Solar and Power FAQ 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. Why does my solar node keep dying at night? Diagnosing Night Drain If your solar node runs fine during daylight but goes offline overnight, you have one of three problems: undersized battery, incorrect charge controller settings, or excessive power draw. Step 1: Verify Actual Battery Capacity First, measure what you actually have. LiPo and LiFePO4 batteries are frequently sold at optimistic ratings. A "5000 mAh" LiPo from an unknown manufacturer may test at 3000-3500 mAh in practice. Test: Fully charge the battery, disconnect solar, and measure how long the node runs. Node runtime (hours) × current draw (mA) = actual battery capacity (mAh). Step 2: Calculate Required Overnight Capacity In winter at high latitudes, "night" can mean 16+ hours of darkness. Your battery must cover: hours of darkness × node current draw. Example: 14 hours dark, 12 mA draw = 168 mAh minimum. Even a small 1000 mAh battery covers this with margin. If your node dies overnight, either the battery is not fully charging during the day or the current draw is much higher than expected. Step 3: Verify the Battery is Actually Charging Check the charge controller status LED or voltage output during the day. Common issues: Panel undersized for the season - A panel just barely adequate in summer may not fully recharge the battery on short winter days. The battery reaches discharge before it can recover overnight. Panel partially shaded - Even partial shading (one corner of the panel) can reduce output by 50-80%. Inspect during the time of day the sun is lowest (winter morning/afternoon). LVD set too high - If the charge controller's Low Voltage Disconnect threshold is set above the battery's actual minimum, it cuts power while charge still remains. For LiFePO4: LVD = 3.0V/cell (12.0V for 4S). For LiPo: LVD = 3.0-3.2V/cell. Charge controller in wrong battery mode - A controller configured for lead acid will overcharge LiPo or undercharge LiFePO4. Verify the battery type setting matches your battery chemistry. Step 4: Reduce Power Draw If the battery and panel are correctly sized but the node still dies, attack the power draw: Disable OLED display (save 15-20 mA) Disable Bluetooth (save 5-15 mA on ESP32) Switch from ESP32 to nRF52840 board (save 30-50 mA average) Reduce TX power to minimum needed (each 3 dB reduction in TX power roughly halves transmit current) What battery chemistry should I use outdoors? Short Answer: LiFePO4 for outdoor deployments Lithium Iron Phosphate (LiFePO4) is the recommended battery chemistry for any permanent outdoor LoRa mesh installation. It is safer, more durable, and handles temperature extremes far better than standard LiPo batteries. Why Not LiPo? LiPo (Lithium Polymer) batteries are the default on most development boards because they're inexpensive and compact. For an outdoor deployment, they have serious limitations: Temperature sensitivity - LiPo loses significant capacity below 0°C and should never be charged below 0°C (causes internal lithium plating and eventual failure). In any climate with freezing winters, an outdoor LiPo battery will degrade rapidly or fail entirely within 1-2 seasons. Thermal runaway risk - LiPo batteries can catch fire if punctured, overcharged, or deeply discharged and then recharged. Not ideal in unattended outdoor enclosures. Short cycle life - 300-500 full charge cycles. A solar node cycling daily would exhaust a LiPo in 1-1.5 years. LiFePO4 Advantages Property LiPo LiFePO4 Operating temperature 0°C to 45°C -20°C to 60°C Cycle life 300-500 cycles 2,000-4,000 cycles Thermal runaway Yes (fire risk) No (thermally stable) Nominal voltage 3.7V/cell 3.2V/cell Energy density ~250 Wh/kg ~130 Wh/kg Cost Lower Higher (but lower cost per cycle) LiFePO4 Products for LoRa Deployments Small cells (3.2V) - EVE LF50K, EVE LF100 18650-format cells; use with a LiFePO4-compatible BMS Integrated packs - Bioenno 3.2V to 12.8V packs with built-in BMS; Antigravity RE-START series; Dakota Lithium packs 12V packs - For larger systems with 12V charge controllers; Battleborn BB10012, Dakota Lithium 10-100Ah options Charge Controller Compatibility LiFePO4 requires a charge controller set to LiFePO4 chemistry. Lead acid charge profiles will undercharge LiFePO4 (not a safety issue, but reduces usable capacity). LiPo charge profiles will overcharge LiFePO4 (potential safety concern). Verify your charge controller supports LiFePO4 mode before purchasing.