Solar Panel Selection and Installation
Panel technologies, Wp ratings, geographic sizing, mounting hardware, orientation, and charge controller selection.
- Choosing a Solar Panel for LoRa Nodes
- Solar Panel Mounting and Orientation
- Charge Controllers: PWM vs MPPT
Choosing a Solar Panel for LoRa Nodes
Solar panel selection involves matching the panel's output to the node's energy needs while accounting for real-world efficiency losses, geographic location, and physical mounting constraints. This page covers panel technology, rating systems, derating factors, geographic sizing, and wiring configurations.
Panel Technologies
| Technology | Efficiency Range | Temperature Coefficient | Low-Light Performance | Physical | Best Use Case |
|---|---|---|---|---|---|
| Monocrystalline silicon | 20 - 24% | −0.35% / °C above STC | Good | Rigid, glass-covered, aluminum frame | Fixed installations, roof/pole mounts |
| Polycrystalline silicon | 15 - 18% | −0.40% / °C above STC | Good | Rigid, glass-covered, aluminum frame | Budget fixed installations |
| Amorphous silicon (thin-film) | 6 - 8% | −0.20% / °C above STC | Excellent (diffuse light) | Flexible or glass, no frame | Curved surfaces, low-light climates |
| CIGS thin-film | 12 - 14% | −0.32% / °C above STC | Very good | Flexible or rigid | Curved surfaces where efficiency matters |
For most LoRa node deployments, monocrystalline panels are the correct choice. Their higher efficiency means a smaller, lighter panel for the same power output - important when mounting on a mast or in a small enclosure. Thin-film flexible panels are useful when the panel must conform to a curved surface (conduit mast, cylindrical enclosure) or when severe vibration makes rigid glass panels impractical.
Understanding Wp (Watt-Peak) Ratings
Panel power is rated in Watts-peak (Wp) at Standard Test Conditions (STC): 1000 W/m² irradiance, 25 °C cell temperature, AM 1.5 spectrum. Real-world conditions deviate from STC in several important ways:
Real-World Derating Factors
| Derating Factor | Typical Value | Explanation |
|---|---|---|
| Temperature (hot day) | 0.80 - 0.90 | Cell temp in direct sun reaches 50 - 75 °C. Monocrystalline loses ~0.35%/°C above 25 °C. At 60 °C: 1 − (35 × 0.0035) = 0.878. |
| Dirt / dust / pollen | 0.90 - 0.97 | Uncleaned outdoor panel loses 3 - 10% annually. Clean panels every 6 - 12 months. |
| Wiring and connection losses | 0.97 - 0.99 | Resistance in MC4 connectors and cable runs. Use AWG 10 - 12 for runs over 5 m. |
| Charge controller efficiency | 0.65 - 0.97 | PWM: 65 - 75%. MPPT: 93 - 97%. See Charge Controllers page. |
| Partial shading | 0.50 - 1.00 | Even 5% shadow on a cell in a string can reduce total output by 50%+ (bypass diodes mitigate but don't eliminate). |
| Spectral mismatch (overcast) | 1.0 - 1.05 for amorphous; 0.95 for mono | Amorphous panels outperform mono in overcast because diffuse light spectrum favors their bandgap. |
| Combined typical derating (MPPT, clean, no shade) | 0.70 - 0.80 | Use 0.75 as a conservative planning factor |
Peak Sun Hours by US Region
Peak sun hours (PSH) is the equivalent number of hours per day at 1000 W/m² irradiance that delivers the same daily energy as the actual variable irradiance. It is the single most important geographic variable in panel sizing.
| Region | Example Cities | Annual Avg PSH | Winter Worst-Month PSH |
|---|---|---|---|
| Southwest Desert | Phoenix, Las Vegas, El Paso | 6.0 - 7.0 | 4.5 - 5.5 |
| Mountain West | Denver, Salt Lake City, Albuquerque | 5.5 - 6.5 | 3.5 - 4.5 |
| Southeast | Miami, Atlanta, Dallas | 5.0 - 6.0 | 4.0 - 5.0 |
| Midwest / Great Plains | Kansas City, Minneapolis, Chicago | 4.5 - 5.5 | 2.5 - 3.5 |
| Mid-Atlantic / Northeast | NYC, Philadelphia, Boston | 4.0 - 4.8 | 2.0 - 3.0 |
| Pacific Northwest | Seattle, Portland, Eugene | 3.5 - 4.2 | 1.5 - 2.5 |
| Alaska (Anchorage) | Anchorage | 3.0 - 4.0 | 0.5 - 1.5 |
Always size for the worst-month PSH, not the annual average, to ensure year-round operation.
