What size UPS do I need? A backup power calculator
Pick the devices you want to keep running and how long for. The calculator gives the output and battery size to look for.
- Format
- Calculator
- Read time
- 4 min
- Updated
- Written by
- Vorax
Add up the watts of everything you want to keep on, then add 25%. That is the output your backup unit must deliver. For battery size, multiply the watts by the hours you need and divide by 0.85. A fibre router and ONT draw about 15 W, so a small mini DC UPS of around 40 Wh covers two hours. A desktop PC and monitor draw about 180 W, which needs roughly 430 Wh for the same two hours.
Power cuts are part of life in South Africa, and the right backup unit depends on two numbers: how much power your devices draw, and how long you need them to stay on. Use the calculator, then read on for what the numbers mean and which kind of unit suits you.
Calculator
How much backup power do I need?
- Total load
- 0 W
- Output to look for
- –
- Battery for 2 hours
- –
Wattages are typical figures; check the label or power adapter of your own devices for exact numbers. The output figure allows 25% headroom and the battery figure allows about 15% for conversion losses. Batteries also hold less as they age. Do not run heaters, kettles or laser printers from a small backup unit.
Watts, watt-hours and VA
Backup power is sold using three different units, which is where most of the confusion starts.
A quick way to estimate runtime: take the battery size in watt-hours, multiply by 0.85 to allow for losses, and divide by the watts you are drawing. A 55 Wh unit running a 15 W router lasts about three hours.
Three kinds of backup, and what each is for
| Type | Rated in | Runs for | Good for |
|---|---|---|---|
| Mini DC UPS | Watts and Wh | Hours | Fibre router, ONT, small cameras |
| Traditional UPS | VA | Minutes | Saving your work and shutting a desktop PC down safely |
| Power station with UPS mode | Watts and Wh | Hours | A PC, monitor, TV and router through a long outage |
Most traditional UPS units are designed to bridge a few minutes, not to work through a two-hour outage. If you want to keep working, look at battery capacity in watt-hours and not only the VA rating.
This is the backup power range in the store, with current prices and stock.
Keeping the Wi-Fi on
For many homes, the router and fibre box are the only things that have to stay on. A mini DC UPS does this without any conversion to mains power, so little energy is wasted.
- Check the voltage. Routers and fibre boxes commonly run on 12V, but some use 9V or 5V. The label on the power adapter tells you.
- Check the plugs. Make sure the supplied cables fit both devices, and that the unit has enough outputs for each of them.
- Check the watts. Add the two devices together and compare the total with the unit's output.
- PoE devices. Some access points and cameras take power through the network cable. They need a mini UPS with a PoE output.
Keeping a computer on
A desktop PC, monitor and router together draw far more than a router alone, and a gaming PC more again. For these, three things matter.
- Output in watts. It must exceed your total load with headroom. The calculator adds 25%.
- Switch-over time. When the power fails, the unit takes a moment to take over. Look for a UPS mode with a short transfer time in the specifications, or the PC may restart.
- Pure sine wave output. Modern desktop power supplies work best with it.
A laptop has its own battery, so it does not need a UPS to survive a power cut. Backing up the router is usually enough to keep working.
Do not run heaters, kettles, hairdryers or laser printers from a small backup unit. They draw far more power than a computer and can overload it.
Working it out by hand
- Find the wattage of each device on its label or power adapter.
- Add them up. That is your load.
- Multiply by 1.25. That is the minimum output to look for.
- Multiply the load by the hours you need, then divide by 0.85. That is the battery size in watt-hours.
Batteries hold less as they age, so a little extra capacity is worth having.
Frequently asked questions
How long will a UPS keep my router on?
Multiply the battery size in watt-hours by 0.85 and divide by the watts your router and fibre box draw. A 55 Wh mini DC UPS running a 15 W load lasts about three hours.
What does VA mean on a UPS?
VA stands for volt-amps and is the rating used on traditional UPS units. The power you can use, in watts, is usually around 60% of the VA figure, so a 1000 VA unit supports roughly 600 W.
Can a UPS run a gaming PC?
Yes, if its output in watts is higher than the PC and monitor draw together, with headroom. Runtime will be short unless the battery is large, because a gaming PC can draw several hundred watts under load.
What is the difference between a UPS and a power station?
A traditional UPS switches over instantly but usually runs for minutes. A power station has a much larger battery and runs for hours. Some power stations include a UPS mode that switches over automatically when the power fails.
Do I need a UPS for a laptop?
No. A laptop's own battery carries it through a power cut. A mini DC UPS for the router is usually all you need to stay online.
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