MPPT Charge Controller Calculator
By Frank Zale · Last updated
Size the charge controller for a van, RV or caravan solar array. Enter your total panel watts and battery voltage, and this gives the amperage with the cold-weather margin already applied.
Add up the rated watts of every panel. Wiring them in series or parallel does not change the total, so it does not change the controller amperage.
Controller size
50 A
- Calculated amps
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This array exceeds a single 100 A controller. Split it across two controllers, or move to a higher system voltage.
Show the math
The formula
Controller amps = array watts ÷ battery voltage × 1.25, rounded up to the next standard size: 20, 30, 40, 50, 60, 80 or 100 A.
The 1.25 is not padding for its own sake. Solar panels produce above their rated output when the cells are cold, so a 400 W array on a bright winter morning can briefly behave like a larger one. A controller sized exactly to the calculated figure will current-limit at exactly the moment the array is doing its best work.
The specification people forget
Amperage is only half of it. Every controller also has a maximum PV input voltage, commonly 100V or 150V. Panels wired in series add their open-circuit voltages together, and that voltage rises as temperature falls. Multiply panel Voc by the number in series, add about 25% cold margin, and keep the result under the controller limit. Exceeding it does not trip anything: it destroys the controller, and that is not covered by warranty.
Why MPPT rather than PWM
A PWM controller pulls panel voltage down to battery voltage and loses the difference as heat. An MPPT controller converts that surplus voltage into usable current, which is worth roughly 15 to 25% more harvest. On a 400 W array that difference is about what a compressor fridge uses in a day.
For the reasoning in full see what size MPPT charge controller, or read MPPT vs PWM compared.
Frequently asked questions
＋ What size MPPT controller do I need for 400W?
At 12V, 400 W needs 400 ÷ 12 × 1.25 = 41.7 A, so a 50 A controller. The same array on a 24V battery needs only about 21 A, so a 30 A controller is enough.
＋ Why multiply by 1.25?
Panels can briefly exceed their rated output when cells are cold and the sun is strong, typically on a clear morning after a frost. Sizing exactly to the calculated amperage means the controller clips output on precisely the days your array performs best.
＋ Can a charge controller be too big?
No. An oversized controller simply runs below its maximum, with no efficiency penalty. The only cost is the purchase price, and it leaves room to add panels later without replacing the unit.
＋ Does wiring panels in series change the controller size?
Not the amperage. The controller amps follow total array watts and battery voltage, so series or parallel wiring gives the same answer. Series wiring does change the PV input voltage the controller sees, which must stay under its maximum rating.