So from some googling it looks like the biggest nuclear subs top out at about 30MWe of output, which is roughly the same as EDFs West Benhar wind farm (https://www.edf-re.uk/what-we-do/onshore-wind/) which has 7 onshore wind turbines and can power about 18,000 homes.

Subs reactors will also be designed around the needs to power the sub with will presumably have much different load requirements to commercial nuclear power station.

I'm also willing to bet it isn't exactly economically viable running it long term.

Slightly related but there was a time they used a engine as an emergency generator for a town https://youtu.be/FWYbD2ga8DM

Why is the sub thing a bad idea?

It's apparently been done in Russia (they also use nuclear-powered ice-breakers) but they don't necessarily care about safety too much. When it is done elsewhere, it tends to be that the marine reactor is no longer in the sub/ship or they modify it heavily and facilities are built to facilitate it's use as a power plant - scroll down to "Marine reactors used for power supply, floating nuclear power plants" here.

Here's something from someone who has thought it through (the ultimate source seems to be this post from a submarine officer):

Can a nuclear submarine [or ship] provide power to the grid in a disaster? I know it was done once after a Hawaii hurricane, but a Navy nuclear power officer recently listed the problems with the idea:

1. MOST IMPORTANTLY - There is no standard procedure to allow this. Submariners don’t brush their teeth without a vetted and approved procedure.

2. The vast majority of the rated power for a reactor is for propulsion and the vast majority of the electric capacity is for plant operations to support 100% reactor power.

3. The shore power breaker is not that big (compared to the needs of a city). It’s only meant to provide ship service power alongside the pier and sufficient power to start the reactor.

4. To jury-rig some cable into the power plant switchgear would violate many design criteria of the plant, the power plant electrical design is evaluated as a component of reactor safety to ensure it is safe. Imagine the vetting required for some Rube Goldberg solution, running an unapproved cable into the ER hatch would violate the plant’s containment. - I can’t imagine NAVSEA would risk a spotless, 100s of millions of hours (on well over 200 ships) of safe operation and a legacy of safety spanning 61 years when other solutions are available.

5. What do you do with the cable on the other side? The power grid has tons of reverse power protections and features to protect vital distribution equipment - how do you get it to the exact location you want it? There are no extension cords capable of handling that many amps long enough and if there are - the line loss would be huge. For the EE [electrical engineers] engineers and people in power generation- I also think it’d be a reactive power nightmare…

Basically, there's no giant socket on a nuclear sub to plug anything into (some recharge the batteries from dockside others just do it from the excess power of the reactor when in dock) and there would need to be a step-up transformer with a sufficiently heavy-duty cable to run it to the grid, where the power breakers would need to be over-ridden. So basically, you'd be jury-rigging a number of complicated systems (including a nuclear reactor) for no good reason. Also if he thinks he's going to try that in the middle of Liverpool then he can do one. I think JRM can now consider himself on the Scouse Shit-list - by default Tories are on the Naughty List and you have to work hard to get off it (basically only Hezza is welcome to come round for a pint, but he still might have to get the ale in to guarantee no-one kicks off).

Why is the sub thing a bad idea?

Because JRM suggested it.