Solar Panel Configuration
MPPT for higher efficiency, PWM for simplicity
Temperature Compensation & Environmental
Used to adjust Voc for cold conditions
Panel Voc change per degree (typically negative)
Leave 0 for sea level
Typical: 5-15% for most installations
Battery & Safety Configuration
Extra buffer to handle overproduction
From panels to charge controller
Calculation Results
Calculation Formulas
-
Adjusted Voc (per panel): Accounts for increased voltage in colder conditions.
Adjusted Voc = Voc × [1 + (TempCoeff% ÷ 100) × (25 − Lowest Temp °C)] -
Total Array Voltage: Maximum voltage of panels connected in series (must stay below the controller’s max voltage rating).
Total Array Voltage = Adjusted Voc × Panels in Series -
Total Array Current: Combined short-circuit current (Isc) of all parallel strings.
Total Array Current = Isc × Panels in Parallel -
Controller Amps (MPPT): Based on solar power output divided by battery voltage.
MPPT Amps = (Total Array Power ÷ Battery Voltage) × (1 + Margin%) -
Controller Amps (PWM): Based on the array’s total short-circuit current.
PWM Amps = Total Array Current × (1 + Margin%)
Example Calculations
MPPT Example
System: 4 × 300W panels (2S × 2P) with a 24V battery bank. Specs: Voc = 38V, Isc = 10A, TempCoeff = −0.3%/°C, Lowest Temp = −10°C, Margin = 25%Adjusted Voc: 38 × [1 + (−0.003 × (25 − (−10)))] = 42V
Total Array Voltage: 42V × 2 (series) = 84V
Controller Amps: (1200W ÷ 24V) × 1.25 = 62.5A
Result → Use a 70A MPPT controller with a minimum 100V rating.
PWM Example
System: 2 × 100W panels (parallel) with a 12V battery. Specs: Voc = 22V, Isc = 6A, Margin = 25%Total Array Current: 6A × 2 (parallel) = 12A
Controller Amps: 12A × 1.25 = 15A
Result → Use a 15A PWM controller. Voc (22V) is within range for charging a 12V battery with PWM.
MPPT vs PWM Charge Controller Comparison
| Feature | MPPT Charge Controller | PWM Charge Controller |
|---|---|---|
| Efficiency | 95%–99% (extracts max power from solar panels) | 75%–80% (less efficient, especially in cold weather) |
| Cost | Higher upfront cost | Lower upfront cost |
| Best For | Larger solar systems & higher voltage panels | Small DIY/off-grid setups with low budgets |
| Battery Compatibility | Works well with lithium & advanced batteries | Mainly suited for lead-acid batteries |
| Performance in Cold/Cloudy Weather | Very good (adapts to changing conditions) | Poor to average |
| Lifespan | Longer (due to efficient charging) | Shorter compared to MPPT |
Frequently Asked Questions (FAQs)
Why adjust Voc for temperature?
Solar panel voltage increases in colder weather. By calculating the coldest-case Voc, you ensure the array won’t exceed the controller’s voltage rating, preventing permanent damage.
How do I choose between %/°C and mV/°C?
This value comes from the panel datasheet. Our calculator auto-converts units, but always match the manufacturer’s specification correctly (%/°C vs mV/°C).
What if my calculated current exceeds controller ratings?
You can either: (a) choose a larger charge controller, or (b) split your array into smaller sub-arrays with their own controllers.
Recommendations
Your ideal charge controller depends on system size, efficiency goals, and budget:
- PWM (Pulse Width Modulation): Best for small, low-cost systems where panel voltage ≈ battery voltage (e.g., a “12V” panel to 12V battery). Common sizes: 10A, 20A.
- MPPT (Maximum Power Point Tracking): Up to 30% more efficient. Essential if array voltage is much higher than battery voltage. Common sizes: 20A, 40A, 60A, 80A, 100A.
- Systems over 100A: Many premium controllers allow stacking in parallel for very large solar arrays.
Complete All-in-One Solution: ⚡ For portable or compact setups, consider a power station with a built-in MPPT, battery, and inverter. Example: BLUETTI EB3A supports up to 200W solar input in one compact unit.
Use Cases
- Off-Grid Living 🏡: Reliable controller sizing for cabins, homesteads, and remote installations.
- RV, Van & Marine 🚐: Accurate sizing for mobile power systems where space & efficiency matter.
- System Upgrades 📈: Check if your current controller supports expansions or if an upgrade is needed.
- Educational Projects 🔬: Ideal for students and DIY hobbyists to learn PV system design.
References
- NREL: Charge Controller Sizing Guide
- IEC 62509 – PV Charge Controllers Standard
- Solar Power World: Choosing the Right Charge Controller
More Tools & Resources
Looking for more ways to plan your off-grid system? Check out our additional calculators and resources: