Free Tool
MPPT Charge Controller Calculator
Match your solar array to the right MPPT controller. Enter your panel watts, battery bank voltage and string layout to get the amp rating and PV input voltage your controller needs.
Size an MPPT controller by output current: divide your total panel wattage by your battery voltage and add about 25 percent headroom. Then check the array’s cold-weather open-circuit voltage stays below the controller’s maximum PV input, because voltage rises as temperature drops.
Interactive Tool
MPPT Charge Controller Calculator
Printed on the panel label
Recommended controller
40A / 53V
A 40A controller with at least 53V PV input. 400W ÷ 12V = 33.3A charge current.
Victron naming tip: a SmartSolar 100/30 accepts up to 100V of panels and outputs 30A, so pick a model where the first number is at least 53 and the second at least 40.
Shop MPPT controllersIncludes a 20% cold weather margin on PV voltage, as panels exceed their rated Voc in cold conditions. Always check the controller datasheet before finalising your array.
How this is calculated
An MPPT controller is sized on two numbers. The first is output current: the controller converts your array's power down to battery voltage, so the maximum charge current is array watts divided by battery voltage. A 400W array on a 12V bank can push around 33A into the batteries, which we round up to the next standard controller size, 40A in that example.
The second is PV input voltage. Panels wired in series add their voltages together, and on a cold, bright morning a panel's open-circuit voltage can climb well above the figure on the label. We multiply your series string voltage (panels in series × Voc) by a 1.2 safety margin, because exceeding a controller's PV input limit even briefly can destroy it.
Controller model names usually encode both limits. A Victron SmartSolar 100/30, for instance, accepts up to 100V from the panels and outputs up to 30A of charge current. Pick a model where both numbers meet or exceed the figures this calculator gives you.
Frequently asked questions
What size MPPT charge controller do I need?
Divide total solar wattage by battery bank voltage to get the charge current, then add roughly 25 percent headroom. A 400W array on a 12V bank needs about a 40A controller. The calculator also checks the voltage limit for you.
MPPT or PWM: which should I choose?
MPPT for almost all systems. It converts excess panel voltage into extra charging current, typically harvesting 20 to 30 percent more than PWM, and it allows higher-voltage arrays. PWM only makes sense for tiny, low-voltage setups.
Why does cold weather voltage matter?
A panel’s open-circuit voltage rises as it gets colder. On a frosty, sunny UK morning an array can exceed its rated voltage, and if that tops the controller’s maximum PV input it can be damaged. Always size with cold-weather headroom.
Can I add more panels to my controller later?
Only up to the controller’s current and voltage limits. If you plan to expand, size the controller for the future array now, or wire additional panels through a second controller.
