Getting Started (For Beginners)
If you’re new to NB-IoT power management, this section will help you understand how NB-IoT devices achieve 10+ year battery life.
What is NB-IoT Power Management? (Simple Explanation)
Analogy: Think of NB-IoT power modes like your smartphone’s power settings, but WAY more extreme:
- Your smartphone: “Sleep mode” saves battery, but the phone still checks for notifications every few seconds - battery lasts 1-2 days
- NB-IoT PSM mode: “Deep sleep” - the device completely turns off the radio and sleeps for hours or days - battery lasts 10-15 years!
The key difference: NB-IoT devices don’t need to be instantly reachable (unlike your phone). They wake up, send data, and go back to deep sleep.
Real-World Examples
Example 1: Smart Water Meter (PSM Mode)
Scenario: Municipal water meter in basement
- Reports water usage once per day (midnight)
- No downlink needed (utility company doesn't send commands)
- Must last 15 years (replacing battery costs $50 labor + truck roll)
Power profile (24 hours):
├─ Sleep (PSM): 23 hours 59 minutes (radio OFF, 5 μA)
├─ Wake up at midnight: 1 second
├─ Send water reading (50 bytes): 5 seconds (200 mA)
├─ Receive ACK: 1 second (50 mA)
└─ Return to PSM: immediate
Daily energy consumption:
- Sleep: 23.98 hours × 5 μA = 120 μAh = 0.12 mAh
- Transmit: 7 seconds × 150 mA (average) = 0.29 mAh
Total per day: 0.41 mAh
Battery life with 6,000 mAh battery (2× AA lithium):
6,000 mAh ÷ 0.41 mAh/day = 14,634 days = **40 years**!
(In practice: 15 years due to battery self-discharge)
Why PSM works here:
- Device doesn’t need to receive commands (uplink-only)
- Reporting schedule is predictable (midnight daily)
- Deep sleep maximizes battery life
Example 2: Smart Parking Sensor (eDRX Mode)
Scenario: Street parking sensor
- Reports occupancy changes immediately (car arrives/leaves)
- Must receive commands from cloud (e.g., "enable maintenance mode")
- Battery replacement every 5 years is acceptable
Power profile (typical day):
├─ Sleep with eDRX listening: 23 hours (15 μA average)
│ └─ Wake every 2.91 hours to check for downlink (100 ms)
├─ Occupancy change events: 4× per day (car in/out)
│ └─ Each transmission: 5 seconds (200 mA)
└─ Total active time: 20 seconds/day
Daily energy consumption:
- eDRX sleep: 23 hours × 15 μA = 345 μAh = 0.345 mAh
- Transmissions: 4 × 5s × 200mA = 1.11 mAh
Total per day: 1.46 mAh
Battery life with 6,000 mAh battery:
6,000 ÷ 1.46 = 4,110 days = **11.3 years** ✅
Why eDRX is needed here:
- Device must be reachable for cloud commands (downlink required)
- Still achieves 10+ year battery life
- Slightly higher power consumption than PSM (15 μA vs 5 μA)
The Two Essential Power Saving Modes
NB-IoT offers two main power-saving modes. Here’s how to choose:
| PSM (Power Saving Mode) |
3-5 μA |
No (radio OFF) |
Hours to days |
15+ years |
Uplink-only sensors (meters, trackers) |
| eDRX (Extended DRX) |
15 μA |
Yes (wakes periodically) |
Seconds to minutes |
5-10 years |
Devices needing downlink (actuators, remote config) |
Key insight: The difference between 5 μA (PSM) and 15 μA (eDRX) seems tiny, but over 10 years: - PSM: 5 μA × 87,600 hours = 438 mAh - eDRX: 15 μA × 87,600 hours = 1,314 mAh - Difference: 876 mAh (can double battery size requirements!)
Why PSM and eDRX Exist When Cellular Networks Already Have DRX
Standard LTE devices already use Discontinuous Reception (DRX) to save power – a smartphone checks for paging messages every 1.28 seconds during idle mode. So why did 3GPP invent two entirely new power-saving mechanisms (PSM in Release 12, eDRX in Release 13) specifically for IoT?
Standard DRX wastes 97% of an IoT device’s energy budget on unnecessary listening. A smart water meter that reports once per day spends 86,398 seconds idle and 2 seconds transmitting. With standard LTE DRX (1.28s paging cycle), the radio wakes up 67,498 times per day to check for paging messages that almost never arrive. Each wake-up draws 30-50 mA for 5-10 ms, consuming roughly 5-9 mAh per day just for paging reception. The actual data transmission uses only 0.08-0.15 mAh. The paging overhead alone would drain a 6,000 mAh battery in under 3 years.
PSM eliminates paging entirely. By telling the network “do not page me until I contact you,” the device avoids all 67,498 daily wake-ups. Sleep current drops from the DRX average of 1-2 mA to the PSM floor of 3-5 uA (set by the voltage regulator leakage, not the radio). This is a 200-400x reduction in idle power, extending battery life from 2-3 years to 15+ years.
eDRX is the compromise for bidirectional devices. Some IoT devices need occasional downlink messages (firmware updates, configuration changes, actuator commands) but not with smartphone-like immediacy. eDRX extends the paging cycle from 1.28 seconds to values ranging from 5.12 seconds up to 2.91 hours. At 40-minute eDRX cycles, a device wakes 36 times per day instead of 67,498 times, reducing paging energy by 99.95% while remaining reachable within 40 minutes.
The three modes form a clear decision hierarchy:
| Standard DRX |
67,500 |
1-2 mA |
< 2 seconds |
Smartphones |
| eDRX (40 min) |
36 |
15 uA |
< 40 minutes |
Parking sensors, fleet trackers |
| PSM |
0 |
3-5 uA |
Only when device wakes |
Water meters, soil sensors |
Common Beginner Questions
Q1: What’s the difference between PSM and “turning the device off”?
A: PSM keeps network registration, “off” loses it:
PSM (Power Saving Mode):
1. Device tells network: "I'm going to sleep for 24 hours"
2. Network says: "OK, I won't page you until then"
3. Device sleeps (5 μA)
4. Wake up → immediate connection (already registered)
└─ No handshake needed, just send data (fast!)
Fully OFF (not registered):
1. Device powers off completely (0 μA)
2. Wake up → must re-register with network
├─ Cell search: 5-10 seconds
├─ Attach procedure: 10-20 seconds
└─ Finally send data: 5 seconds
Total: 20-35 seconds active (vs 5 seconds with PSM)
Battery impact:
- PSM: 5 seconds active (0.28 mAh)
- Full OFF/ON: 25 seconds active (1.39 mAh)
→ PSM uses 80% less energy per transmission!
Q2: Can I use Wi-Fi-style “sleep mode” with NB-IoT to save power?
A: No, NB-IoT requires PSM or eDRX for long battery life:
Wi-Fi sleep (like ESP32 light sleep):
- Radio stays synchronized with access point
- Must wake every 100 ms to receive beacon
- Sleep current: 800 μA - 15 mA (depending on implementation)
- Battery life: weeks to months
NB-IoT PSM (deep sleep):
- Radio completely OFF (loses synchronization)
- No periodic wake-ups required
- Sleep current: 3-5 μA
- Battery life: 10-15 years
Why 100× difference?
- Wi-Fi: Designed for high-speed, always-on communication
- NB-IoT: Designed for infrequent, scheduled communication
Q3: How do I choose between PSM and eDRX?
A: Ask: “Does the cloud need to send commands to the device?”
