What factors affect the solar energy earth receives?

The Sun's solar output is more or less constant (it is getting hotter very slowly but over human time scales this is irrelevant) and the planet Earth is not growing in diameter. This means that the solar irradiation Earth receives is dependent on its position on its orbit. The orbit is slightly elliptical and therefore in December it is closer to the Sun than in June. This distance is the only factor that affects the solar energy impinging on the planet.