One electronvolt is exactly 1.602176634 × 10⁻¹⁹ joules. Since 2019 that figure has been exact rather than measured, because the elementary charge was fixed as a defining constant of the SI.
About the units
An electronvolt is the energy gained by a single electron accelerated through a potential difference of one volt. It is a natural unit for atomic and particle physics, where joules would require exponents around minus nineteen in every equation. The 2019 redefinition of the SI fixed the elementary charge at exactly 1.602176634 × 10⁻¹⁹ coulombs, which made the electronvolt an exactly defined quantity as a side effect. Before that it carried a measurement uncertainty, and published values shifted slightly with each improvement in the measurement of the charge.
The exact factor
The value is exact by definition. The scale is what makes the unit useful: visible light photons carry 1.6 to 3.3 eV, chemical bonds are a few eV, X-rays are thousands, and the Large Hadron Collider accelerates protons to 6.8 tera-electronvolts. Mass is often expressed in the same currency through E = mc², so an electron's rest mass is 511 keV/c² and a proton's is 938 MeV/c². Room temperature corresponds to about 0.025 eV of thermal energy per particle, which is the number that explains why chemical bonds of a few eV do not spontaneously break.
Where you meet this conversion
Particle physics, atomic physics, semiconductors and spectroscopy. Band gaps in semiconductor materials are quoted in electronvolts — silicon at 1.12 eV, gallium nitride at 3.4 — and that figure determines what wavelengths a device can absorb or emit. X-ray and gamma spectroscopy label peaks in keV. Photovoltaic research, radiation dosimetry and electron microscopy all use the unit. Converting to joules is needed whenever a quantum-scale figure enters a macroscopic energy balance.