I'm finally getting pushed off my copper phone line to fiber/VoIP.

Grump, grump.

The faster internet will be nice, I suppose, but I'm unhappy with the available power backup options for keeping a phone going during a power failure.

My provider offers for sale an 8-hour backup device with a rechargeable battery and a 24 hour one that runs on a big pile of non-rechargeable D cells.

8 hours isn't really long enough. We've had power failures of about that length within recent years.

The usual computer Uninterruptible Power Supplies are not designed to keep a low power device (e.g. 4 watts) going for a long time - they're designed to supply a goodly amount of power for keeping your computer going long enough to shut down cleanly. You can't just assume that if you draw 1/10th the power of your computer that the UPS will run for 10 times as long; I'm not sure it's linear like that at low levels.

I'd like a 24 hour backup that uses a rechargeable battery. (Reliability would be nice, too.) If I can't find one, I may have to build my own.

Do you have a solution that you like?

I'm not too sure you're correct regarding the UPS not being ok for the 4w. devices. I assumed it depleted what was drawn. I use an old UPS to power my router and phone and it worked fine for a 2day outage.

That's good to know! Do you recall the model and/or size of your UPS? I may need to look for that one.

I've got an APC SUA1000 for my computer. It's a 670 Watt 1000 VA UPS.

They provide a web page for estimating runtime, at SUA1000 Features. It says the min-max range is 67-670W. When I put in 4W, it says, "The total load (power draw) entered is less than the minimum value for this product (67 W)." Not sure exactly what that means; will it not work, or is it just that they don't recommend it?

I also looked at the Jackery Explorer 160 Portable Power Station (which looks pretty nice!), but it automatically powers off after 6 hours if drawing less than 10W.

So it appears the behavior depends a lot on the particular model.

Correction: my ISP's 24 hour solution DOES use rechargables. (It looked similar to a different model that takes D cells. I got confused.)

It costs $300 and is single-purpose for the VoIP connection box only.

We kept a back-up copper line for about 5 years. In the end we decided that our cell phones were fine since 90% of calls go through them anyway. If cell service is unavailable, my UPS seems to work fine for the sorts of minimal power outages we get here. Don't see why a low draw wouldn't last about what calcs would show given the battery size you work with, the UPS only sees the draw and that is normally pretty linear effect on the battery.

The battery itself might be linear, but the UPS isn't - not at low power levels. The UPS itself uses power - I've seen numbers of about 10 watts for a 1000VA UPS. So if your load is 5W, then runtime calculations will be off by a factor of three (since the battery needs to supply 15W, not 5W). There may be other effects in either direction that I don't know about yet.

I just unplugged everything from my APC SUA1000, plugged in a 4W nightlight, and pulled the power plug. It stayed on, for 3 minutes at least. Yay.

However there are plenty of reports of other makes and models that DON'T work at low power levels, in addition to some reports that representatives of APC itself said, "their UPS won't function properly with such a low load."

There are lots of people claiming, with great confidence, exactly opposite opinions about this topic ( UPS for tiny load , How To Disable UPS Auto Turnoff? , etc.)

Unless you're the engineer that designed the thing, I think a few experiments on actual devices is worth any number of guesses.

The UPS's inverter will draw some power like you said. Your other option is to fix up a low voltage UPS. For example my router takes 12v in from a little 12v 1A transformer. So its pretty easy to run from a 12v SLA battery eliminating the inverter loss of an off the shelf ups.

USB powerbank with pass through charging?

It's not an uninterruptible power supply as likely drop for a fraction of second switching between input power and battery.

This seem sufficiently answered, but a few thoughts.

First, internet and VOIP are immediately backed up by UPS at my house. These tend to last several hours for me. You might plug your 8 hours backed up device into a UPS and stretch it to near the 24 hours you want.

Second, you can buy rechargeable D cells. So you could buy the 24 hour backup system and periodically top up the D cells. In fact, you could store more D cells and make it last even longer.

