin (typically 25%) covers cold-weather voltage boost, irradiance spikes and panel-to-controller voltage drop.
How it works
- Sum your array's STC wattage. For mixed orientations, sum the panels you want on one MPPT input.
- Pick your battery bank voltage: 12 V for small DIY, 24 V for medium, 48 V for anything > 1 kW.
- Apply 25% safety margin (NEC default). Round up to the next standard model (30 A, 40 A, 60 A, 100 A).
Frequently asked questions
Should I get an MPPT or PWM controller?+
MPPT for any system above 200 W or where panel voltage doesn't match battery voltage. PWM only for tiny 12 V trickle systems with a matched 12 V panel. For 90% of off-grid installs in 2026, MPPT is the right choice.
What size MPPT for a 400 W solar array on a 12 V battery?+
400 W ÷ 12 V = 33 A theoretical. With 25% margin → 41 A. Round up to a 40 A or 50 A MPPT charge controller. Make sure max input voltage handles your panels' Voc × 1.25 cold-temp factor.
Can I parallel two charge controllers?+
Yes, on the battery side, with separate panel arrays per controller. Most modern controllers communicate over CAN/RS485 to coordinate. Avoid combining their outputs into one DC bus without isolation.
Why does my array produce less than its rated wattage?+
Real-world output is 75–85% of STC rating due to temperature, irradiance and wiring losses. A 400 W array typically peaks at 320–340 W on a clear, cool day.