EL = Encounter Level

[d20Dwarf]

A CR 4 creature doesn't really increase the EL of an encounter with a CR 10 one, so the EL would still be 10. If you can't come up with a combination of CRs (through the method I explained) that would increase the EL, then just use the highest creature's CR as the EL. There are times when this method might produce an EL a bit too low for the reality of the situation, but that's primarily going to be subjective to your group and their abilities.

Prime example, my Dawnforge players are 6th level (ECL 7 by Dawnforge standards) and today fought a level 7 night elf wizard, level 6 night elf fighter, 2 gargoyles, and 3 level 4 orc fighters. That's CR 7 + CR 6 + (EL) 6 + EL (6), or 8+7+6, or 9+7, or 10. Final EL of 10 for 4 7th-level characters should be TOUGH, but because they had found a staff of frost with a few charges left earlier in the adventure, they were able to blow through the staff and obliterate their foes. Even with 2 cones of cold, though, 1 party member went down and another was at 3hp before the final gargoyle was finally dusted.

 

[Silveras]

Something else I use EL for in my homebrew is to give the PLAYERS an idea of how dangerous an area is (to represent what their CHARACTERS would understand about it). The Falor Woods, for example, is an area where the average EL is 9. Putting the EL for each area in my world into the player handouts gives them an idea of which ones are too tough for them to visit "just yet". Of course, it is an average, so they know there are some encounters that are quite a bit weaker than the listed number, and a few that are quite a bit stronger.

 

[dreaded_beast]

Great replies everyone!

I think I have a better understanding of EL and it's usefulness in planning my future adventures and encounter tables.