I mean, let’s be honest: People do crimes even when they have no material need to do so, so you will always need some form of law enforcement, how should a solarpunk society handle this?
The same is true for a military: As long as there is no world peace or some magical way to shut the outside world out, and as long as other societies exist somewhere nearby (and in today’s world this means somewhere on the same planet), a society needs the means to defend itself… what would this look like?
I reject the idea that law enforcement is always necessary. Laws are frequently unjust and anti-solarpunk. I don’t care about people breaking laws if the laws don’t serve the people. But let’s say all the laws are just - I also reject the idea that we need a bunch of power-hungry assholes with weapons and legal immunity to “enforce” them.
Here’s the thing - we rely on the police to do a broad range of tasks and, at least in the US, we hand them a gun to do all of those tasks. The traffic lights are down? Take your gun and go stand in the intersection. Parking tickets? You’ll need your gun for that. Homeless people on the streets? Don’t forget your gun. House broken into? Here’s a gun, try not to shoot the wrong person.
The simple answer is that we should have a different team of (trained, specialized) people for each of those scenarios. Police don’t receive training in each of those areas which means they are frequently in danger and frequently relying on violence or the threat of violence to get things done. It’s not fair to the officers and its not fair to the people they “serve.” I want to see a team who handles traffic, a team to respond to homeless people (though hopefully we’ll be on our way to finally providing housing for all), a separate team for crimes like burglary and theft, etc etc.
Here’s a resource for you (and others) to check out, they explain things much more succinctly than me: https://defundthepolice.org/alternatives-to-police-services/
As for a military - I will be completely honest, I haven’t educated myself as much on alternatives to that, but I do question your assumption that we will always need to defend ourselves. It assumes that people will always be attacking us.
Solarpunk and its Anarchist ideals are a pretty direct threat to virtually every other societal system. Fascists, Capitalists, and Authoritarians all either see it as an enemy to starve out, or even directly attack in an attempt to prevent its spread, as was seen during the Russian Revolution with Mahkno’s Black Army, and the Spanish Civil War.
As long as there are still state governments with military might at their disposal, a Solarpunk society is likely going to need to be able to defend itself against them. The Anarchists of Spain organized their voluntary militias from the bottom up, where the soldiers themselves would elect someone to lead the group (but with the ability to immediately recall them outside of battle if they didn’t perform their duties to the satisfaction of the soldiers who elected them), and those elected leaders would then elect a commander to help organize them more effectively, with the same recall rules applied. It seemed to work out quite well for them.
However, there may be a point in time where the advantages of the alternative solarpunk system are too apparent to ignore, which could spark mass-civil disobediance or even revolution in those other countries causing their governments and capitalist economies to collapse, hopefully then leading to them joining in with their own solarpunk movements. Were that to eventually happen the world over, the need for militias to exists would likely dwindle.