Panel Sizing Calculation
Required_Wp = Daily_Wh / (PSH_worst_month × overall_derating) Example: 5.75 Wh/day node, Seattle (1.8 PSH winter), MPPT controller (0.95), other derating (0.85): Combined derating = 0.95 × 0.85 = 0.808 Required_Wp = 5.75 / (1.8 × 0.808) = 5.75 / 1.454 = 3.95 Wp → use 5 Wp panel
Panel Sizing by Latitude (Rule of Thumb)
| Latitude (°N) | Panel Wp Required per 1 Wh/day node load | Notes |
|---|---|---|
| 25 - 30° (South Florida, Texas) | 0.5 - 0.7 Wp | Year-round high sun |
| 30 - 37° (Southeast, Southwest) | 0.6 - 0.9 Wp | Good solar resource |
| 37 - 42° (Mid-Atlantic, Midwest) | 0.9 - 1.3 Wp | Moderate winter derating |
| 42 - 48° (New England, Northwest) | 1.3 - 2.0 Wp | Poor winter sun |
| 48 - 65° (Northern US, Alaska) | 2.0 - 5.0 Wp | Size for worst month or use large battery |
Wiring: 5 V USB Charging vs 12 V Systems
5 V USB Charging (small panels, direct LiPo charging)
Panels rated 5 - 6 V open-circuit (e.g., 0.5 - 2 W "USB solar panels") are designed to pair with TP4056 or CN3791 LiPo charger ICs. These work only in full sun - the panel voltage drops below the charger's minimum input at partial cloud cover. Acceptable for supplemental trickle charging of small nodes but not reliable primary power.
12 V Nominal Systems
Panels rated 18 V open-circuit (12 V nominal, e.g., 10 W, 20 W, 40 W monocrystalline) are the standard for serious solar deployments. These pair with a dedicated charge controller (PWM or MPPT) that regulates voltage down to the battery charge voltage. MC4 connectors are the industry standard for these panels.
Series vs Parallel Configuration
| Configuration | Effect on Voltage | Effect on Current | When to Use |
|---|---|---|---|
| Series (panels in series) | Voltages add (2× 18 V = 36 V) | Current stays same | Higher voltage charge controllers; longer cable runs (less current = thinner wire) |
| Parallel (panels in parallel) | Voltage stays same | Currents add (2× 5 A = 10 A) | Same voltage system but need more current; partial shading (each panel has independent MPPT) |
For small LoRa deployments (5 - 40 Wp), a single panel in direct connection to a 12 V charge controller is the simplest and most reliable approach.
Recommended Panels for LoRa Deployments
| Panel | Power | Dimensions | Best For | Approximate Cost |
|---|---|---|---|---|
| Voltaic P110 (monocrystalline) | 2 W, 6 V | 132 × 91 mm | nRF52840 trickle charge, USB-C output | $25 |
| Newpowa NPA10-12MBK (mono) | 10 W, 12 V nominal | 340 × 235 mm | ESP32 nodes, primary solar | $20 - 25 |
| Renogy RNG-100D (mono) | 100 W, 12 V nominal | 1050 × 540 mm | Pi gateway installations | $85 - 100 |
| SunPower Flexible 50 W | 50 W, 12 V nominal | 710 × 540 mm | Curved mast mounting, marine | $90 - 110 |
| BougeRV 30 W (flexible CIGS) | 30 W, 12 V nominal | 580 × 350 mm | Curved enclosures, portable | $45 - 55 |
Solar Panel Mounting and Orientation
The mechanical installation of a solar panel is as important as the panel selection itself. A correctly sized panel pointed in the wrong direction, partially shaded, or insufficiently secured will fail to meet its energy budget. This page covers orientation rules, mounting hardware options, shading avoidance, and special-purpose installations.
