# Micro energy harvesting: Efficiency

From 0.1% to 85%: why each harvester has its own ceiling

Source page: https://www.materialinsights.org/energy/e_micro_harvest/efficiency/

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 Record

#### Efficiency by measure

| Measure | Range | Type | What is measured |
|---|---|---|---|
| **Thermoelectric wristwatch (body heat)** | 0.1%^[1](https://www.electrochem.org/dl/interface/fal/fal08/fal08_p54-56.pdf) | 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](https://science.nasa.gov/wp-content/uploads/2024/02/mmrtg-factsheet-updated-5-18-20-1.pdf) | 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](https://www.energy.ca.gov/sites/default/files/2023-06/CEC-500-2023-036.pdf) | 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](https://www.energy.ca.gov/sites/default/files/2023-03/CEC-500-2023-008.pdf) | Record | Electrical input to output of the conditioning circuit at 10 to 100 mW |
| **Triboelectric nanogenerator (laboratory)** | 70 to 85%^[6](https://pubs.rsc.org/en/Content/ArticleLanding/2015/EE/C5EE01532D) | Record | Mechanical to electrical energy; 70% instantaneous, up to 85% total conversion |
| **Pyroelectric thin film, Olsen cycle (laboratory)** | 27% of Carnot^[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC11079096/) | 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](https://spiral.imperial.ac.uk/handle/10044/1/10270) | 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](https://m.researching.cn/articles/OJ7d84641c3d0cbec) | 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](https://science.nasa.gov/wp-content/uploads/2024/02/mmrtg-factsheet-updated-5-18-20-1.pdf) |
| **Vehicle motion not converted by a roadway piezo harvester** | about 90% (calculated)^[3](https://www.energy.ca.gov/sites/default/files/2023-06/CEC-500-2023-036.pdf) |

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

1. [Small Thermoelectric Generators (G. J. Snyder, The Electrochemical Society Interface)](https://www.electrochem.org/dl/interface/fal/fal08/fal08_p54-56.pdf) The Electrochemical Society, 2008
2. [Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) fact sheet](https://science.nasa.gov/wp-content/uploads/2024/02/mmrtg-factsheet-updated-5-18-20-1.pdf) NASA, 2020
3. [Ultra-High Power Density Roadway Piezoelectric Energy Harvesting System (CEC-500-2023-036)](https://www.energy.ca.gov/sites/default/files/2023-06/CEC-500-2023-036.pdf) California Energy Commission / University of California, Merced, 2023
4. [Force Multiplier Actuated Piezoelectric Energy Harvester for Roadway Energy Recovery (CEC-500-2023-008)](https://www.energy.ca.gov/sites/default/files/2023-03/CEC-500-2023-008.pdf) California Energy Commission / Pyro-E, 2023
5. [Progress in triboelectric nanogenerators as a new energy technology and self-powered sensors (Wang, Chen and Lin, Energy & Environmental Science 8)](https://pubs.rsc.org/en/Content/ArticleLanding/2015/EE/C5EE01532D) Royal Society of Chemistry, 2015
6. [Large pyroelectric energy conversion in lead scandium tantalate thin films (Aravindhan et al., Heliyon 10)](https://pmc.ncbi.nlm.nih.gov/articles/PMC11079096/) Elsevier / PMC, 2024
7. [Current State of Research at Imperial College London in RF Harvesting and Inductive Power Transfer (Pinuela et al.)](https://spiral.imperial.ac.uk/handle/10044/1/10270) Imperial College London, 2012
8. [Fabrication and performance optimization of SiC-based betavoltaic batteries (He et al., Nuclear Techniques 48)](https://m.researching.cn/articles/OJ7d84641c3d0cbec) Nuclear Techniques (Shanghai Institute of Applied Physics), 2025

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_micro_harvest/index.md)
- [Overview](https://www.materialinsights.org/energy/e_micro_harvest/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_micro_harvest/correlation/index.md)
- [Techniques](https://www.materialinsights.org/energy/e_micro_harvest/techniques/index.md)
- [Resources and labour](https://www.materialinsights.org/energy/e_micro_harvest/resources/index.md)
- [Impact on the biosphere](https://www.materialinsights.org/energy/e_micro_harvest/biosphere/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_micro_harvest/links/index.md)