Well, I calculated this by assuming 1kw of light energy is what falls onto a square meter in MC from the sun and assuming an overall efficiency of 19.99%.
1EU/t = 121.8140625 watts Whoops Watts is J/s, not J/(s/20) 6.090703125 is the correct value.
I'll do it again using a different, likely better suited value as the power of sunlight (750 watts per meter squared).
A more likely value: 91.360546875 watts in an EU/s or 4.56802734375 Joules per tick = 1EU/t
looks a lot like a calorie (the proper one, the one defined as the amount of energy required to heat one gram of water by one kelvin)