Seconded. Note that the vast majority of heat released by these processes is transferred to rivers, ultimately making its way to the ocean. A cloudy day over the ocean would have a greater effect.
To put it in perspective, a typical, 1200 MWe nuke plant generates about 3500 MWt, so it throws away 2300 MWt. Let's round up and say 2500. At about 100 nuclear plants in the USA, and 20% of US power coming from nuclear, and the US making up about 25% of the worlds GDP, that gives you 5 million megawatts being thrown away. I know this isn't quite precise, but it should be within an order of magnitude.
The solar energy constant at Earth's orbit is 430 BTU/hr/square foot, or 126 watts/ft 2. Give that Earth is 8000 miles in diameter, earth sucks down 1.76 x 10^11 megawatts of solar energy. That's 5 orders of magnitude of difference. Granted some of that is reflected back, and there are other effects to consider, but not 5 orders of magnitude of effects.
Thus, we can say that the waste heat generated by commercial power is less than a drop in the bucket.