What Is RSSI and Why Does It Matter?

RSSI stands for Received Signal Strength Indicator. It is measured in dBm (decibels relative to 1 milliwatt) and tells you how strongly a sensor’s radio signal is being received at the LoRaWAN gateway. The scale is negative — the closer to zero, the stronger the signal.

RSSI Range	Signal Quality	LoRa Spreading Factor	What It Means
-40 to -70 dBm	Good	SF7	Sensor is close or has clear line-of-sight to gateway
-71 to -90 dBm	Acceptable	SF9-10	Walls, floors or distance are attenuating the signal
-91 to -120 dBm	Critical	SF11-12	Sensor is struggling — retransmissions likely

KEY	RSSI does not just affect data reliability — it directly controls how much energy each transmission uses, which determines battery life.

2. LoRaWAN Adaptive Data Rate (ADR)

LoRaWAN uses a mechanism called Adaptive Data Rate (ADR). The network server continuously monitors signal quality and instructs each sensor to use the most efficient radio settings for its environment.

How ADR Works

• Good signal (RSSI > -70 dBm) → network assigns SF7 (fastest, lowest energy)
• Medium signal (-70 to -90 dBm) → network steps up to SF9 or SF10
• Poor signal (< -90 dBm) → network assigns SF11 or SF12 (slowest, highest energy)
• Very poor signal → sensor may retransmit the same packet 2-3 times per cycle

The Energy Cost of Higher Spreading Factors

Each step up in Spreading Factor roughly doubles the time-on-air of each packet. Since radio transmission is by far the largest energy drain on a battery-powered sensor, this has a compounding effect on battery life.

Spreading Factor	Time on Air (approx.)	Energy vs SF7	Relative Battery Impact
SF7	~56 ms	1×	Baseline
SF8	~103 ms	2×	+100%
SF9	~185 ms	3.3×	+230%
SF10	~370 ms	6.6×	+560%
SF11	~741 ms	13×	+1,200%
SF12	~1,483 ms	26×	+2,500%

3. Elsys ERS Lite — Battery Specifications

The Elsys ERS Lite is a compact indoor environment sensor using a single AA Lithium 3.6V battery (Saft LS14500 or equivalent, ~2,400 mAh).

Energy Budget Breakdown

Operating Mode	Current Draw	Duration	Notes
Deep sleep	~5 µA	~14.9 min/cycle	Between transmissions
MCU wake + measure	~2 mA	~200 ms/cycle	Sensor reading
LoRa TX (SF7)	~40 mA	~56 ms	Best-case signal
LoRa TX (SF9)	~40 mA	~185 ms	Typical indoor
LoRa TX (SF12)	~40 mA	~1,483 ms	Worst-case signal

At SF7 with 15-minute intervals the sensor spends over 99.9% of its time in deep sleep. As the Spreading Factor increases, the active radio time per day grows significantly, accelerating battery depletion.

4. Battery Life Calculation

Step 1 — Transmissions per day

96 transmissions/day  =  (24 hours × 60 min) / 15 min interval

Step 2 — Energy per transmission (SF9 example)

Each transmission consists of: sleep current between cycles + sensor measurement + LoRa radio burst.

E_tx = (40 mA × 185 ms)  +  (2 mA × 200 ms)  +  (0.005 mA × 900 s)  ≈  12.9 mAh/day

At SF7 (RSSI > -70 dBm): E_tx ≈ 7.2 mAh/day  →  battery life ≈ ~6.5 years

At SF9 (RSSI ≈ -80 dBm): E_tx ≈ 12.9 mAh/day  →  battery life ≈ ~4.0 years

At SF12 (RSSI < -100 dBm): E_tx ≈ 30+ mAh/day  →  battery life ≈ ~1.1 years

Step 3 — Battery life formula

Battery Life (years)  =  Battery Capacity (mAh)  /  (Daily Consumption (mAh/day) × 365)

Reference Battery Life Table

RSSI (dBm)	ADR Setting	Est. Battery Life	Swaps / Year (1,000 sensors)
-40	SF7	7.5 years	133
-60	SF7	6.5 years	154
-75	SF8	5.0 years	200
-80	SF9	4.0 years	250
-90	SF10	2.8 years	357
-100	SF11	1.8 years	556
-110	SF12	1.1 years	909
-120	SF12 + retrans	0.6 years	1,667

5. Annual Cost Model

The total annual cost of battery maintenance is made up of two components:

Cost Component	Value Used	Formula
Battery (AA Lithium)	SEK 50 per sensor	Swaps/year × SEK 50
Labour (technician)	SEK 420/hour, 20 min/swap	Swaps/year × (20/60) × SEK 420
Labour cost per swap	SEK 140	20 min × SEK 420/60 min
Total cost per swap	SEK 190	SEK 50 + SEK 140

Annual Cost Formula

Annual Cost (SEK)  =  (1,000 / Battery Life in years)  ×  (SEK 50 + (20/60 × SEK 420))

Cost Comparison by Signal Quality

RSSI	Battery Life	Swaps / Year	Battery Cost	Labour Cost	Total / Year
-60 dBm	6.5 yrs	154	SEK 7,700	SEK 21,560	SEK 29,260
-80 dBm	4.0 yrs	250	SEK 12,500	SEK 35,000	SEK 47,500
-100 dBm	1.8 yrs	556	SEK 27,800	SEK 77,840	SEK 105,640
-120 dBm	0.6 yrs	1,667	SEK 83,350	SEK 233,380	SEK 316,730

IMPACT

Moving from RSSI -100 dBm to -60 dBm saves approximately SEK 76,000 per year across 1,000 sensors. A single additional LoRaWAN gateway costs SEK 3,000-8,000 — it pays for itself within weeks.

6. What Can a Property Manager Do?

Improve RSSI by 10-20 dBm with low-cost actions:

• Reposition existing gateway higher (each floor gained = ~3-5 dBm improvement)
• Move gateway away from metal cabinets, lift shafts and electrical panels
• Install an outdoor or ceiling-mounted directional antenna
• Add a second LoRaWAN gateway to cover a problematic zone
• Use an antenna extension cable to reach a better mounting location

Monitor and act proactively with Sensor-online.se:

• Set RSSI alert thresholds — get notified when a sensor drops below -85 dBm
• Use the battery voltage trend to predict replacements 3-6 months in advance
• Plan batch replacements by floor or zone to minimise technician travel time
• Review gateway coverage map after any building renovation

7. Summary

Variable	Value / Assumption
Sensor model	Elsys ERS Lite (indoor environment)
Battery type	1× AA Lithium 3.6V, ~2,400 mAh
Transmission interval	Every 15 minutes (96×/day)
Protocol	LoRaWAN 868 MHz, ADR enabled
Fleet size	1,000 sensors
Battery unit cost	SEK 50 per sensor
Swap labour time	20 minutes per sensor
Technician rate	SEK 420/hour → SEK 140 per swap
Battery life model	Interpolated from Elsys datasheet + LoRa air-time calculator
RSSI range modelled	-40 dBm (excellent) to -120 dBm (critical)

This analysis was produced using the Sensor-online.se IoT Platform. For questions about your specific installation, gateway coverage, or battery monitoring configuration, contact your Sensor-online.se account manager.