OK, let's face it, XP rounding is a game mechanic that won't go away.... it's an accepted part of the game, so let's make full use of it....

When contributing to the base some levels give fractional XP per energy dropped in... so to maximize your XP

No XP modifiers [XP+0%]
Level 1 to Level 2, 1.8 XP per energy... putting in 12 energy at a time will give you 1.833 xp per energy
Level 2 to Level 3, 1.5 XP per energy... putting in 11 energy at a time will give you 1.545 xp per energy
Level 3 to Level 4, 1.3 XP per energy... putting in 12 energy at a time will give you 1.333 xp per energy
Level 4 to Level 5, 1.1 XP per energy... putting in 15 energy at a time will give you 1.133 xp per energy
Level 5+ to next lvl, 1.0 XP per energy... putting in any amount you want will give you 1.000 xp per energy

Human (10%) OR RSL implant (10%) (NOT BOTH) [XP+10%]
Level 1 to Level 2, 1.98 XP per energy... putting in 10-25 energy at a time will give you 2 xp per energy
Level 2 to Level 3, 1.65 XP per energy... putting in 10 energy at a time will give you 1.7 xp per energy
Level 3 to Level 4, 1.43 XP per energy... putting in 10 or 15 energy at a time will give you 1.4 xp per energy
Level 4 to Level 5, 1.21 XP per energy... putting in 12 energy at a time will give you 1.25 xp per energy
Level 5+ to next lvl, 1.1 XP per energy... putting in 15 energy at a time will give you 1.133 xp per energy

Neural Interface [XP+15%]
Level 1 to Level 2, 2.070 XP per energy... putting in 10 energy at a time will give you 2.1 xp per energy
Level 2 to Level 3, 1.725 XP per energy... putting in 12, 16, or 20 energy at a time will give you 1.75 xp per energy
Level 3 to Level 4, 1.495 XP per energy... putting in even numbered amounts of energy between 10 and 100 energy at a time will give you 1.5 xp per energy
Level 4 to Level 5, 1.265 XP per energy... putting in 10 energy at a time will give you 1.3 xp per energy
Level 5+ to next lvl, 1.15 XP per energy... putting in 10 energy at a time will give you 1.2 xp per energy

Human AND RSL implant [XP+21%]
Level 1 to Level 2, 2.178 XP per energy... putting in 10, 15, or 20 energy at a time will give you 2.2 xp per energy
Level 2 to Level 3, 1.815 XP per energy... putting in 13 energy at a time will give you 1.846153 xp per energy
Level 3 to Level 4, 1.573 XP per energy... putting in 10 or 15 energy at a time will give you 1.6 xp per energy
Level 4 to Level 5, 1.331 XP per energy... putting in 11 energy at a time will give you 1.3636 xp per energy
Level 5+ to next lvl, 1.21 XP per energy... putting in 12 energy at a time will give you 1.25 xp per energy

(Human (10%) OR RSL implant (10%) (NOT BOTH)) AND Neural Interface [XP+26.5%]
Level 1 to Level 2, 2.2770 XP per energy... putting in 13 energy at a time will give you 2.307692 xp per energy
Level 2 to Level 3, 1.8975 XP per energy... putting in 14 energy at a time will give you 1.9285714 xp per energy
Level 3 to Level 4, 1.6445 XP per energy... putting in 12, 15, 18, or 21 energy at a time will give you 1.666 xp per energy
Level 4 to Level 5, 1.3915 XP per energy... putting in 12 energy at a time will give you 1.41666 xp per energy
Level 5+ to next lvl, 1.265 XP per energy... putting in 10 energy at a time will give you 1.3 xp per energy

Human AND RSL implant AND Neural Interface [XP+39.15%]
Level 1 to Level 2, 2.50470 XP per energy... putting in 11 energy at a time will give you 2.545454 xp per energy
Level 2 to Level 3, 2.08725 XP per energy... putting in 18 energy at a time will give you 2.111111 xp per energy
Level 3 to Level 4, 1.80895 XP per energy... putting in 13 energy at a time will give you 1.846153 xp per energy
Level 4 to Level 5, 1.53065 XP per energy... putting in 18 energy at a time will give you 1.555555 xp per energy
Level 5+ to next lvl, 1.3915 XP per energy... putting in 12 energy at a time will give you 1.41666 xp per energy

Edit: Updated with optimum chunks for lvl 5+ XP modifiers

Nice finds there.

of course you'll be cursing me after doing a couple thousand energy in chunks between 10 and 20 at a time...

yeah i had made something like this for reference in excel, but it does get annoying how at higher levels xp return goes down. I donated 21k energy and got about overall a 1.1 xp return from it. I don't really mind seeing as it helps my legion but it does get a little annoying over time.

nope... it loses momentum after 100...

100 * 1.495 = 149.5, rounds up to 150... 150 / 100 = 1.5
102 * 1.495 = 152.49, rounds down to 152... 152 / 102 = 1.49

but 100 is a decent sized chunk that you won't get aggrevated with