Experience is gained from killing champions in single battles, arenas, and events. Losing will give half. Kills don't increase exp only winning or losing. Experience gained is based on your player level.
Below is experience gained from levels. Note that base xp is the xp gained from ai win. A +25% is gain on the base for pvp wins and mini boss wins. A lose is -50% of base xp.
Guild Experience is also gained from this but gives just -50% of the xp unless using Spirit of growth.
So at level 20 a player won't gain extra xp to their player level but it is still gained. Its only use is to level up guilds. At level 20 you will get 1513xp for ai wins to guild.