Sizing a Solar System for Your Climate

Solar panel sizing depends heavily on your geographic location. The same 5W panel produces dramatically different energy in Portland, Oregon vs. Phoenix, Arizona. This guide walks through climate-specific sizing calculations.

Peak Sun Hours by Region

Peak Sun Hours (PSH) is the number of hours per day when solar irradiance averages 1,000 W/m². It's the key variable in solar sizing calculations.

Design for the worst month: Use winter PSH for sizing, not annual average. A system that works in December will have abundant power all other months.

Solar Sizing Calculation

# Formula: Panel watts needed = Daily energy need / (PSH * system efficiency)
# System efficiency accounts for MPPT losses, wiring losses, temp derating: ~0.75-0.85

# Example: RAK4631 repeater in Chicago winter
Daily energy: 15mA avg * 3.7V * 24hrs = 1.33 Wh/day
PSH (Chicago winter): 2.5 hours
System efficiency: 0.80

Panel size = 1.33 Wh / (2.5 h * 0.80) = 0.67W

# Result: Even a 1W panel is 1.5x the minimum needed for Chicago winter!
# In practice, use at minimum 5W to account for cloudy day sequences.

# For Raspberry Pi room server + RAK4631 in Chicago winter:
Pi Zero 2W avg: 300mA * 5V = 1.5W = 36 Wh/day
RAK4631 avg: 15mA * 3.7V = 0.055W = 1.33 Wh/day
Total: 37.33 Wh/day

Panel needed: 37.33 / (2.5 * 0.80) = 18.7W → use 25W panel minimum

Battery Sizing for Autonomy

# Formula: Battery capacity = Daily energy * Autonomy days / (DOD * voltage)
# DOD (Depth of Discharge): LiPo 80%, LiFePO4 80%, SLA 50%

# RAK4631 repeater with 3-day autonomy, 3.7V LiPo:
Battery Ah = (1.33 Wh/day * 3 days) / (0.80 * 3.7V)
           = 3.99 Wh / 2.96
           = 1.35 Ah minimum

# With 10Ah LiPo: 10Ah * 0.80 * 3.7V / 1.33 Wh/day = 22 days autonomy!
# Very comfortable margin for most climates.

# For Pi + RAK at 3-day autonomy, 12V LiFePO4:
Battery Ah = (37.33 * 3) / (0.80 * 12V) = 11.7 Ah minimum
# Use 20Ah battery for comfortable margin.

Seasonal Charging Strategies

• High-sun climates (AZ, NM, TX): Standard sizing with modest battery. More at risk from heat-related battery degradation than insufficient solar.
• Variable climates (CO, GA, NY): Size for winter worst-case. Summer excess charging is harmless with a good charge controller.
• Low-sun climates (PNW, AK): Over-size significantly. A 20W panel for a 1W load is not overkill in Portland. Alternatively, use a second backup charging source (wind generator, AC trickle charge from nearby structure).