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A typical modem + router will usually run on a UPS for:
| UPS size | Typical runtime for modem + router only |
|---|---|
| Small 400–600 VA UPS | ~1–3 hours |
| Medium 750–1000 VA UPS | ~2–5 hours |
| Large 1350–1500 VA UPS | ~4–8 hours |
| Large power station / high-capacity battery | 10+ hours to multiple days |
For most home setups, assume the modem and router together draw about 15–30 W. A dedicated 750–1000 VA UPS often gives roughly 3–5 hours if the battery is healthy.
The runtime depends mainly on three things:
The basic estimate is:
\[ \text{Runtime in hours} \approx \frac{\text{usable battery energy in Wh}}{\text{load in W}} \]
For example, if your modem and router consume 20 W, and the UPS can deliver about 80 Wh of usable energy:
\[ \frac{80 \text{ Wh}}{20 \text{ W}} = 4 \text{ hours} \]
So the modem and router would run for approximately 4 hours.
| Device | Typical power draw |
|---|---|
| Cable/DSL/fiber modem | 5–20 W |
| Basic Wi-Fi router | 5–15 W |
| High-performance Wi-Fi 6/6E/7 router | 15–35 W |
| Combined modem/router gateway | 10–30 W |
| Small Ethernet switch | 3–10 W |
A common home modem plus router combination is usually around:
\[ 15–30 \text{ W total} \]
If you have a high-end gaming router, mesh node, PoE switch, NAS, or ONT, the load can be higher.
One common mistake is assuming that a 1000 VA UPS has twice the runtime of a 500 VA UPS. The VA rating mainly tells you the maximum load the UPS inverter can support, not directly how much energy the battery stores.
For runtime, the more useful value is watt-hours, or Wh.
If the UPS battery is listed as:
\[ 12 \text{ V}, 9 \text{ Ah} \]
then the theoretical battery energy is:
\[ 12 \times 9 = 108 \text{ Wh} \]
However, you usually cannot use all of that energy at the AC output. Losses occur in the inverter, the router’s power adapter, and the UPS control electronics.
A practical derating factor is often:
\[ 0.5–0.7 \]
So a 108 Wh battery may provide only about:
\[ 108 \times 0.6 \approx 65 \text{ Wh} \]
of useful output energy.
Assume the modem and router together draw 20 W.
| UPS battery configuration | Theoretical battery energy | Usable energy estimate | Approx. runtime |
|---|---|---|---|
| 12 V, 5 Ah | 60 Wh | ~35 Wh | ~1.75 hours |
| 12 V, 7 Ah | 84 Wh | ~50 Wh | ~2.5 hours |
| 12 V, 9 Ah | 108 Wh | ~65 Wh | ~3.25 hours |
| 2 × 12 V, 9 Ah | 216 Wh | ~130 Wh | ~6.5 hours |
| 250 Wh power station | 250 Wh | ~200 Wh | ~10 hours |
| 500 Wh power station | 500 Wh | ~400 Wh | ~20 hours |
These are estimates. Real runtime may be lower if the UPS battery is old, the load is higher, or the UPS is inefficient at low power.
Most consumer UPS units use sealed lead-acid batteries. These degrade significantly over time.
A UPS battery that gave 4 hours when new may give only 2 hours or less after several years.
Typical replacement interval:
A normal AC UPS converts:
\[ \text{Battery DC} \rightarrow \text{AC output} \rightarrow \text{router power adapter DC} \]
This double conversion wastes energy.
At low loads such as a modem and router, some UPS units are not very efficient. The effective efficiency may be around 50–80%, depending on the model.
Some UPS models have an energy-saving feature that shuts the UPS down when it thinks there is “no load.” A modem and router may draw so little power that the UPS misinterprets them as no load.
If that happens, the UPS may shut off even though the battery still has energy.
Check the UPS manual for settings such as:
Disable that feature if possible.
Only plug essential network equipment into the battery-backed outlets if you want maximum runtime.
Good candidates:
Avoid plugging these into the same UPS if internet runtime is the goal:
A 200 W desktop computer can drain a UPS many times faster than a 20 W network setup.
Best method: use a plug-in power meter.
Measure the modem and router together at the wall outlet.
If you do not have a meter, check the power adapters.
Example:
Router adapter:
\[ 12 \text{ V} \times 1.5 \text{ A} = 18 \text{ W} \]
Modem adapter:
\[ 12 \text{ V} \times 1.0 \text{ A} = 12 \text{ W} \]
Maximum combined power:
\[ 18 + 12 = 30 \text{ W} \]
Actual power may be lower, perhaps 15–25 W, because adapter ratings are usually maximum ratings.
Look for the battery specification in the UPS manual or on the battery itself.
Examples:
Calculate:
\[ \text{Wh} = \text{Voltage} \times \text{Ah} \]
Example:
\[ 12 \text{ V} \times 9 \text{ Ah} = 108 \text{ Wh} \]
If there are two 12 V, 9 Ah batteries:
\[ 2 \times 12 \times 9 = 216 \text{ Wh} \]
Use:
\[ \text{usable Wh} = \text{battery Wh} \times 0.5 \text{ to } 0.7 \]
For a healthy UPS, using 0.6 is a reasonable planning value.
Example:
\[ 108 \text{ Wh} \times 0.6 = 64.8 \text{ Wh} \]
If the modem and router draw 20 W:
\[ \frac{64.8}{20} = 3.24 \text{ hours} \]
So the estimated runtime is about 3 hours.
If you want:
| Desired internet runtime | Recommended backup approach |
|---|---|
| 30–60 minutes | Small 400–600 VA UPS |
| 2–4 hours | 750–1000 VA UPS |
| 4–8 hours | 1350–1500 VA UPS dedicated to networking |
| 8–24 hours | Large UPS, DC UPS, or portable power station |
| Multi-day runtime | LiFePO₄ battery system or power station with solar/generator recharge |
For best results:
For only a modem and router, a DC mini-UPS can be more efficient than a standard AC UPS.
A standard UPS does this:
\[ \text{Battery DC} \rightarrow \text{120 V AC} \rightarrow \text{adapter DC} \]
A DC mini-UPS does this:
\[ \text{Battery DC} \rightarrow \text{regulated DC output} \]
That avoids one conversion stage and can improve runtime. However, you must make sure:
For networking-only backup, a DC UPS or LiFePO₄ battery system is often technically better than a conventional desktop UPS.
A modem and router usually draw 15–30 W total. On a typical consumer UPS, expect approximately:
For a quick estimate:
\[ \text{Runtime} \approx \frac{\text{battery Wh} \times 0.6}{\text{modem/router watts}} \]
If you give me the UPS model and the power adapter ratings for your modem and router, I can estimate your runtime more accurately.
