Milliwatts to Microwatts: Power Budgeting for Wireless and Energy Harvesting

A structural health monitoring node for a bridge was designed around a photovoltaic energy harvesting budget. The solar cell produced a measured 500 µW in indoor diffuse light conditions. The power budget spreadsheet was built in milliwatts. The firmware engineer entered the harvest figure as 500 mW — reading the datasheet value without noting the µW unit. The system appeared to have ample margin. The node was deployed. It went dark within six hours.

500 µW is not 500 mW. It is one one-thousandth of that. The actual power available was 0.5 mW — barely enough to run the microcontroller in active mode for a single measurement cycle per hour. The power budget was 1,000× wrong. The node was recovering from that error for the remainder of the field trial.

To convert milliwatts (mW) to microwatts (µW), multiply by 1,000. To convert microwatts (µW) back to milliwatts (mW), divide by 1,000. Use the mW to µW converter to verify any harvest or consumption figure before it enters a budget model.

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When comparing RF transmit power, sensor consumption, and harvest figures, convert mW to µW without risk.

The Power Scale in Wireless and IoT Design

Power design for battery-powered and energy-harvesting systems spans three orders of magnitude that cross the mW/µW boundary constantly. Understanding which side of that boundary your system operates on determines the entire architecture.

Notice the boundary: transmit events happen in mW; sleep states and harvest sources operate in µW. The entire power architecture of an energy-harvesting device is an exercise in reconciling these two scales.

For the current draw equivalent (mA vs µA), which is how power manifests at a fixed supply voltage, see milliampere vs microampere in low-power design. For timing implications (duty cycle in ms vs µs), see milliseconds vs microseconds.

1 mW = 1,000 µW

graph LR
    A[BLE TX Burst<br>15 mW] -->|"÷ 1,000"| B[Deep Sleep<br>15 µW... equivalent]
    C[Solar Harvest<br>500 µW] -->|"÷ 1,000"| D[0.5 mW<br>Available to budget]
    style A fill:#7c3aed,color:#fff
    style C fill:#22d3ee,color:#111

For the underlying SI prefix rule, see milli to micro conversion.

RF Power: mW and dBm

RF engineers express transmit power in both mW and dBm. The relationship is:

P(dBm) = 10 × log₁₀(P(mW))

A received signal at −100 dBm is 0.1 picowatt — ten billionths of a milliwatt. A transmitter at +20 dBm is 100 mW. Understanding where mW and µW sit in the dBm scale prevents gain budget errors in link margin calculations.

For reverse power conversions, use the µW to mW converter.

mW to µW Conversion Table

Frequently Asked Questions

How many microwatts are in a milliwatt? Exactly 1,000 µW = 1 mW. This is an SI-defined exact relationship. Use the mW to µW converter for specific values.

What is the power consumption of a BLE device in µW? A BLE device transmitting at a low duty cycle (e.g., advertising every 1 second) draws approximately 15–20 mW during a 2 ms transmission burst and 1–5 µW in deep sleep between bursts. The average power over 1 second with a 2 ms burst is approximately 15 mW × 0.002 + 0.003 mW × 0.998 ≈ 33 µW average — a figure that must be calculated in consistent units.

How does mW/µW relate to mA/µA for power budgeting? Power (mW) = Voltage (V) × Current (mA). At 3.3 V supply, 1 mA = 3.3 mW and 1 µA = 3.3 µW. Most embedded power budgets convert between mA and mW freely using the supply voltage — but only if the unit is consistent throughout. A mA figure and a µW figure cannot be compared directly without conversion.

What energy harvesting sources produce µW vs mW? Typical indoor photovoltaic (small panel, diffuse light): 100–500 µW. Thermal harvesting (low ΔT, wearable): 10–100 µW. Piezoelectric vibration: 1–100 µW. RF energy harvesting: 0.1–10 µW. These are firmly in the µW regime — which is why confusing them with mW figures produces 1,000× optimistic power budgets.

How is transmit power related to regulatory limits? Most BLE and Zigbee devices operate at +0 dBm to +10 dBm (1–10 mW). FCC/CE regulatory limits are typically +20 dBm (100 mW) for unlicensed ISM bands. LoRa operates at +14 to +27 dBm (25–500 mW) depending on regional regulations. All regulatory limits and link budget calculations require consistent units — mW or dBm, but not mixed.

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Next: Power consumption directly drives battery life at the mA/µA scale — see Milliampere vs Microampere: Measuring Low-Power Electronics.

Sources

• NIST Special Publication 811 — Guide for the Use of the International System of Units
• BIPM: SI Prefixes
• IEEE 802.15.1 Bluetooth Low Energy Standard — transmit power specifications
• Texas Instruments: CC2340R5 BLE SoC Datasheet — power consumption specifications
• Analog Devices: Energy Harvesting — Power Management for Wireless Systems