To add, although not quite the same as running a full server closet, I've also repurposed an old laptop as an always-on server and run a handful of web services for both professional and personal purposes from my home connection:
- Some utility applications I maintain for consulting clients. These tend to be incredibly low volume, accessed a handful of times a month at most.
- I host some analytics infrastructure for clients. It's for reporting, rather than data capture, so latency and uptime aren't super critical.
- I run a personal zerotier[1] network, which I use as both a virtual LAN across all my devices hosted everywhere as well as to tunnel my laptop and mobile traffic when I'm not at home. My internet gateway is hosted at home, so all my mobile and public wifi traffic routes through my home connection.
- I do a minor bit of web scraping. This is fairly low volume, and a mix of both personal and professional work. If it was higher volume I wouldn't host it at home, due purely to the risk/potential hassle of IP blocks and complaints.
I have a fairly stable symmetrical 1Gbps fiber connection (plus a block of static IPs). It's a residential connection so has no firm SLA, but still achieves ~500Mbps when experiencing "congestion" (in whatever form that means for fiber to the premise) and has only been down twice (once during a total power outage and another an attempt to use the parental control features for one device resulted in every device being MitM'd by an invalid SSL cert).
I also have a mobile hotspot I use when traveling for work, which I have connected as a failover ISP when I'm at home. This covered both instances of downtime I've experienced. And in the case when it doesn't and a client needs to access a service, I maintain external backups that I can spin up on VPS somewhere in a pinch. Probably not enough guarantees for a primarily SaaS based business, but has worked without a hitch when user-facing SaaS websites/apps are only a minor secondary component of the work.
[1] https://www.zerotier.com/