Realistically Possible - Combined UPS/Solar/Mains power?

[badass]

In theory it would make sense for my server to be solar powered where possible. However it needs no power interruptions. It currently uses an APC UPS. Pretend I have no intention of using it here.

For solar power to work, the power must be uninterrupted. In theory it's possible to have a decent capacity battery with the right chemistry (probably LFP) that uses mains power if both the charge drops below a certain percentage and the battery is not charging enough through solar.
However I cannot find any charge controllers that do that. They all seem designed for true off grid usage.

What I am after is this:
Solar can charge the battery to it's max capacity (that doesn't cause premature battery degradation)
Server can deplete battery overnight
Solar charges battery during the day.
When solar hasn't charged the battery enough, use mains power to maintain a minimum charge level that gives the server enough time to gracefully shut down is the power is cut.

I intend to use this as a mini project that should reduce energy bills a bit. If it can also act as a UPS then it's a bonus.

My questions:
Is there a solar charge controller that can choose to use mains power but only if the battery charge is below a certain percentage?
Is there one that also can report on the mains supply and battery percentage via an API or similar - i.e. I can programmatically check to see if a graceful shutdown is required. This requirement is less important because I can simply use my existing UPS to do that if necessary - ie. then the solar battery is discharged, the UPS kicks in and graceful shutdown begins.
Is using DC an option for power from said charge controller? Solar Panels are DC, Batteries are DC and electronics internally is DC. I suspect significant waste from using an inverter and then a rectifier when all that actually needs changing is the voltage.

[kalniel]

I've come across similar for hot water control - common in houses with solar panels - only the battery is water tank temperature with a minimum temperature causing mains to kick in. You might be able to re-purpose one of those, otherwise this sounds like the sort of thing Raspberry Pis could be useful for - check their forums to see if someone has done the same.

[Jonatron]

Does your server have redundant PSUs? You could use a smart plug on the PSU connected to the grid.

12v DC input PSUs do exist, but seem to be rare and low power: https://www.ebay.co.uk/p/3022058440

[jimbouk]

A few options for DC PSUs, but yeah mostly aimed at ITX computers. I had once (with a large laptop style block) for an APU/ITX mini build previously.

The 'dumb' solution (not calling API's, etc) would be a car battery and just to monitor the battery voltage - nice and high, disconnect the solar; too low, connect the mains adapter. Obviously would require something to do the monitoring of voltages. Maybe something for an Arduino? Would be good to have monitoring though so you can tune the on/off values.