How to size a battery backup for a modem and WiFi router

Last updated September 26, 2026. Not a lab test. Hours below are arithmetic: published watt-hours ÷ named device watts (or V×A adapter ceilings). We did not run a modem/router outage on a station or UPS.

Size backup power from two published numbers: each networking box’s named watts (or adapter volts × amps), and the battery / UPS / portable power station’s watt-hours.

Hours ≈ battery Wh ÷ stack watts.

Outage internet is a chain: cable modem or fiber ONT → WiFi router (or a gateway that combines both). Miss one box and WiFi can look “up” while the WAN is dead. Stack the named figures, then divide.

Adapter labels are usually maximum supply ratings, not continuous draw. Where a maker also publishes idle / typical watts (ASUS SNE; Nokia ONT), prefer those for Wh math and keep the adapter rating for clearance. Prefer your label when it differs.

Affiliate note: Off Grid Power Boom may earn a commission from qualifying purchases. The site header already carries the disclosure—this is the one body reminder for this draft.

Named draws (manufacturer sources)

Cable modems — adapter V×A ceilings

These watt figures are adapter ceilings (V × A). Continuous modem draw is often lower; do not invent a measured idle. For pessimistic Wh÷W, use the ceiling. For a tighter plan, meter your wall wart with a plug-in watt meter and re-run the divide.

WiFi 6 routers — PSU rating + published idle where available

Use the SNE idle (or your meter) for runtime arithmetic when you have it. Use the PSU max when checking that a DC mini-UPS or AC inverter continuous rating clears the brick with headroom.

Fiber ONT (add this on fiber ISPs)

Fiber homes: stack ONT + router (and any separate WiFi nodes). Cable homes: stack modem + router. Gateway combos (modem+WiFi in one chassis) are one PSU—use that gateway’s ARRIS/NETGEAR table row instead of double-counting.

DC mini-UPS vs AC portable power station

Two honest paths. Same Wh÷W math; different conversion losses.

Path A — DC mini-UPS (12 V barrel class)

Many cable modems and some routers ship with 12 V wall warts (SB8200 / S33 / CM1000 / Archer AX55 in the table above). A DC mini-UPS or Li-ion pack with matching barrel voltage can feed those brick inputs (or replace the wall wart with a maker-approved 12 V path) and skip AC inversion.

• Pros: high efficiency on low continuous loads; small footprint for networking shelves.
• Cons: must match voltage, polarity, and connector; multi-voltage stacks (12 V modem + 19 V ASUS router) need two DC rails or stay on AC for the odd brick; ISP support may frown on non-OEM supplies—prefer pass-through of the stock adapter into a DC UPS that accepts the wall wart, when the UPS is designed for that.

Soft brand class (no invented affiliate links): APC / CyberPower compact AC UPS units, and third-party 12 V DC mini-UPS marketed for modem/router (confirm your V/A/polarity before buying). We are not inventing ASINs for those SKUs here.

Path B — AC portable power station (PPS)

Plug each factory wall adapter into a pure-sine AC outlet on a Jackery / BLUETTI-class station.

• Pros: works with any stock adapter (12 V or 19 V); one battery can also cover phones, a CPAP, or a nebulizer the same night.
• Cons: inverter idle + efficiency shorten real hours vs the raw Wh÷W divide; leave AC enabled for the whole outage or the stack drops when eco-mode kills low loads—read the station manual for low-load auto-off.

Path C — Traditional AC UPS (short bridge)

A line-interactive UPS keeps the wall warts alive through brief blips. Named maker examples (soft—no Boom affiliate invent):

These clear a modem+router stack on watts easily. Multi-hour outages still need Wh math (or a PPS). Maker half-load runtimes on lead-acid UPS pages are for much higher loads than a 15–50 W networking stack—do not paste those minutes as “modem runtime.” Re-run Wh÷W from the UPS’s usable Wh if the maker publishes it, or meter your stack.

Worked stacks (arithmetic only)

Stations already used on Boom’s CPAP / nebulizer shortlists:

• Jackery Explorer 300 Plus — 288 Wh, 300 W AC
• BLUETTI AC70 — 768 Wh, 1,000 W AC
• Jackery Explorer 1000 v2 — 1070 Wh

Example A — Fiber: Nokia ONT typical + ASUS RT-AX86U SNE idle

Stack = 3.66 + 8.75 = 12.41 W ≈ 12.4 W.

Example B — Cable adapter ceilings: ARRIS SB8200 + ASUS RT-AX86U PSU max

Stack = 18 + 45 = 63 W (pessimistic adapter-ceiling sum).

