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:

1. 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.
2. Calculate daily energy consumption - Current (mA) × 24 hours = mAh per day. Example: 12 mA × 24 = 288 mAh/day = 0.288 Ah/day.
3. 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).
4. Panel size - Daily consumption (Ah) ÷ peak sun hours × 1.25 (efficiency factor) = panel Ah output needed. Convert to Watts at your system voltage. Example:The 0.288bare Ah/dayarithmetic ÷for 3.5an PSHultra-low-power nRF52840 node yields a tiny number (Seattlewell December)under ×1 Wh/day), but do not use that as your panel size. The 1.25 =factor 0.103does Ahnot neededaccount fromfor real-world losses: cloudy stretches well below the average peak-sun-hours figure, dust and dirt on the panel, panel →aging, atcharge-controller 5Vand USBbattery charging,inefficiency, 0.103and Ahmulti-day ×overcast 5Vperiods. =After 0.51those Whlosses, → a 1W5W panel is theoreticallythe enough,practical butminimum sizeeven for the smallest nodes in poor-insolation regions like the Pacific Northwest in winter. Size up to 5Wfurther for margin.ESP32 and Pi-class nodes per the table below.

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.