you can think of the units of measure as multiplying and dividing sort of like the numbers themselves.

So if the nuclear battery continuously delivers 1 watt...

In one hour it would have delivered 1 Wh or watt-hour, because 1W * 1h = 1Wh.

And it works in reverse. If it takes 2 hours to deliver that 1 Wh? That's 1Wh per 2 hours or 1Wh/2h=0.5W!