How does the sun generate energy today?

Hint: Lord Kelvin was the first to point out that the present energy content of the sun is calculated from thermodynamics. The energy of the sun can be calculated with help of some of the fusion reactions that take place.

Complete step by step answer:
As we look outward from the earth and beyond the moon, the most obvious object in the sky is the sun. It is important to us for several reasons. It sustains a comfortable average temperature on the earth’s surface and it is the ultimate source of virtually all sources of our energy.
Every day the sun radiates or generates very enormous energy. The radiation of the energy is so large that it radiates the energy that can be used in one year in a single day. The energy from the sun is usually called renewable energy.
Now let us try to answer the given energy that is how the sun generates the energy.
Like most of the stars, the sun is made up of a plasma state that contains hydrogen and helium atoms.
Some fusion reaction cycles can be used to describe the energy of the sun. When the temperature of the core is reached about $1.5 \times {10^7}K$, which is enough for the hydrogen nuclei that are also called as protons in plasma to have sufficient energy on the average to fuse into helium nuclei. This reaction is a chain of reactions, was first proposed by H.A Bethe and it is referred to as the proton-proton cycle. The first reaction in the chain is:
$ \Rightarrow {}^1H + {}^1H \to {}^2H + {e^ + } + {\upsilon _e} + 0.42MeV$
The fusion is only possible because of quantum mechanical tunneling; this tunneling sets a limit on the rate at which the sun can produce energy. The limit is called the bottleneck of the solar fusion cycle. The reaction can be reduced into,
\[ \Rightarrow {}^2H + {}^1H \to {}^3He + \gamma + 5.49MeV\]
The given reaction is followed by,
\[ \Rightarrow {}^3He + {}^3He \to {}^4He + 2{}^1H + \gamma + 12.86MeV\]
The proton-proton cycle is the primary source of the sun’s energy. The neutrino created in the initial step escapes from the core. The net energy produced, including that released from the orbital electron binding.
Another most important cycle is the CNO cycle. That is known as the Carbon Nitrogen Oxygen cycle that accounts for about 1.5 percent of the total Luminosity. This cycle is very temperature dependent and a dominant H-fusion cycle.

Note:
Only a small portion of the energy of the sun strikes the earth through space. That is one part in two billion. Yet the energy is very enormous. The neutrinos produced in the proton-proton cycle escape from the core, providing the only means for direct observation of the sun’s interior.