How big a solar panel do I need?

Short Answer 
 For most LoRa mesh nodes: a 5W panel for nRF52840-based nodes, 10-20W for ESP32-based nodes. For Raspberry Pi gateways: 20-40W. 

 The Calculation 
 Solar system sizing is a four-step calculation: 
 
 Measure your node's current draw - Use a USB inline power meter. Real measurements beat estimates. Typical values: nRF52840 repeater: 8-15 mA average; ESP32 repeater with OLED: 40-70 mA; Pi Zero 2W gateway: 100-150 mA. 
 Calculate daily energy consumption - Current (mA) × 24 hours = mAh per day. Example: 12 mA × 24 = 288 mAh/day = 0.288 Ah/day. 
 Find peak sun hours for your location - This is the key local variable. A panel receives "peak sun hours" as an energy-equivalent of full-rated output. US values: Miami: 5.5 (annual average), Denver: 5.3, Seattle: 3.6, Boston: 4.2, Phoenix: 6.1. Use the worst month value for sizing (typically December for most US locations). 
 Panel size - Daily consumption (Ah) ÷ peak sun hours × 1.25 (efficiency factor) = panel Ah output needed. Convert to Watts at your system voltage. Example: 0.288 Ah/day ÷ 3.5 PSH (Seattle December) × 1.25 = 0.103 Ah needed from panel → at 5V USB charging, 0.103 Ah × 5V = 0.51 Wh → a 1W panel is theoretically enough, but size up to 5W for margin. 
 

 Practical Sizing Recommendations 
 
 Node Type Average Draw Panel (temperate US) Panel (PNW winter) 
 
 nRF52840 repeater, no display 10-15 mA 5W 5W 
 ESP32 repeater, no display 40-55 mA 10W 20W 
 ESP32 repeater, OLED on 60-80 mA 15W 30W 
 Pi Zero 2W + LoRa HAT 120-160 mA 20W 40W 
 
 

 Battery Sizing 
 Size the battery for 3-5 days of autonomy (no solar input). This covers cloudy periods and seasonal weather patterns. Battery (Ah) = Daily consumption (Ah) × autonomy days × 1.2 (derating for depth of discharge). 
 Example: nRF52840 at 12 mA × 24h = 0.29 Ah/day × 5 days × 1.2 = 1.73 Ah minimum. Use a 3.7V 4000 mAh LiPo or a LiFePO4 equivalent for comfortable margin.