The units used to measure energy in everyday life, like the Joule or the Calorie, are not well suited to elementary particles with very small such as protons or electrons, or nuclei of cosmic rays.
In this case particle physicists measure the energy in terms of electronVolt (eV). 1 eV is the energy acquired by a free electron when accelerated by an electrical potential difference of 1 Volt in vacuum. To make an example, a free electron placed in a field generated by a 12V battery would be accelerated by the potential difference and would acquire an energy of 12 eV. To convert eV to Joule we must recall that electrical potential is calculated dividing potential energy by electrical charge. So multiplying the electron charge by one Volt will do the job:
1 eV = e * 1V = 1.6 * 10-19 C * 1 V = 1.6 * 10-19 J
This is a very low energy. Electrons and protons in the hot solar corona, where temperature is about 1 million degree, have a mean energy of 100 eV. But as we already saw here, cosmic rays have much higher energies. To be detectable on ground, for example, cosmic rays impinging on Earth atmosphere must have a minimum energy of 450 MeV (450 millions eV).
To know how particle energy is related to its speed, stay tuned. I will talk about it in a future post: How to calculate speed of cosmic ray particles from kinetic energy.