South-Facing Tilt: The Fundamental Rule
In the Northern Hemisphere, solar panels produce maximum annual energy when facing true south (azimuth 180°) and tilted at an angle approximately equal to the installation latitude. This is not magnetic south - use a compass corrected for magnetic declination or use a sun-path tool (NREL's PVWatts, Solargis) to confirm true south orientation at your specific location.
| Latitude (°N) | Optimal Fixed Tilt | Winter-Optimized Tilt | Example Locations |
|---|---|---|---|
| 25° | 25° | 35° | Miami FL, Key West FL |
| 30° | 30° | 42° | Houston TX, Jacksonville FL |
| 35° | 35° | 48° | Los Angeles CA, Memphis TN |
| 40° | 40° | 53° | Denver CO, Columbus OH, NYC |
| 45° | 45° | 58° | Minneapolis MN, Portland OR |
| 47° | 47° | 61° | Seattle WA |
| 61° | 61° | 75° | Anchorage AK |
For fixed-tilt installations in climates with significant winter operation (Pacific NW, New England, Mountain states), set the panel at latitude + 10 - 15° to capture more low winter sun. This sacrifices some summer production but improves the worst-month (winter) performance critical for battery sizing.
A ±15° deviation from true south reduces annual yield by only 2 - 3%. A ±30° deviation reduces it by about 7 - 8%. East or west orientations (90° from south) reduce yield by approximately 20%.
Fixed Tilt vs Seasonal Adjustment
Single-axis seasonal adjustment (adjusting tilt twice a year - summer and winter) improves annual yield by 5 - 10% compared to a fixed optimal tilt. For most unattended LoRa deployments, this tradeoff is not worth the maintenance visit. The exception is a high-latitude deployment (above 45°N) where winter sun angles are very low and a steep winter tilt meaningfully improves worst-month performance.
Mounting Hardware Options
Pole and Mast Mounts
Mast mounting is ideal for LoRa repeaters that are already on a mast or tripod. The same structure that supports the LoRa antenna can support the solar panel, reducing site footprint. Two common approaches:
- Side-of-pole (SOP) mount: A steel bracket clamps to the mast with U-bolts and holds the panel at an adjustable tilt. Products like the Renogy SOP Mount (fits 1.5 - 2.5" OD mast) and IronRidge SRP-01 are widely available at $15 - 30. The panel faces south and the bracket tilt is fixed at installation.
- Top-of-pole mount: Panel mounts above the mast on a swiveling head. Larger capacity (40 - 100 Wp panels), more wind load. Unirac and IronRidge make residential-grade top-of-pole mounts; for smaller panels, Renogy's 100 W single-panel top-of-pole mount ($40 - 60) is a popular choice.
J-Mounts (Roof Rafter Mounts)
J-mounts bolt through the roof into rafters and hold the panel parallel to the roof surface, adding 2 - 4 inches of standoff for airflow. IronRidge, Unirac, and Quick Mount PV are the main suppliers. J-mounts fix the panel tilt to the roof pitch - acceptable for slopes between 15° and 40° facing south.
Flush Roof Brackets
For panels mounted flat on a low-slope roof or equipment enclosure lid, Velcro-based adhesive mounts (Voltek VP7000 series) or through-bolted HDPE brackets work for panels up to 40 Wp. Adequate ventilation between panel and surface is critical - cells at 75 °C lose 17% of Wp rating.
Ground-Mount Frames
For larger gateway installations (100 - 400 Wp), commercial aluminum ground-mount frames (Ironridge GMI, Arctech, Shoals ground mount kits) provide adjustable tilt and wind resistance up to 140 mph with proper anchoring. Concrete ballast blocks or driven ground screws provide the foundation.
Keeping Panels Shadow-Free Throughout the Day
Shade is the single largest cause of underperforming solar nodes. A shadow covering just one cell in a 36-cell string drops the entire string's output by 50 - 70% (partial bypass diode protection reduces this but cannot eliminate it).
- Use the Sun Surveyor app or SOLMETRIC SunEye to measure shade at the proposed mounting location at both 9 AM and 3 PM local solar time on the worst day (December 21). Any obstruction that casts shadow during these hours will significantly reduce winter output.
- Trees grow. A tree 30 ft away that doesn't shade the panel today may shade it in 5 years. Add 30% to the estimated shadow cone radius when evaluating obstructions.
- Nearby LoRa antennas, lightning rods, conduit runs, and fence posts can all cast thin shadows that track across the panel during the day. Route everything above or well to the side of the panel.
Clearing Snow
In snow climates, steep tilt angles (above 45°) allow snow to slide off naturally. Panels tilted below 30° will accumulate snow and may be buried for days. Solutions:
- Set winter tilt to 60° or greater if the installation allows adjustment.
- Mount the bottom edge of the panel at least 18 inches above the expected snow depth to prevent burial.
- Do not scrape panels with metal scrapers - use a soft brush or wait for natural sliding (the panel warms as soon as any light penetrates the snow cover).
- Black-frame monocrystalline panels absorb more heat and shed snow faster than white-back polycrystalline panels.
