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How long will my power bank or mini-UPS run the Wi-Fi?

By Bulan Sarkar, founder of ElectronicsInfoline (since 2000)

When the power goes, the Wi-Fi goes with it, unless the router and the fibre box run from a battery. This works out how long a power bank, a DC mini-UPS or any battery keeps them going, from the numbers printed on the labels. It works the same for a power cut in India or an outage anywhere else.

Runtime in hours = the battery's usable watt-hours ÷ the watts your devices draw. One maker's 20,000 mAh, 3.7 V bank holds 74 Wh and is rated 13,000 mAh at 5 V, which is 65 Wh out. A router with a 12 W adapter and a fibre box with a 7.2 W adapter draw at most 19.2 W: 65 Wh ÷ 19.2 W = at least 3 h 23 min.

1. What runs during the cut

Device 1

adapter 12 V × 1 A = 12 W (most it can draw), from the maker's spec page.

Device 2

adapter 12 V × 0.6 A = 7.2 W (most it can draw), from the maker's spec page.

Full charges from the same battery.

mAh
V

Both from the phone maker's spec.

2. The battery

mAh
V

The voltage printed beside the mAh.

mAh

= 87.8 % of the cell energy

hours

YesAbout 3 h 23 min

Yes: about 3 h 23 min against the 3 h you need.

Battery 74.00 Wh · load 19.20 W

Show the working
  1. Battery energy = 20000 mAh ÷ 1000 × 3.7 V = 74 Wh
  2. Usable energy = 74 Wh × 87.8 % = 65 Wh
  3. Load = 12 W + 7.2 W = 19.2 W
  4. Runtime = 65 Wh ÷ 19.2 W = 3.39 h (3 h 23 min)
Runtime against load for 65 Wh of usable energyHours = 65 Wh ÷ watts. At 5 W: 13 h; 10 W: 6 h 30 min; 20 W: 3 h 15 min; 40 W: 1 h 38 min. Marked: 19.2 W gives 3 h 23 min.5 W13 h10 W6 h 30 min20 W3 h 15 min40 W1 h 38 minhoursload
Runtime falls as the load grows: double the watts, half the hours. Curve for 65 Wh of usable energy.

Cell capacity 74.00 Wh. An adapter rating is the most a device can draw through it, so with adapter figures the real runtime can only be the same or longer. Nothing here is measured.

How the calculation works

Three lines of arithmetic, all shown under the result:

  1. Battery energy (Wh) = mAh ÷ 1000 × the voltage printed beside it.
  2. Usable energy = battery energy × the converter's efficiency.
  3. Runtime (h) = usable energy ÷ the total watts of everything on the battery.

The efficiency matters most, and it is the number makers print least. Many power bank labels give it away, though: they print a rated capacity at 5 V next to the cell capacity, and the ratio of the two is the maker's own figure for what comes out. If your label has no such line and the maker states no efficiency, pick “Not stated”: the calculator then shows the most any converter could give, and the efficiency you would need to reach your target.

Where the energy goes between the cells and your deviceLithium cells at about 3.7 V feed a converter that raises the voltage to 5, 9 or 12 V. Some energy turns into heat in the converter, so less comes out than the cells hold.Cellse.g. 3.7 VConverterraises voltageDevice5 / 9 / 12 Vheat (the loss)
Figure 1. The cells sit near their printed voltage; a converter raises it to what the device needs, and loses some energy as heat on the way. That loss is the efficiency figure in the calculator.
Runtime against load for 65 Wh of usable energyHours = 65 Wh ÷ watts. At 5 W: 13 h; 10 W: 6 h 30 min; 20 W: 3 h 15 min; 40 W: 1 h 38 min. Marked: 19.2 W gives 3 h 23 min.5 W13 h10 W6 h 30 min20 W3 h 15 min40 W1 h 38 minhoursload
Figure 2. Runtime falls as the load grows: double the watts, half the hours. Curve for 65 Wh of usable energy.

