2.8 kw 12volt solar pannel earthing wire

If you are in an area where lightning is likely to hit, I would suggest that 6 AWG or heavier wire is good. 8 AWG and lighter has been known to "fuse".

If you are not in the US, here is a handy conversion chart:
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=guide_awg_to_metric

AWG Number	Ø [Inch]	Ø [mm]	Ø [mm²]
6/0 = 000000	0.580	14.73	170.30
5/0 = 00000	0.517	13.12	135.10
4/0 = 0000	0.460	11.7	107
3/0 = 000	0.410	10.4	85.0
2/0 = 00	0.365	9.26	67.4
1/0 = 0	0.325	8.25	53.5
1	0.289	7.35	42.4
2	0.258	6.54	33.6
3	0.229	5.83	26.7
4	0.204	5.19	21.1
5	0.182	4.62	16.8
6	0.162	4.11	13.3
7	0.144	3.66	10.5
8	0.128	3.26	8.36
9	0.114	2.91	6.63
10	0.102	2.59	5.26
11	0.0907	2.30	4.17
12	0.0808	2.05	3.31
13	0.0720	1.83	2.62
14	0.0641	1.63	2.08

Make sure you run the 6 awg straight or in nice rounded turns--Sharp corners can cause the lightning to take different paths.

Off Grid Grounding Technique?
Another Question, this time about Lightning
Lightning Protection for PV Systems

From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

Towards the end of this thread is a very nice discussion of proper generator grounding.

-Bill