Solar Sizing for Smart Locks: Off-Grid Power
Your smart lock needs 5-15 Wh per day. Here's how to size solar that works in winter storms and summer heat.
Understanding Your Load
Smart locks draw power 24/7, but the consumption varies by type and usage:
- Bluetooth-only: 0.05-0.2W standby
- WiFi-enabled: 0.3-0.5W standby
- Daily use: 5-15 cycles adds 0.5-2Wh
- Total daily need: 2-8Wh typical
Solar Panel Sizing
Worst Case Planning
Size for winter, not summer. In December you might get: - Northern US: 1.5-2.5 sun hours - Central US: 2.5-3.5 sun hours - Southern US: 3.5-4.5 sun hours
Panel Calculation
Panel Watts = Daily Wh ÷ (Sun hours × Derate factor)
- Use 0.75 derate for real-world conditions
- Include 20% buffer for cloudy days
- Round up to available panel sizes
Typical Sizes
| Location | Bluetooth Lock | WiFi Lock |
|---|---|---|
| Northern US | 10W | 20W |
| Central US | 5W | 10W |
| Southern US | 5W | 10W |
Battery Sizing
Autonomy Days
How many cloudy days can you survive?
- Minimum: 3 days
- Recommended: 5-7 days
- Full off-grid: 10-14 days
Battery Math
Ah Needed = (Daily Wh × Autonomy days) ÷ (System voltage × DoD)
- Use 80% DoD for LiFePO4
- Use 50% DoD for lead-acid
- System voltage: 12V typical
Recommended Batteries
- LiFePO4 12V 7Ah: 3-4 days autonomy
- LiFePO4 12V 12Ah: 5-7 days autonomy
- LiFePO4 12V 20Ah: 10+ days autonomy
Charge Controllers
PWM vs MPPT
For small systems, PWM is usually fine:
- PWM: Cheaper, simpler, 70-80% efficient
- MPPT: 90-95% efficient, costs 2-3x more
- Recommendation: PWM for <20W systems
Controller Features
- Low voltage disconnect (LVD)
- Load control output
- Battery type selection
- Temperature compensation
Installation Tips
Panel Placement
- South-facing (northern hemisphere)
- Tilt = latitude + 15° for winter
- No shade from 9am-3pm
- Allow air circulation behind panel
Wiring Guidelines
- Keep wires short and direct
- Use 18AWG for <5A runs
- Include fuses near battery
- Seal all outdoor connections
Mounting
- Use corrosion-resistant hardware
- Allow for panel expansion
- Secure against wind uplift
- Consider theft prevention
Seasonal Adjustments
Winter Strategy
- Increase panel tilt angle
- Clean snow accumulation
- Check battery capacity weekly
- Reduce lock usage if needed
Summer Strategy
- Check for overcharging
- Ensure battery ventilation
- Monitor temperature
- Adjust tilt to latitude - 15°
Related Reading
- Energy Calculator — Run the numbers for your setup.
- Winter Battery Care — Protect batteries in cold.
- Battery Life Calculator — Estimate runtime.
System Examples
Minimal Bluetooth System
- 5W panel
- 12V 7Ah LiFePO4 battery
- 5A PWM controller
- Total cost: $80-100
- Works in most US locations
Full WiFi System
- 20W panel
- 12V 20Ah LiFePO4 battery
- 10A PWM controller
- Total cost: $180-220
- Year-round reliability
Monitoring
- Install simple voltage meter
- Log readings weekly
- Note unusual patterns
- Check connections monthly
Bottom Line
Start with your daily load, size for winter, and add battery autonomy. A 10W panel with 12Ah battery covers most needs. Oversize slightly - the cost difference is minimal compared to the reliability gain.
Quick Calculator
For a WiFi lock (6Wh/day) in Central US (3 sun hours):
Panel: 6Wh ÷ (3h × 0.75) = 2.7W → Use 5W panel
Battery: 6Wh × 5 days ÷ (12V × 0.8) = 3.1Ah → Use 7Ah battery