Bird Deterrents
Bird droppings on panels cause significant localised shading. Common solutions:
- Bird wire / netting: Stainless steel spikes (Daddi Long Legs, Bird-B-Gone) or polycarbonate mesh installed under and around the panel perimeter. Do not run wires over the glass surface.
- Owl decoys: Marginally effective for the first few weeks, then ignored.
- Regular cleaning with deionised water and a soft sponge removes existing droppings. A Rain-X solar panel coating reduces adhesion.
Marine and Boat Mounting for Floating Nodes
Floating LoRa nodes on buoys, boats, or floating platforms require stainless steel or anodized aluminum hardware throughout. Standard steel J-mounts will rust within one season in saltwater. Key requirements:
- All fasteners: 316 stainless steel (not 304).
- Tilt adjustment: Bobstay-style adjustable bracket allows panel to be angled away from most wave wash directions.
- Strain relief: Marine-grade cable glands (Blue Sea, Wiley) with EPDM seals. Cables must have enough slack to accommodate platform motion without straining MC4 connectors.
- Anti-corrosion treatment: Corrosion-X or Boeshield T-9 applied to all metal-to-metal contacts and terminal blocks annually.
- The panel itself should be rated IP65 or IP67 and marked "marine grade" - standard terrestrial panels allow moisture intrusion at the junction box in salt-spray conditions.
Charge Controllers: PWM vs MPPT
The charge controller sits between the solar panel and the battery. It regulates current flow to prevent battery overcharge and manages the charging profile. Two fundamentally different control topologies are in common use: Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT). Selecting the wrong type can waste 20 - 35% of available solar energy or damage batteries.
How PWM Controllers Work
A PWM charge controller connects the solar panel directly to the battery through a switch (MOSFET or relay). When the battery voltage is low, the switch is fully on - the panel feeds the battery at whatever current the panel can supply at the battery's current voltage. As the battery approaches full charge, the controller begins pulsing the switch on and off at a duty cycle proportional to the difference between target and actual battery voltage. This reduces average current flow, preventing overcharge.
The critical limitation of PWM: the panel is forced to operate at battery voltage, not at its own maximum power point (MPP). A 12 V nominal panel has an MPP voltage (Vmpp) around 17 - 18 V but the battery sits at 12 - 14.6 V. The panel is clamped to the lower voltage, operating well off its power curve. This is the root cause of PWM's lower efficiency.
How MPPT Controllers Work
An MPPT controller inserts a DC-DC buck (or boost) converter between the panel and battery. The controller continuously monitors the panel's voltage and current output, computing P = V × I. It then incrementally adjusts the panel's operating point (by changing the duty cycle of the converter's switching transistor) to locate and track the voltage at which the panel delivers maximum power - the Maximum Power Point.
Because the converter can step voltage down (or up) at high efficiency, the panel operates at its optimal Vmpp (~17 - 18 V for a 12 V panel) while the battery receives the current it needs at battery voltage. This voltage step-down comes with a compensating current increase, delivering more total watts to the battery.
Efficiency Comparison
| Parameter | PWM Controller | MPPT Controller |
|---|---|---|
| Typical conversion efficiency | 65 - 75% of panel STC rating | 93 - 97% of panel STC rating |
| Panel voltage utilisation | Poor - clamped to battery voltage | Excellent - panel at MPP |
| Cold weather advantage | None | Significant - cold panels have higher Voc/Vmpp, MPPT captures this gain |
| Partial cloud benefit | None | Moderate - can still track the shifted MPP under clouds |
| Typical cost (5 - 20 A range) | $5 - 15 | $20 - 60 |
| Quiescent current consumption | 5 - 15 mA | 10 - 30 mA |
| Complexity / failure modes | Low - simple circuit | Moderate - switching converter can fail; firmware-dependent tracking |
When the Difference Matters
Small Panels (under ~10 Wp)
For very small panels (2 - 5 Wp paired with a small LiPo and an ESP32 or nRF52840 node), the absolute watt improvement from MPPT is tiny - perhaps 1 - 2 W - and the MPPT controller's own quiescent current (20 - 30 mA = 0.07 - 0.11 Wh/h) becomes a significant fraction of the total load. For these micro-installations, a dedicated LiPo solar charger IC such as the CN3791 (MPPT-capable IC, ~$1) or TP4056 (simple CC/CV, no MPPT, ~$0.30) is the appropriate solution. Full-featured MPPT controllers add cost, quiescent drain, and complexity with minimal return.
