I snagged a used Belkin BU3DC001-12V "Residential Battery Back-up" unit. This thing is the bomb. It replaces the 12V power brick for my router and my cable modem. It has 12v regulated output at 3amps, and my modem has a 2A brick and my router a 1A brick... so I got a splitter and an adapter and now both are powered by this thing. (I needed an adapter because my modem has a 5.5mm x 2.5mm socket, and the unit has a 5.5mm x 2.1mm plug. My router didn't need an adapter). If power goes out, I get about 3 hours of internet usage before the battery dies. That's about how long the battery in my big laptop lasts, so that works out well. But this thing is more than that, it's so much better than the absolutely DINKY brick that came with my TP-Link Archer C1200 router. In fact, the last time the power flikered my modem was unfazed, but the brick to my archer took a dump, and took the router out with it. Everything else that was plugged into that power strip and that outlet survived, but not the TP-Link brick. (The brick is light as a feather, compared to the brick for the cable modem, which feels much like an actual brick in weight).
I liked it so much, I snagged two more. Instead of running a splitter I'm going to run the modem and the router each from their own dedicated backup unit, and I plan to use the third backup unit to power the LED lighting in my living room. I also plan to wire up a 12V USB port so I can charge my phone when the power goes out.
We've been having absolutely torrential rains here recently, and power has flickered quite a bit... and each time it resets the router, a good 3 minutes before it's back online... and often there's another power flicker right after. As I work from home, this is a huge frustration. Also, hurricane season is coming up soon, and it'll be nice to have internet and living room lights when everyone else is using candles and flashlights. One of these days I'll get a generator... but even then, probably only for the fridge and/or the furnace fan. Only the essentials, as the cost and gas consumption makes a whole-house generator impractical.
So yeah, these things are great. They'll power anything that runs off of 12V, they condition the input current and highly regulate the output voltage. And unlike a regular 120V UPS, they're better than 90% efficient when running off of batteries. (A UPS that has to convert internal battery voltage to 120V is around 30% efficient on a good day. If you're then converting back to 12V to run your wifi... well, then... um... yeah, that sounds like the least efficient thing possible)
Also, they go for around $20-$25 shipped from eBay.