Worked examples

A router and fibre box sold in India

TP-Link Archer C6: adapter 12 V × 1 A = 12 W (most it can draw) (TP-Link). TP-Link XN020-G3 (GPON/XPON ONT router): adapter 12 V × 0.6 A = 7.2 W (most it can draw) (TP-Link). Total 19.2 W.

Power bank label: 20,000 mAh, 3.7 V, 74 Wh; rated 13,000 mAh at 5 V (Xiaomi). Efficiency from the label: 65 Wh ÷ 74 Wh = 87.8 %.

Runtime: 65 Wh ÷ 19.2 W = 3 h 23 min at least.

A bigger router sold in the US

TP-Link Archer AX55: adapter 12 V × 2 A = 24 W (most it can draw) (TP-Link). Huawei EchoLife EG8145V5 (GPON ONT): maker: maximum 12 W (Huawei). Total 36 W.

Power bank label: 30,000 mAh, 3.7 V, 111 Wh; rated 18,000 mAh at 5 V (Xiaomi), so 90 Wh out (81.1 %).

Runtime: 90 Wh ÷ 36 W = 2 h 30 min at least. The battery is 1.4 times bigger, but the load is 1.9 times bigger, so the hours come out shorter.

Reading the labels

Two labels give you every number the calculator needs. On each adapter, read the OUTPUT line. On the battery, read the cell capacity and, if it is there, the rated capacity.

Reading the output line on an adapter labelAn example adapter label. The OUTPUT line reads 12 V and 1 A; multiplied, 12 W is the most the device can draw through it.SWITCHING ADAPTERINPUT: 100-240V~ 50/60HzOUTPUT: 12.0V ⎓ 1.0A⊖-◐-⊕ centre positive12 V × 1 A= 12 Wthe mostit can drawINPUT is the wall side. Only OUTPUT matters here.
Figure 3. Read the OUTPUT line, not INPUT. Volts × amps gives the most the device can draw through that adapter: 12 V × 1 A = 12 W. Example label; yours will differ.
Reading a power bank label: cell capacity and rated capacityA maker's label: cell capacity 20000 mAh at 3.7 V, 74 Wh; rated capacity 13000 mAh at 5 V, which is 65 Wh. The maker's own figures put the output at 87.8 % of the cell energy.Cell capacity20000mAh 3.7V74WhRated capacity13000mAh (5V)Energy in the cells74 WhEnergy out at 5 V13 Ah × 5 V= 65 Wh65 ÷ 74 = 87.8 %Label lines as printed in the maker's spec
Figure 4. Two lines from a maker's power bank spec: the cells hold 74 Wh; the rated capacity at 5 V is 13,000 mAh × 5 V = 65 Wh. Out over in: 87.8 %.

Then wire it on the low-voltage side only: each output of the battery unit goes where that device's own adapter went. Nothing here touches mains wiring.

A DC mini-UPS or power bank feeding the router, fibre box and phoneThe battery unit has a 12 V output to the router, a 12 V output to the fibre box and a USB output for the phone. Each output voltage must match the device's own adapter label.Battery unitpower bank orDC mini-UPSWh on the label12 V DCWi-Fi router12 V DCFibre box (ONT)USB 5 VPhone
Figure 5. Low-voltage side only: the battery unit replaces each device's adapter. Match each output to the voltage on that device's own adapter label, and check the plug size and polarity.

Tips and tricks

  • Read OUTPUT on the adapter, not INPUT. INPUT is the wall side and tells you nothing about what the router uses.
  • Volts × amps on the adapter is a ceiling. The router can never draw more than that through it, so a runtime worked out from it is on the safe side.
  • Put the fibre box on the battery too. A router on battery with the fibre box off still loses the internet.
  • Match the voltage and the plug. A 9 V fibre box on a 12 V output is the wrong supply, even if the plug fits.
  • Count phone charging first. A full phone charge takes a fixed share of the battery before the router gets any.
  • Use the rated-capacity line when your power bank has one. It already includes the converter’s loss, so you don’t have to guess at it.