Medium Panels (10 - 100 Wp)
This is where MPPT begins to pay for itself. A 20 Wp panel with a PWM controller delivers approximately 15 Wp to the battery in ideal conditions. The same panel with an MPPT controller delivers approximately 19 Wp - a 27% improvement. Over a 5-day winter week at Seattle's 1.8 PSH:
PWM: 20 Wp × 0.70 × 1.8 PSH × 5 days = 126 Wh MPPT: 20 Wp × 0.95 × 1.8 PSH × 5 days = 171 Wh Difference: 45 Wh - enough to run an extra day of autonomy
Large Panels (over 100 Wp) and Cold Climates
MPPT is the only correct choice. In cold climates (below freezing), panel Vmpp rises significantly - a 36-cell panel that has Vmpp = 17 V at 25 °C may have Vmpp = 20 - 21 V at −10 °C. A PWM controller cannot exploit this; an MPPT controller captures all of it.
LVD Settings for LiFePO4 Packs
Low Voltage Disconnect (LVD) is the battery voltage at which the charge controller cuts power to the load, protecting the battery from over-discharge. Setting LVD correctly for LiFePO4 is critical - these batteries have flat discharge curves that make "soft" voltage warnings less useful.
| Parameter | 12 V LiFePO4 (4S, 12.8 V nominal) | 12 V Lead Acid (12 V nominal) |
|---|---|---|
| LVD (disconnect) voltage | 11.5 - 11.8 V (≈ 10 - 15% SoC) | 11.4 - 11.8 V (≈ 40 - 50% SoC) |
| LVD reconnect voltage (hysteresis) | 12.5 - 12.8 V (≈ 30 - 50% SoC) | 12.2 - 12.5 V |
| Absorption charge voltage | 14.2 - 14.6 V (3.55 - 3.65 V/cell) | 14.4 - 14.8 V |
| Float voltage | 13.5 - 13.8 V (3.375 - 3.45 V/cell) | 13.2 - 13.8 V |
| Equalization | Do NOT equalize LiFePO4 | 15.0 - 16.0 V periodically |
Critical: Many PWM controllers sold as "12 V" ship with default lead-acid charge profiles. If used with LiFePO4, the float voltage will be too low and the absorption voltage may be set for gel/AGM lead acid (14.1 V) which undercharges LiFePO4. Always verify and configure the LiFePO4 profile. Renogy Rover, Victron BlueSolar, and Epever controllers have configurable user-defined battery profiles.
Common Charge Controllers for Small LoRa Deployments
IC-Level (for integration into custom PCBs)
| IC | Type | Input Voltage | Max Charge Current | Chemistry | Cost |
|---|---|---|---|---|---|
| TP4056 | CC/CV linear (no MPPT) | 4.5 - 8 V | 1 A | LiPo (4.2 V cutoff) | $0.25 - 0.40 |
| CN3791 | MPPT, switching | 4.5 - 6 V | 2 A | LiPo (4.2 V cutoff) | $0.80 - 1.20 |
| BQ24650 | MPPT, synchronous buck | Up to 28 V | Up to 10 A | Configurable (Li, LiFePO4) | $2 - 4 |
| SPV1040 | MPPT, boost converter | 0.3 - 5 V | 1.8 A out | LiPo, NiMH | $1.50 - 2.50 |
Module-Level (drop-in for 12 V systems)
| Module | Type | Panel Watts (max) | Features | Cost |
|---|---|---|---|---|
| Renogy Wanderer 10A PWM | PWM | 120 W | LCD, LVD, USB output | $20 |
| Epever Tracer AN 10A MPPT | MPPT | 130 W | RS485 MODBUS, LCD, configurable profiles | $35 - 45 |
| Victron SmartSolar 75/10 | MPPT | 145 W @ 12 V | Bluetooth, VictronConnect app, LiFePO4 profile | $55 - 65 |
| Genasun GVB-8 (8A MPPT) | MPPT | 110 W | Purpose-built LiFePO4 profiles, waterproof | $75 - 90 |
| SRNE ML2430 30A MPPT | MPPT | 390 W @ 12 V | LCD, multiple battery profiles, USB | $45 - 55 |
For most LoRa gateway installations (20 - 100 Wp), the Victron SmartSolar 75/10 is the recommended choice: Bluetooth monitoring allows verifying charge behaviour remotely, and the LiFePO4 profile is well-tested. For budget-constrained multi-node deployments, the Epever Tracer AN 10A provides RS485 telemetry for integration with Grafana monitoring stacks.