Sizing Your Solar System

Sizing Your Solar System

Proper solar sizing means the system produces enough energy to run indefinitely through your worst-case season while maintaining enough battery reserve to survive multiple consecutive cloudy days.

Step-by-Step Sizing Process

Step 1: Determine Daily Energy Consumption

From the Current Draw page: a typical LoRa repeater consumes approximately 2.22 Wh/day (25 mA average at 3.7V over 24 hours). Use your actual measured value if available.

Step 2: Find Worst-Case Peak Sun Hours

Peak sun hours (PSH) vary by location and season. Use the December value for year-round reliability in the northern hemisphere. Look up your location on the NREL PVWatts calculator or use these reference values:

Step 3: Calculate Required Panel Size

Panel size (W) = Daily energy (Wh) ÷ (PSH × efficiency factor)

Use efficiency factor 0.70 to account for panel temperature, wiring losses, and charge controller losses.

Example (North Dakota repeater):
Panel = 2.22 Wh ÷ (2.5h × 0.70) = 2.22 ÷ 1.75 = 1.27W minimum

A 6W panel provides 4.7× the minimum required - a comfortable margin that accounts for snow shading, panel degradation, and future firmware changes that may increase power use.

Step 4: Calculate Battery Reserve

Battery capacity (Wh) = Daily energy × Reserve days ÷ Usable fraction

• Reserve days: 3 days for most locations; 5 - 7 days for high-latitude winter deployments
• Usable fraction: 0.80 for Li-ion (discharge to 20% to preserve cell life); 0.90 for LiFePO4

Example (3-day reserve, Li-ion):
Battery = 2.22 × 3 ÷ 0.80 = 8.33 Wh minimum

A single 3500mAh 18650 cell = 3.5Ah × 3.7V = 12.95 Wh. This provides the required 3-day reserve with margin.

For 5-day reserve in cold climate with 50% cold-weather derate:
Battery = 2.22 × 5 ÷ 0.80 ÷ 0.50 = 27.75 Wh minimum
→ Two 3500mAh 18650 cells in parallel = 25.9 Wh (marginally adequate); three cells = 38.85 Wh (comfortable)

Complete Sizing Example: North Dakota Year-Round Repeater