This is all pointless if the equipment on the other end of the line is down. That's what tends to happen where I live. Everything (cable and cell systems) has backup power for a few hours but eventually fails. Most service provider sites are rigged for generators but don't have one onsite, so unless one is brought in, the system stays down. Last time we lost power my cable modem was blinking but there was no one on the other end.

To complete my answer your actual question, I have a generator that recharges the UPS(es) and I can keep things going for a few days if needed. For a while I had starlink internet, which would in theory stay up during an outage. I switched back to cable, unfortunately.

Final comment, I'm getting solar and batteries. With that the house should be able to run quite a while w/o grid power.

I once played my digital piano for several hours on a 450 watt (?) UPS during a blackout. That was with headphones, not the speakers. So a UPS can indeed last a long time powering a low wattage device.

But rather than trying to provide backup power for VOIP, I'd just go for a cellphone. And have a separate rechargeable power bank. And use text messages rather than voice calls. For one, text messages take a lot less power. And second, they spool up at your ISP if they can't be delivered immediately (say if you only want to only turn your cellphone on for five minutes at the top of every hour to save even more power). If the power outage takes down your cell service ISP this won't work. But in that case, any VOIP provider you have would probably be down as well.

We have a whole house generator that runs on natural gas. When we first built our retirement home power outages were a somewhat common event, and with a well and septic system we decided that the cost was justifiable.

All of our communication (internet, phone, TV) comes & goes via satellite dishes. In the middle of a big rain/snow storm there is no communication. We have to find out if a landline phone service is even available here.

Here in northest Wisconsin even weather radio reception is marginal. Sometimes we'll get alerts, but most of the time we don't.

Oddly enough the state passed one of the largest legislative bills ever to provide broadband access to rural areas. A wireless broadband tower was built less than a mile north of us, but the trees completely block access, and I have no interest in building or maintaining a 100+ foot tall tower.

Even Elon Musk's system won't work here given the tree canopy. I've been told it'll work if we put the antenna on the peak of the roof, but I won't go on our roof to maintain it, so that's a no go.

That makes me wonder then; what UPS have you tried that won't run Michael2? Any I have will run unloaded even without issues and once you subtract the internal inverter load the runtime/power drawn is a pretty linear curve.

None! The whole point is that buying one and finding out after the fact that it doesn't work is a crummy plan. I'd like to know what the facts are instead of doing trial and error purchasing.

It's not a fiction that SOME UPS models and portable power stations shut down under low load. This is noted in the links I gave previously. Sometimes this "no-load shutdown" feature can be disabled, e.g., https://www.se.com/in/en/faqs/FA280865/ and sometimes it can't.

Sometimes people faced with this problem (having already shelled out money for a UPS) resort to electronic hacks to solve the problem.

To make things more confusing, sometimes the manufacturer (APC) says specifically that the UPS will shut down under low load, but users say it doesn't happen, and others say it happened to them. Need I mention that random sites on the internet do not always provide 100% reliable information? That's why I asked here, because I think it might be a bit more trustworthy.

And that's why I am interested in people's personal experiences with particular named UPS models that they personally have verified will run a 5W load. I did that with the big expensive and heavy one I have for my computer, as I mentioned above. But I'm looking for a cheaper solution that can be permanently installed next to my VoIP box.

I have two APC and one other brand that I've ran with no load to test them, so I haven't run across any issues personally with home/consumer models.
I could see the really big ones being like that.

But what power does your VIOP modem take, 5v, 12v, 19v?

Found a solution that met my specs:

APC CP12142LI Network UPS 12Vdc.

APC is a well-known reputable brand, this unit is specifically designed for 12v network devices (so it avoids the inefficiency of converting a DC battery voltage up to 120 VAC and then back down to 12 VDC) and it will keep my fiber terminal going for well over 24 hours. List price is $130; I got it for about $5 less (with free shipping).

I also bought a separate power supply for it (about $20) and an adapter cable to fit the terminal (about $7).

I had spotted this UPS a while ago, but it was out of stock all spring - supply chain issues I presume. I finally got it and installed it a few days ago. So far so good.

I've never seen that specific unit face to face, but APC is an excellent brand and they stand behind their stuff. I've sold and deployed quite a bit of APC gear in previous jobs.