Help with key of a song?!

Yes, the tonal centre is E.

I wouldn't get too cut up about how some of the "out of key" chords can fit in and sound good. Any number of musicians will use such chords to add chromatic interest to a song, and if it sounds ok then just sit back and enjoy the song.

Try playing jazz one day - you'll be gobsmacked at some of the chord progressions you get there.

Thanks for your reply, so there is not any rule of thumb to what ''out of key chords'" I can add to a key? Also would a E major scale be best to use to solo over this song?
Thanks

No, there is no magic formula about which chords to use. So long as it sounds ok.....

You can't solo over the whole song in E because of the dissonances with the out-of-key chords. Time to get experimental on that as well.

Haha thanks! I'll definetly try.

There are rules of thumb... problem is, there's a whole bunch of them.

You've got E as your tonal center, and the E chord is major. So you're right in assuming your "core" chords are E, F#m, G#m, A, B, C#m and D#º. That accounts for some of the chords, but you've got some that don't fit: G#, C, and D.

G# has the notes G#, B# and D#. The B# note is outside the key. C has C, E, and G - C and G are outside the key. And D has D, F# and A - D is outside the key. So we have to account for the use of b6 (B#/C), b3 (G) and b7 (D). Let's look at how they're used in the tune...

You've got a change E-G#-C#m. The notes in those chords are E-G#-B, G#-B#-D#, and C#-E-G#. Notice that G# appears in all three chords. Now look at what the other notes are doing: E moves down to D# and back to E. B moves up to B#, and up again to C#. What you're seeing is "voice leading" - simultaneous melodic lines that combine to make up a chord change. When each voice is moving in a singable line, the chords make up a natural flow... E moves to its neighbor, D#, and then back; B does a chromatic step up to another scale tone. It sounds natural, even though B# is out of key.

This sets up the next change, which goes A-C-D-E. Notice that three of those four chords are in the key of A: A is I, D is IV, and E is V. You've modulated to a new key - jazzers would call A the "key of the moment", but the C note in the second chord doesn't fit either the main key of E or the temporary key of A. But also notice that C is the same pitch as the B# you heard earlier in the G# chord!

A great deal of a musical experience is rooted in short-term memory. No note sounded by itself will ever sound bad or "out of key". We only experience a note being out of key in context, and the earlier voice leading has prepared you to accept B#/C as having a place in the tune. So when we move to the key of A, the substitution of C for C#m is noticeable, but not jarring.

Chord changes like this are usually the result of experimentation on the part of the writer. But if they flow, you can almost always take them apart and develop a rationale for why they work (as I just did). If you're trying to come up with a way to use outside chords, the common modulations for a song in a major key are to the IV, V, relative minor, or tonic minor. Since there's more than one minor scale, that gives you a lot of possibilities. For a song in E, chords that might work are:

E major: E-F#m-G#m-A-B-C#m-D#º
A major (the IV): A-Bm-C#m-D-E-F#m-G#º
B major (the V): B-C#m-D#m-E-F#-G#m-A#º
C# harmonic minor: C#m-D#º-E+-F#m-G#-A-Bº
C# melodic minor: C#m-D#m-E+-F#-G#-A#-B#
E natural minor: Em-F#º-G-Am-Bm-C-D
E harmonic minor: Em-F#º-G+-Am-B-C-D#º
E melodic minor: Em-F#m-G+-A-B-C#º-D#º

I've put the three "outside" chords in bold so you can easily see how they can fit.

Thanks very much for your answer NoteBoat I couldn't find anything on the web quite as detailed as your answer. Ive got a lot of experementing to do now haha!
Much appreciated Matty.

A lot of times players do chord borrowing from the minor key. So the last 2 chords (C & D) work because they do relate to Emin. The G# is a bit of and oddity, but again it is pretty common to swap a major chord and a minor chord without causing too much disruption to the ear.

There is really better though of as a list of common observations than a set of rules. Ultimately your ears will tell you what "works".