Micro energy harvesting: Efficiency
From 0.1% to 85%: why each harvester has its own ceiling
What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.Micro harvesters turn small flows of motion, heat, radio waves or moisture into microwatts to watts. Conversion efficiencies range from about 0.1% for a body-heat watch to over 80% for some laboratory triboelectric devices, but the energy available to them is tiny, so output stays small whatever the efficiency.
Chart: Efficiency ranges
Commercial In service RecordEfficiency by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| Thermoelectric wristwatch (body heat) | 0.1%1 | Commercial | Heat flowing from skin to air converted to electricity across a 1.5 K temperature drop (22 microwatts) |
| Radioisotope thermoelectric generator (NASA MMRTG) | 6%2 | In service | Decay heat (about 2,000 W thermal) to electricity (about 110 W) at beginning of life |
| Roadway piezoelectric harvester (road test) | 10%3 | In service | Mechanical energy from passing vehicles to electricity, dynamic case at 65 mph (about 105 km/h) |
| Piezoelectric harvester power electronics (prototype) | 65 to 82%4 | Record | Electrical input to output of the conditioning circuit at 10 to 100 mW |
| Triboelectric nanogenerator (laboratory) | 70 to 85%6 | Record | Mechanical to electrical energy; 70% instantaneous, up to 85% total conversion |
| Pyroelectric thin film, Olsen cycle (laboratory) | 27% of Carnot7 | Record | Share of the Carnot limit achieved over a 10 K span (30 to 40 C) |
| Ambient radio-frequency rectenna (London field trials) | 20 to 40%8 | In service | Received radio power to DC; up to 40% for GSM 900, above 20% for TV and 3G bands |
| Betavoltaic cell, nickel-63 on silicon carbide (laboratory) | 2.2 to 2.8%15 | Record | Beta-particle energy to electricity; earlier published devices 0.5-2% |
Where the energy is lost
| Step | Loss |
|---|---|
| Heat passing through thermoelectric legs without being converted (MMRTG) | about 94% (calculated)2 |
| Vehicle motion not converted by a roadway piezo harvester | about 90% (calculated)3 |
Each method faces a different ceiling. Heat-driven devices (thermoelectric, pyroelectric) are capped by the Carnot limit, which is small across a few degrees: a 10% thermoelectric generator can need a hot side of at least 500 C. Mechanical harvesters can convert efficiently but only capture the slight deflection of a floor or road. Ambient radio waves are weak: only just over 50 of 270 London Underground stations had usable levels in each band surveyed.
Figures across methods are not directly comparable, because each measures a different input.
Sources
- Small Thermoelectric Generators (G. J. Snyder, The Electrochemical Society Interface) The Electrochemical Society, 2008
- Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) fact sheet NASA, 2020
- Ultra-High Power Density Roadway Piezoelectric Energy Harvesting System (CEC-500-2023-036) California Energy Commission / University of California, Merced, 2023
- Force Multiplier Actuated Piezoelectric Energy Harvester for Roadway Energy Recovery (CEC-500-2023-008) California Energy Commission / Pyro-E, 2023
- Progress in triboelectric nanogenerators as a new energy technology and self-powered sensors (Wang, Chen and Lin, Energy & Environmental Science 8) Royal Society of Chemistry, 2015
- Large pyroelectric energy conversion in lead scandium tantalate thin films (Aravindhan et al., Heliyon 10) Elsevier / PMC, 2024
- Current State of Research at Imperial College London in RF Harvesting and Inductive Power Transfer (Pinuela et al.) Imperial College London, 2012
- Fabrication and performance optimization of SiC-based betavoltaic batteries (He et al., Nuclear Techniques 48) Nuclear Techniques (Shanghai Institute of Applied Physics), 2025