Solar Power Calculator
Calculates the solar power required to charge battery backup systems and power telecommunications, radio, repeater, emergency, and other DC systems.
What This Calculator Does
-
Calculates required solar array power
-
Supports 12 V, 24 V, and 48 V systems
-
Uses battery capacity and daily load requirements
-
Calculates required solar panel quantity
-
Uses the selected panel wattage
-
Provides basic controller, fuse, and conductor sizing guidance
-
Designed for backup and RF communication systems
Engineering Notes
Design Recommendations For a Solar Power System
-
Account for panel aging.
-
Consider low-temperature derating.
-
Oversize solar panels for mission-critical systems.
Low-Light Array Turn-On Limits (The MPPT Awake Constraint)
Charge controllers initialize a charging state only if the operating voltage stays higher than the battery bus potential (typically 2V to 5V higher).
The Mitigation: The operating string voltage must be high enough to ensure the battery can still charge during cloudy days and peak summer heat, when solar panel voltage drops.
Extreme Cold-Weather Voltage Spikes (Voc String Planning)
PV module open-circuit voltage increases as temperature decreases because of the module's negative voltage temperature coefficient.
The Mitigation: Size the panels in series according to the panel cold temperature Voc and/or the charge controller Maximum Input Voltage Rating must be higher then possible voltage spike during cold temperatures. Use the following formula for calculation:
Total Panels in Series * Voc Tlow * 1.2 < Controller Maximum Input Voltage
Low-Voltage Disconnect (LVD) & Voltage Sag
High peak current can cause a temporary voltage drop (voltage sag) that may trigger the LVD and prevent the charging system from starting, particularly when the backup batteries are deeply discharged.
The Mitigation: Use an LVD/controller with a 30–60 second time delay or an appropriate voltage margin to prevent transient current spikes from causing an unnecessary shutdown.
📘 Solar System Sizing Calculation Workflow
The calculator determines the solar array required to supply the continuous equipment load and replenish the battery energy used during the daily discharge period. It then calculates the required panel count and provides guidance for charge-controller, overcurrent protection, and conductor sizing.
​​
Initial Parameters
Enter:
System Voltage, Power (Watts) or Current (Amperes)
Calculated backup battery capacity (use the Battery Backup Sizing Calculator)
Estimated battery discharged level %
Daily sun hours
Solar Panel Power Rating (solar panels power intended to be used)
Estimated percent power loss in wires and distribution equipment
​​
Output:
Total Battery Raw Energy:
The maximum battery bank potential energy (Ah*V).
Daily Battery Recharge Requirement:
The amount of energy (kWh) needed for recharging battery bank from scheduled nighttime discharge cycle.
Daily Equipment Energy:
The total energy (in kWh) needed for the entire 24-hour period, which must generated by solar system.
Solar Power for Battery Charging:
The solar power required for recharging the battery during the limited daylight time.
Solar Power Required for Daytime Equipment Load:
The solar power required to run the equipment during the limited daylight time.
Total Combined Solar Array Power:
The minimum solar panel installation power (including safety factor to overcome charge controller and wiring losses).
Solar Panels Required:
The panel count required to build the solar system using entered panel rating.
Estimated Battery Reserve Autonomy:
Estimated number of hours or days the equipment can run if solar panel generation is cut off.
Overcurrent Protection Device (OCPD) Guide:
Provides guide for solar system charge controller, wiring and protection
​
Frequently Asked Questions
Why can the solar charger repeatedly start and stop?
High current demand can cause voltage sag that temporarily reduces the battery or bus voltage below the controller's operating threshold. This can cause repeated starting and stopping.
Why does the solar system produce little or no power on overcast days?
The PV string voltage may fall below the charge controller's minimum start or operating voltage. Increasing the number of panels in series can increase the string voltage, but the resulting cold-weather Voc must remain below the controller's maximum input voltage.
What is an LVD?
LVD stands for Low Voltage Disconnect. It protects the battery by disconnecting the load when the battery voltage falls below a specified threshold.
Why can a solar charge controller burn out during cold weather ?
Solar panels can produce higher open-circuit voltage at low temperatures. If the PV string voltage exceeds the charge controller's maximum input voltage, the controller can be damaged.