The cheapest form of energy is solar and wind energy.
Solar and wind energy are renewable energy sources that have low operating costs which are even lower than fossil fuels such as oil, natural gas, coal etc.
However installing solar panels and wind generators can be expensive, especially when you need to install a huge solar farm or install large wind turbines.
For homes you would need to install a lot of solar panels as well if using solar energy along with the battery banks and inverters that can raise the cost into thousands of dollars which can take over a decade or longer to pay back.
However for small solar panel setups the cost can be recovered pretty quickly and you can use solar part time and use your normal electricity when you need more power.
Also improved efficiency of solar panels and wind turbines, along with the competition among renewable energy providers have also lead to lower costs.
Natural gas for electricity generation is sometimes cheaper in some areas for a power plant to operate steam turbine generators and in other places the coal can be cheaper to burn to make steam to power the steam turbine generators.
We still do need natural gas and coal power plants as well as nuclear power plants because solar panels and wind turbines cannot provide the entire world with enough power.
And solar only works when the sun is shining and wind turbines only work when the wind is blowing enough to spin the wind turbines.