Example C — Cable adapter ceilings: NETGEAR CM1000 + TP-Link Archer AX55

Stack = 30 + 24 = 54 W.

Example D — Mixed honesty band: CM1000 adapter ceiling + RT-AX86U SNE idle

Stack = 30 + 8.75 = 38.75 W ≈ 39 W. Useful when you trust ASUS’s idle publish but only have the NETGEAR adapter rating for the modem.

These tables are ceilings from published Wh ÷ published W. They are not measured outage hours. Inverter efficiency and idle shorten AC results. Mesh nodes, VoIP ATAs, and PoE switches add more watts—add their labels before you trust a day-long claim.

Inverter idle and low-load cutoffs

A networking stack often sits under 20–60 W. Some portable power stations auto-shutdown AC below a watt threshold. If that happens mid-outage, the modem reboots and you lose the session. Check the station manual for eco / EPS / UPS modes and either disable low-load cutoff for the outage or use a small always-on load the maker documents as safe.

DC mini-UPS paths avoid that class of inverter eco-cut—but still need a charged pack and the correct barrel.

Soft station shortlist (makers’ Wh; Awin Jackery + BLUETTI)

Prefer Awin Jackery/BLUETTI CTAs (publisher aff 2016467). Soft maker links are fine if a listing drifts. No Amazon shortcodes on this niche page.

Jackery Explorer 300 Plus — 288 Wh

Jackery US: 288 Wh, 300 W AC. At Example A (12.4 W) ≈ 23 h arithmetic. At Example C (54 W) ≈ 5.3 h. Continuous AC easily clears modem+router adapter ceilings.

Check Jackery Explorer 300 Plus (Jackery US)

BLUETTI AC70 — 768 Wh

BLUETTI US: 768 Wh, 1,000 W AC. At 12.4 W ≈ 62 h; at 54 W ≈ 14 h; at 63 W ≈ 12 h. Same AC70 class cited on the CPAP page.

Check price at BLUETTI (Awin)

Jackery Explorer 1000 v2 — 1070 Wh (step-up)

Jackery US: 1070 Wh. At 12.4 W ≈ 86 h; at 54 W ≈ 20 h. Useful when the same station also covers lights, phones, a CPAP, or fridge planning the same night—still arithmetic, not a shared-load test.

Check Jackery Explorer 1000 v2 (Awin)

Safety and ISP notes

• Use only the OEM power supply or a replacement that matches the maker’s V / A / polarity / connector. Wrong polarity bricks gear.
• Do not open ISP-leased modems/ONTs or splice barrel leads.
• Backup keeps your LAN/WAN path powered. It does not restore a dead cable head-end or fiber OLT—if the neighborhood plant is down, local WiFi still will not reach the internet.
• For house-level generator hookups, see transfer switch safety (UL 1008)—do not backfeed a panel with a suicide cord.
• Related sizing: CPAP backup battery, nebulizer backup battery, 12V fridge / freezer runtime.

Related Boom job pages

• How to size a CPAP backup battery
• How to size a battery for a nebulizer compressor
• How to size a battery for a 12V fridge
• Transfer switch safety

Guides hub: how-to. Portable power stations hub: portable power stations.

What this page does not claim

• No Boom outage / uptime tests or invented “kept Zoom alive for N hours”
• No invented ASINs or watt-hours
• No claim that adapter V×A equals continuous draw
• No medical / life-support claims (see CPAP / nebulizer pages for those jobs)
• ISP plant outages are out of scope for a home battery

FAQ

How many watts does a cable modem and WiFi router use?
It depends. Named examples: ARRIS SB8200 / S33 12 V × 1.5 A (18 W) adapter; NETGEAR CM1000 12 V × 2.5 A (30 W); ASUS RT-AX86U PSU ~45 W max with 8.75 W SNE idle; TP-Link Archer AX55 12 V × 2 A (24 W). Read your labels.

How long will a portable power station run a modem and router?
Hours ≈ Wh ÷ stacked watts from cited sources. Expect fewer real AC hours once inverter losses and idle are included.

Should I use a DC mini-UPS or an AC power station?
DC when every brick shares a supported voltage and you want efficiency. AC PPS when adapters differ (12 V + 19 V) or you want one battery for multiple jobs. Traditional UPS units bridge short blips; multi-hour outages need Wh.

Do I need to power the fiber ONT?
Yes on fiber. Nokia G-010G-A lists about 3.66 W typical. Without the ONT, the router’s WAN has nowhere to go.