Neutrinos were theorized when some energy was noticed to be 'missing' from beta decays, correct? But until recently, they were thought to be massless.
So, how much energy does a single neutrino have?
I can find many places to look up the energy (in eV, or whatever units) of a photon of a particular frequency, and the electrical charge/energy of a single electron, and so on.
But I have found no site that states the amount of energy in a neutrino....
There are two types of energy a free particle can have, kinetic energy and the rest mass energy. The rest mass energy is the energy given by Einstein's famous mass-energy equation $E=mc^2$. When we are dealing with relativistic particles, and neutrinos are almost invariably travelling near the speed of light. we don't make this distinction and simply write the total energy of the particle as:
$$ E^2 = p^2c^2 + m^2c^4 $$
where $p$ is the relativistic expression for the momentum:
$$ p = \frac{mv}{\sqrt{1 - v^2/c^2}} $$
Neutrinos have a non-zero rest mass (the $m$ in the above equation) but this mass is very small and we don't know its values for the three types of neutrino. At the moment we only have upper limits. Anyhow the non-zero rest mass means there will be a minimum energy the neutrinos can have of $mc^2$.
But in general there is no typical energy for a neutrino. It's like asking what the energy of a bullet is - it depends on how fast the bullet is travelling. For example the neutrinos produced by free neutron decay have energies around $0.5$ MeV, while the Icecube observatory has detected a neutrino with an energy of several PeV (a petaelectronvolt is $10^{15}$eV or about a hundred times greater than the LHC energy).
