Looking for a low-power cable modem? Ah, there’s this technique of turning off channels to save power. Alas, the power savings are dismal: 0.5 W in the best case. So, lesson learned, the bottom line of a low power cable modem is go for one.
20180912/DuckDuckGo low power consumption cable modem
20180912/https://www.excentis.com/blog/can-you-really-reduce-cable-modem-power-usage-using-energy-management-feature
Now you’re wondering about lead-acid battery capacity, size, weight, and cost? You’re in good luck. For being an uninterruptible power supply for small low-power network equipment, their capacity is pretty good: 12 volts, 7 amps, 20 hours. Size and weight are acceptable: 5.9 x 2.5 x 3.7 inches, 5 pounds. Cost is acceptable: $20. Safety is acceptable: lead-acid batteries don’t catch fire easily, don’t require charge control circuitry, and can’t overcharge.
20180912/DuckDuckGo 12 volt uninterruptible power supply lead acid battery
20180912/https://www.amazon.com/ExpertPower-EXP1270-Rechargeable-Lead-Battery/dp/B003S1RQ2S
This is a helpful article on building an uninterruptable power supply, for HAM radio.
20180912/http://www.westmountainradio.com/pdf/BuildYourOwnUPS.pdf
Make sure to use sealed lead-acid batteries, otherwise they can generate dangerous fumes when charging. “Two types of sealed lead acid batteries are Absorbed Glass Mat (AGM) and Gel.”
However, further investigation makes this obvious. How long will the battery last? Will the cost still be worth it adjusted for the longevity of the battery? Expect the battery to last between 6 months and 48 months, though if you’re lucky the battery may last you a few more years. Also note that having a smarter battery charge controller can help prevent over-discharge to your battery, though this is not strictly required with lead-acid batteries connecting wholly to switched-mode power supply circuits.