How much battery do a modem and router need for eight hours?
For an illustrative combined 20W load in one location, our two scenarios call for about 217–343Wh of rated battery capacity. Your measured load and the battery’s behavior can change that answer substantially.
Published October 24, 2026 · Calculated scenarios, not a runtime guarantee
Start with operating watts, not the adapter label
Measure the equipment while it is doing the work you need during an outage. Use a suitable plug-in power meter if you already have one, or an existing device readout when it can measure the load reliably. Leave unknown values unknown until measured; guessing from an adapter’s maximum output can badly overstate normal use.
Include a separate modem or ONT, the router, and any switch or mesh node your connection requires. If one gateway combines the modem and router, measure and count that physical unit once. Group equipment by the battery it will actually use.
A worked 20W example
Suppose a modem uses 8W and a router uses 12W, both on one battery. These are sample inputs. The familiar calculation, 20W × 8 hours = 160Wh, describes energy at the load. It leaves out energy that never reaches the equipment and the battery’s own idle consumption.
The planner uses this relationship for each location:
Required rated capacity = hours × (equipment watts + idle overhead) ÷ delivered-energy fraction
| Scenario | Calculation | Capacity, rounded up |
|---|---|---|
| 85% delivered energy, 3W idle | 8 × (20 + 3) ÷ 0.85 | 217Wh |
| 70% delivered energy, 10W idle | 8 × (20 + 10) ÷ 0.70 | 343Wh |
The delivered-energy fraction combines conversion losses and unavailable capacity. The extra idle term represents battery-side overhead. These values are editable assumptions; they are not a measured efficiency rating or statistical confidence interval.
Why a 256Wh battery is a borderline example
| Rated capacity | Modeled runtime | Eight-hour target |
|---|---|---|
| 160Wh | 3.7–5.9 hours | Below target in both scenarios |
| 256Wh | 6.0–9.5 hours | Only the higher-runtime scenario reaches it |
| 512Wh | 11.9–18.9 hours | Both scenarios reach it |
This table compares energy capacity only. It is not a recommendation for a specific model. Also check continuous output, the correct voltage and outlets, battery condition, and supported low-load behavior. For example, the EcoFlow RIVER 3 Plus manual documents model-specific timeout and EPS considerations that a simple capacity figure cannot capture.
Separate rooms need separate calculations
If the modem is on a hallway battery and the router on an office battery, do not enter 20W on one shared battery unless they really share that source. Calculate each location independently. The shortest required runtime limits the connection, and a location with no backup brings it to zero. See the two-room ONT and router example.
Calculate my eight-hour backup plan ↗
In the calculator, choose “One room,” remove or exclude the sample switch, and enter your modem and router’s measured watts. Set the target to eight hours. Use “My own battery” to compare capacities and output ratings without choosing a listed product.
Your provider’s outside network must remain available. AT&T’s voice-backup guidance also illustrates why a separate ONT can require its own backup. Check your actual service’s documentation, plus transfer behavior and automatic shutdown settings, before relying on a plan.