I'm coming up with a tagline for a solar company logo that says 'All you need is the Sun'. I am however confused with the use of 'the' and the capitalisation of 'sun' here. Are they necessary or 'All you need is sun' sufficient enough?
The capitalization or otherwise of 'sun' is a useful (but optional) device for distinguishing between our sun (i.e. the one at the centre of our own solar system) and suns somewhere else.
'The Sun' thus refers specifically to our sun, whereas 'sun' can mean both a sun (e.g. "That solar system has two suns"), our sun (e.g. "Remember not to look directly at the sun!") and sunlight (e.g. "It's time you came in out of the sun"), and is therefore ambiguous.
So how you decide to answer the question you have asked will depend on what you are trying to convey.
If you mean "All you need is (our) sun", then the most appropriate wording is "All you need is the Sun" or "All you need is the sun".
If you mean "All you need is sunlight", then I'd suggest you use that form of words if you want to avoid the ambiguity of "All you need is sun".