Personally, I was often curious how much of 'global warming' is due to human energy generation (30%~40% efficiency), which all ends up as heat anyway.
Also, the average temperature in cities is way way higher than it is in more natural locations.
It shouldn't be significant because GW is all about how bad Earth is in dissipating energy into space (in comparison to black body), rather than how much energy flows through. Nevertheless, let's count: according to Wiki, 2012 World energy consumption is 5.6*10^20J; this means we were producing at an average rate of this/seconds in year/area of Earth, which is 0.034W/m^2. Solar input (and thermal radiation output, since Earth is roughly in an equilibrium) is 340W/m^2, making our input 0.1%, about the same as variations due to solar cycles.