CALL US TODAY
Open M-F, 9-5 CST
Charge controller sizing is one formula and one voltage check. Get both right and the controller runs cool for years; get either wrong and you either cap your solar harvest or risk damaging the controller. This guide gives you the formula, worked examples for common array sizes, and the controllers we carry at Krato Power for each.
Controller amps = total solar watts, divided by battery voltage, times 1.25.
The 1.25 is a safety margin, because panels can briefly produce more than their rated output on cold, bright days. Then round up to the next controller size: common ratings are 10A, 20A, 30A, 40A, 50A, 60A, 80A, and 100A.
The battery voltage matters as much as the panel wattage. The same array needs half the controller amps on a 24V bank as on a 12V bank, and a quarter on 48V. That is one reason larger systems move to higher battery voltages.
Amps are only half of it. Your panel string's open-circuit voltage (Voc) must stay under the controller's maximum input voltage, and Voc rises in cold weather. Add the Voc of panels wired in series, then leave headroom for your coldest mornings.
Input limits vary a lot by controller. For example, the Outback FLEXmax 80 accepts up to 150VDC, the Outback FLEXmax FM100 up to 300VDC, and the EG4 100A MPPT up to 500VDC. Check the spec sheet of any controller before you wire a long series string into it.
Yes. A controller only pushes the current your panels actually produce, so going one size up costs a little more but gives you room to add panels later. Undersizing is the real problem: the controller caps your output and runs hot.
Prices are current at Krato Power. Browse them all in Solar Charge Controllers.
RV, van, and boat (12V, up to about 400W):
Cabins and mid-size systems (12V or 24V, about 400W to 1,500W):
Off-grid homes (48V, 1,500W and up):
Not sure whether you need MPPT or PWM? Read our MPPT vs PWM guide. Building a full system? See 48V Server Rack Batteries and Solar Panel Kits.
On a 12V battery, about 42A minimum, so a 50A or 60A controller. On a 24V battery, about 21A, so a 30A controller works.
On 12V, about 104A, which is more than most single controllers handle, so move to 24V or 48V. On 24V, about 52A (choose 60A). On 48V, about 26A (choose 30A or 40A).
Not in a harmful way. An oversized controller only delivers what your panels produce. The tradeoff is cost, and the benefit is room to expand.
Most RV arrays run 200W to 600W on a 12V lithium bank, which points to a 30A to 60A MPPT controller. Use the formula with your exact panel watts.
Send the Krato Power team your panel wattage, panel Voc, how the panels are wired, and your battery voltage, and we will confirm the right controller. Call 888-855-7286 or browse Solar Charge Controllers.
This is general information, not electrical or professional advice. Have a licensed electrician verify wiring, fusing, and installation per local code.