Cold string voltage
Panel Voc rises as temperature falls. Correct the panel Voc for the lowest expected temperature, then multiply by panels in series.
The four checks
Wattage alone does not size a controller.
The panel label supplies the electrical values. The battery bank and proposed wiring determine what the controller will actually see.
Operating voltage
Series wiring adds Vmp. The operating string voltage must stay high enough above battery charging voltage for the MPPT controller to work properly.
Array current
Parallel strings add current. String current does not increase when identical panels are placed in series.
Charging output
The controller converts higher PV voltage into battery-charging current. Lower-voltage battery banks require more controller output current for the same array wattage.
Interactive design check
Charge-controller sizing calculator
Enter the values from one panel’s label, the proposed array wiring, and the battery-bank voltage.
1Panel specifications
2Array and battery
Your preliminary result
Recalculates automatically as values change.
Cold-corrected string Voc——
Operating string voltage—Series Vmp at STC
Array power——
Estimated peak charge current——
Array operating current—Parallel strings add Imp
PV source-circuit check—125% of combined Isc
Proposed wiring—
—
Controllers that fit this preliminary calculation
This calculator is an educational preliminary check—not a permit design or substitute for the current manufacturer manual. Confirm maximum PV open-circuit voltage, MPPT operating range, input-current limits, maximum array wattage, conductor ampacity, overcurrent protection, disconnects, grounding, and applicable electrical requirements before installation.
How the calculation works
Series raises voltage. Parallel raises current.
Cold-temperature voltage correction
PV module Voc is rated at 25°C cell temperature. Because voltage rises in cold weather, the coldest expected condition—not the average daytime temperature—sets the maximum series-string length.
Cold Voc = Voc × [1 + (Voc coefficient ÷ 100) × (cold °C − 25°C)]
Then multiply the corrected module Voc by the number of modules in series. That result must remain below the controller’s absolute PV-input limit. Good designs leave practical headroom rather than landing on the limit.
What series wiring does
With identical panels in series, voltage adds while current stays approximately equal to one panel.
Panel 1
Panel 2
Panel 3
String Voc = corrected panel Voc × panels in series
String Vmp = panel Vmp × panels in series
String Vmp = panel Vmp × panels in series
What parallel wiring does
When identical series strings are paralleled, string current adds while string voltage stays the same.
String 1 →
String 2 → combiner → controller
String 3 →
Array Imp = panel Imp × parallel strings
Array Isc = panel Isc × parallel strings
Array Isc = panel Isc × parallel strings
Why battery voltage changes controller size
An MPPT controller converts PV power to the lower battery voltage. The same PV array produces roughly twice the charging current into a 24-volt bank as it does into a 48-volt bank.
Estimated output amps = array watts ÷ charging voltage
- Use the actual battery charging voltage—not only the nominal bank voltage.
- Check the battery manufacturer’s maximum allowable charge current.
- Large arrays may need two or more controllers with the array divided between them.
Worked example
How a complete answer is built
Example input
- Panels
- 12 × 440 W
- Panel Voc
- 41.0 V
- Panel Vmp
- 34.4 V
- Voc coefficient
- −0.26%/°C
- Lowest temperature
- 50°F / 10°C
- Battery bank
- 48 V
- Proposed wiring
- 3S × 4P
First check the voltage
At 10°C, each panel’s corrected Voc is about 42.6 volts. Three in series produce about 127.8 volts. That fits beneath a 150-volt controller limit with useful headroom; four in series would not.
Then check current and wattage
The four parallel strings add their current. Total array power is 5,280 watts. At a 57.6-volt charging target, the array represents about 91.7 amps before real-world losses and controller limiting.
Finally choose and divide the equipment
The voltage result allows a 150-volt-class controller, but the output calculation eliminates undersized models. The final design may use one controller with adequate output and permitted array wattage, or divide the four strings evenly between two controllers.
Never solve an over-voltage string by assuming wire loss will protect the controller. Open-circuit voltage occurs with essentially no load, so conductor voltage drop is not a dependable safeguard.