What we would do

Add up the adapter labels of the router and the fibre box, multiply by the hours your usual cut lasts, and divide by the efficiency the battery's own label implies. That is the Wh to look for. Then buy a unit whose outputs match each device's voltage, rather than the biggest number on the box.

If the labels show different voltages (say 12 V for the router and 9 V for the fibre box), a DC mini-UPS with both outputs is simpler than adapters and step-up cables. If your cuts run longer than this calculator says the unit can manage, a bigger battery is the only fix. A long cut and a home inverter are a different job: the inverter backup calculator covers that.

Questions people ask

How long will a 20,000 mAh power bank run a Wi-Fi router? It depends on the router. One maker's 20,000 mAh bank holds 74 Wh in its cells and is rated 13,000 mAh at 5 V, which is 65 Wh. A router whose adapter is rated 12 W plus a fibre box rated 7.2 W draw at most 19.2 W, so it runs them for at least 3 h 23 min.

How do I find out how many watts my router uses? Look at the OUTPUT line on its power adapter and multiply the volts by the amps: 12 V and 1 A is 12 W. That is the most it can draw through that adapter, so a runtime worked out from it is on the safe side.

Why is a power bank’s real capacity less than the mAh on the box? The mAh on the box is counted at the cell voltage, about 3.7 V. The output runs at 5 V or more, and the converter that raises the voltage loses some energy as heat. One maker’s own label puts the output at 87.8 % of the cell energy.

Does the fibre box need backup too? Yes. Without power the fibre box (ONT) cannot pass the internet to the router, so a router on battery with the fibre box off gives you Wi-Fi with no internet. Put both on the battery and add both to the calculator.

Can I run a 12 V router from a USB power bank? Only through something that gives 12 V: a DC mini-UPS with a 12 V output, or a USB cable with a step-up converter set to 12 V. A plain USB port gives 5 V. Match the voltage on the router’s own adapter label, and the plug size.

Sources

Each figure on this page is quoted from the source below or worked out from them. Sources checked , each quote read again against the source in a second pass.

  1. Xiaomi, Xiaomi Xiaomi Power Bank 20000 (Integrated Cable) PB2020MI: Cell Capacity 20000mAh 3.7V 74Wh Rated capacity 13000mAh (5V) mi.com/global/product/xiaomi-power-bank-20000/specs/
  2. Xiaomi, Xiaomi Xiaomi 18W Power Bank 30000mAh PB3018ZM: Battery power 111Wh 3.7V 30000mAh Rated capacity 18000mAh (5V/3.6A) mi.com/global/product/xiaomi-18w-power-bank-30000mah/specs/
  3. TP-Link, TP-Link Archer C6: Power 12 V ⎓ 1 A tp-link.com/in/home-networking/wifi-router/archer-c6/
  4. TP-Link, TP-Link XN020-G3 (GPON/XPON ONT router): External Power Supply 12V/0.6A tp-link.com/in/service-provider/gpon/xn020-g3/
  5. TP-Link, TP-Link Archer AX55: Power 12 V ⎓ 2 A tp-link.com/us/home-networking/wifi-router/archer-ax55/
  6. Huawei, Huawei EchoLife EG8145V5 (GPON ONT): System power supply 12 V DC, 1.5 A Static power consumption 5 W Maximum power consumption 12 W e.huawei.com/eu/products/optical-terminal/echolife-eg8145v5
  7. USB-IF, USB Type-C Cable and Connector Specification, Release 2.0 (August 2019): The following order of precedence of power negotiation shall be followed: USB BC 1.2 supersedes the USB 2.0 and USB 3.2 specifications, USB Type-C Current at 1.5 A and 3.0 A supersedes USB BC 1.2, and USB Power Delivery supersedes USB Type-C Current. usb.org/sites/default/files/USB%20Type-C%20Spec%20R2.0%20-%2…

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