The upper limit is calculated in this thread:

https://bitcointalk.org/index.php?topic=281279.0

Based on $100 per BTC and $0.10 per kWh the electrical cost break even point is about 179.7 PH/s.

Adjusting for $600 per BTC (x6) and $0.02 per kWh (x5) would give us 179.7 * 6 * 5 = 5,391 PH/s.

The same thread assumes 0.8 J/GH so:

    5,391 PH/s = 5.391 * 10¹⁸ H/s
    0.8 J/GH = 0.8 * 10^-9 J/H

    (5.391 * 10¹⁸ H/s) * (0.8 * 10^-9 J/H)
 
    = ( 5.391 * 0.8 ) * ( 10¹⁸ * 10^-9 )  ( H/s * J/H )

    = 4.3128 * 10^(18-9)  (J/s)

    = 4.3128 GW

World power production from:

http://en.wikipedia.org/wiki/Electricity_generation

is about 2311.4 GW.

So Bitcoin would use about 0.1866 % of world electrical output in this maximum build out scenario.

BTW in the above scenario the difficulty would be 753,000,000,000