What I need is:
- Protection for various always-on SMPSU devices, with a total load of between 15W and 100W, against brief outages of a few cycles to a few seconds
- For a subset of these loads (50W max), autonomy of at least 40 minutes and preferably more than one hour
- Advance warning of impending shutdown, preferably in some standard way, so RPi's running Linux can be safely shutdown without peril to their SD cards
- Not to have the house set on fire, nor to risk electrocution
- Not to have damagingly high voltages, generated by the UPS itself, applied to the 'protected' loads
- Small size and quiet operation (beeps that can be turned off, if a fan is fitted, it must be quiet & speed-controlled)
What I do not need is:
- 400W or more of standby power, for a short period
- Less-than-one-cycle switchover times
- Protection against abnormally low or high line voltages - the loads can pretty much look after themselves, within limits that are exceeded here only once a decade or so
What I want is:
- A standby power consumption (with fully charged battery) that is as low as possible, and certainly less than the normal power consumption of the protected loads
- A user-replaceable battery, preferably hot replaceable
- Built in battery condition monitoring and/or test
I think we need to revisit the concept of want & need, but before I do that, I want to define what a UPS is designed to do.
The majority of off-the-shelf consumer UPSs are used with compute devices, and the intent is to allow the orderly shut down of the system in the event of a power outage so as to avoid the loss of data, this generally takes no more than fifteen to thirty minutes, so this is what the product is designed to provide. There are "mission critical" systems where downtime is not acceptable, and use cases where the cost of the downtime can be enumerated in hundreds or even thousands of dollars in lost revenue, and for these type of installations, standby power is what is required, and the UPS is needed to bridge the gap between the loss of the utility power and the standby power coming online.
As mentioned in an earlier post, off-the-shelf consumer UPSs are built to compete on price and most UPS manufacturers & engineers will advise against running their product for the sort of time frame listed in "Need #2", it's not that they don't have a product or they can't build a product that can deliver the requirement, but rather the use case is not the norm, so it requires a specialized product or configuration - extended run times can be achieved by specifying a unit that allows full time operation of the inverters (so double conversion), equipping the unit with additional battery capacity, and a suitable charger - it also needs to be made clear to the customer that the batteries will need periodic replacement, and frequent deep discharge of the batteries will necessitate more frequent battery replacement. These "extended run" systems are going to cost considerably more than a conventional off the shelf product, not only to purchase, but also to operate and maintain, which is the primary reason that the professionals will advise against them.
Back to need & want - I see much of your list as "nice to have", which is more of a want than a need - what will it cost you if the entire load goes down for more than a few seconds (Need #1)? what will it cost you per minute if the "subset of the load" goes down for over forty minutes (Need #2)? For Need #3, I can't think of a commercial product that doesn't come with some sort of AC failure and/or low battery signaling right out of the box, getting it to work with a RaspberryPi is slightly more challenging (again not the normal usage case), but not impossible (let me know if you want details, I have done it)
I can understand that you don't want the house to catch fire (Need #4), but that's easily resolved, don't install whatever it is in the house, install it in a data center with a fire suppression system - I'm going to stop here rather than go through your list item by item, because I'm sure you have recognized that I distinguish the difference between need & want based on a monetary value - you can get almost everything you've listed, provided you're willing to pay for it, which, based on "Want #1", I'd say you're not.
Bottom line - if you're running a business, you build a use case based on loss of revenue to explain to management why you
need a UPS, if it's a hobby, then it's no longer a need, it's just "nice to have".