Can someone unpack that? What precisely is an average capacity factor?
Some day it will be near 0 (0% efficiency), or near 0.9 (90% efficiency). It depend on weather conditions.
A real world fusion power plant ("nuclear") basically runs as close as they can to 100% flat out 24/7 so their capacity factor is the fraction of the time their plant is not undergoing maintenance, refuelling or other disruptions. They might have a 95% capacity factor. They do this because their fuel (e.g. uranium) is so cheap compared to the cost of owning a power plant that it always makes sense to make electricity at any price.
An old-fashioned coal plant uses relatively expensive fuel, coal, which somebody has to be paid a lot of money to dig out of the ground and transport to them. It also costs money to clean the smoke from the plant so that it's less toxic as required by environmental laws in most places. Their plant could achieve nearly the same ratios as the fusion plant, but it would only make sense to do that if electricity was very expensive - when prices are low the plant idles, since running it would cost more than you'd earn selling electricity. As a result capacity factor might be 10% or lower at an older coal plant. This is why nobody (to a first approximation) is building new ones.
The photovoltaic solar plant produces "free" electricity when the sun shines. It's fairly low maintenance, and so basically you get a capacity factor that depends almost directly on how much the sun shines. When it's cloudy or the sun is low in the sky it produces much less power, at night it produces nothing. Capacity factor for such a plant may be about 20%.
Electricity from a Wind turbine is also nearly free. Sometimes it runs at night, but it's even less predictable. July is always good for PV panels in the North, but this July might bring a whole month of calm winds or several epic storms for the wind turbines and we've no way to know. Capacity factors for a particular turbine depend on location a lot, but 30% as here would be fairly typical.
Modern Combined Cycle Gas turbines again could hit a fairly high capacity factor, but since operating them is far from free (gas is cheaper than coal, but not THAT much cheaper) they are used to make up the difference between demand and cheap supply because they can be "spun up" quickly and you can quickly build as many as your country needs if you have a way to feed them gas. So in practice you might see 50% capacity factor but that could fall a lot if storage (for the cheaper renewals and fusion) undercuts it.
>A real world fusion power plant ("nuclear")
Certainly you meant to write fission power plant (splitting of heavy nuclei) as the real world nuclear we're presently using.
However, the point will probably also stand for the future fusion (combining light nuclei) power plants. Presuming we'll run them mostly with the cheap hydrogen for fuel (as opposed to the expensive deuterium), and presuming the operations will not require frequent, expensive maintenance.