Step 1: Install In-Game NBTEdit.(Here)
Step 2: Make sure you're in creative mode and cheats are enabled.(or you have admin powers)
Step 3: Place down an MFSU, this will be your box of infinite energy.
Step 4: While looking at the MFSU, type in /nbtedit.
Step 5: Click the arrow next to ROOT, click the word energy, then click the I icon at the top.
Step 6: Type in 1,999,999,999(No commas) then click save twice.
Step 7: Enjoy 2 billion EU
Note: This may not be too useful in creative mode as you can easily set up compact solar panels but this is still awesome.
Note 2: This works with anything that can store EU.