Fission: Nuclear power stations have relatively low capacity factors (a measurement of how much of time they're actually connected to the grid and producing power). This is typically 70% over the last few decades for Western countries (i.e. a third of the time, a nuclear power station is not producing anything). As well as unintended maintenance shut down periods, most reactor designs require a multi-month shut down to refuel.
In recent years some countries have managed closer to 90%.
Fusion: By '24/7' you mean the 0.5 s sustained burn at JET in the 90s. Humanity has not yet managed to sustain fusion for a full second on the Earth, let alone get any useful power out of it.
ITER (the next generation Tokamak test reactor) isn't built yet. The intent is for it to achieve 1000 s of fusion burn time. Its over budget and behind time, and they've just had a management reshuffle to try and deal with this.
* Edited & extended to try and incorporate nbouscal's feedback from below
Do you mean to include the entire U.S. as one of those states? Wikipedia lists nuclear's capacity factor as 90.3% in the U.S. in 2009, and 88.7% averaged over 2006-2012, according to the DOE: https://en.wikipedia.org/wiki/Capacity_factor#United_States
The Helion design which Ycombinator funds is pulsed, so sustained burn time isn't applicable. It's also much smaller and cheaper than mainstream tokamaks.
I think you would like to know that your comment is completely impenetrable to an average intelligent layperson. All the words are reasonably understandable, but I have no idea how they translate into a response to the parent comment. My guess is that you're falling prey to the illusion of transparency [1], though I suppose it's possible that you were only intending your comment to be meaningful to people familiar with the industry jargon.
For over ten years the US nuclear fleet has maintained an average capacity factor of about 90%, and that is nearly 1/4 of the world's nuclear plants - http://www.nei.org/Knowledge-Center/Nuclear-Statistics/US-Nu.... It's not "some countries" it's most plants at around 90%.
