4s 11A is 820g but i wouldnt use 4s on the low Kv motors that the S800 will have, you should stay with 6s which is better anyways. The link above shows you a 6s 11A lipo with weight 1.2 KG 400$ compared to a 15A 6S but 2.2 KG. The 11A probably will give you the same same flight time as the 15A...