Power Outages and Internet: Why Fiber Modems Need a Battery Backup

August 18, 2026 · Fiber internet for real communities

Public domain, via Wikimedia Commons

If the power goes out, your fiber internet goes out too, unless the equipment in your house has its own battery. This matters most for anyone who relies on a landline replacement for 911 calls, a medical alert system, a security system, or a home business phone. The single decision that matters is how many hours of runtime you actually need, because that number decides everything else about the battery you buy.

Decide this first

Figure out what you're actually trying to keep alive: just phone service for emergency calls, or phone plus Wi-Fi so you can work or check email during a multi-hour outage. A battery backup sized for a few hours of phone calls is small, cheap, and simple. A battery backup sized to keep a router and Wi-Fi running for half a day is bigger, heavier, and needs more planning. Most residential outages in the U.S. last under four hours, but ice storms, hurricanes, and grid failures can knock out power for a day or more, so decide which scenario you're actually planning for before you shop.

What to look for

Runtime versus load

Every battery backup unit is rated for a certain runtime at a certain load, usually stated in watts or amps. The number on the box assumes a specific device draw, and your ONT (the box that turns the fiber light signal into an electrical one) plus a router will draw more than the ONT alone. As a rough range, a small dedicated ONT battery backup might give you 4 to 8 hours powering just the ONT for phone service, and 2 to 4 hours if you also try to run a router and Wi-Fi off the same unit. Treat any number on packaging as a best-case estimate, not a guarantee, since real runtime drops as the battery ages.

What the unit actually powers

Some backup units are built specifically for the ONT and phone adapter, with just enough capacity for voice calls and nothing else. Others are small UPS units meant to also carry a router and maybe a switch. Check the outlets and the wattage rating, not just the brand name. If you want Wi-Fi during an outage, you need enough capacity for the ONT, the router, and possibly a mesh access point, and that changes which unit you need.

Battery type and replacement plan

Most backup units use sealed lead-acid batteries, which are cheap and reliable but typically last 3 to 5 years before they need replacing, sooner if the unit sits in a hot closet or garage. Some newer units use lithium batteries, which cost more up front but tend to hold capacity longer and handle heat better. Either way, ask what a replacement battery costs and how hard it is to swap, because a backup unit with a dead battery is just a box that does nothing when you need it most. A realistic replacement cost range is $20 to $60 for a basic sealed lead-acid battery, more for lithium.

What to ignore

Skip units marketed with vague claims like "whole home internet backup" without a wattage spec attached. There's no such thing as a battery backup that powers your entire home network indefinitely on residential hardware, so any claim that sounds unlimited is marketing, not engineering. Also ignore fancy LCD displays and app integrations if the core battery and outlet count don't match what you actually need to power. A backup unit with a bright screen and a weak battery is worse than a plain one that actually lasts through your outage.

Common mistakes

The most common mistake is assuming fiber itself carries power the way old copper phone lines used to. It doesn't. Copper landlines got their electricity from the phone company's central office, which is why an old-fashioned wall phone kept working when the power went out at home. Fiber carries light, not electricity, so the ONT in your house needs local power to convert that light into a signal your devices can use. No local power, no ONT, no internet or phone, unless a battery is doing the job.

Another common mistake is sizing the backup for the ONT alone and then being surprised when Wi-Fi doesn't work during an outage. The ONT staying powered keeps a wired phone line active if you have a phone adapter, but it does nothing for your Wi-Fi network unless the router is also on backup power. If you need internet access, not just a dial tone, plan for the router's draw too.

The third mistake is buying a backup unit once and never touching it again. Batteries degrade with age and heat, and a unit that gave you 6 hours of runtime when new might give you 90 minutes three years later. Test it periodically by unplugging the unit from wall power for a few minutes and confirming your phone or router actually stays up. If it doesn't, the battery likely needs replacing, and that's a five-minute fix if you catch it before an actual outage.

FAQ

Does fiber internet stop working the moment the power goes out?

Yes, unless the ONT is on a battery backup. The fiber cable itself doesn't carry electricity, so the equipment converting the signal in your house needs its own power source. Without a battery backup, an outage means no internet and no phone service tied to that connection, even though the fiber line outside is completely undamaged.

How long will a typical battery backup actually last during an outage?

For a unit powering just the ONT for phone service, a realistic range is 4 to 8 hours when the battery is new, less as it ages. If you're also running a router for Wi-Fi off the same unit, expect something closer to 2 to 4 hours. These are rough ranges, not promises, since actual runtime depends on battery age, temperature, and exactly what's plugged in.

Do I still need a battery backup if I have a home generator?

If your generator kicks in automatically within a minute or two of an outage, you may not need a separate battery for short outages, since the generator will keep the ONT and router powered directly. But generators can fail to start, run out of fuel, or take a few minutes to kick on, so a small battery backup is still cheap insurance for that gap, especially if 911 calls depend on the connection.

Will 911 calls work if my internet phone service loses power?

Not if the ONT and phone adapter lose power and have no battery backup. This is the main reason providers, including GigSpeed, recommend a battery backup for anyone using internet-based phone service as their primary line, particularly households with medical alert devices, security systems, or anyone in an area prone to long outages.

Is it worth it for a small business to invest in a bigger backup setup?

If phone lines, point-of-sale systems, or security systems depend on the internet connection, yes. A short outage that knocks out card payments or phone lines for a few hours can cost more than a decent battery backup unit, and municipal or commercial UPS units built for higher loads and longer runtime are a reasonable investment for a business that can't afford to go dark.