A UPS keeps your equipment running for a few minutes when the power cuts, and shields it from voltage dips and spikes. Choosing one comes down to three numbers: the watts your devices draw, the runtime you need, and the UPS technology. This guide shows how to read them and how to size a UPS for a desktop PC, an internet box or a CCTV kit, using the UPS units in our shop as examples.
VA and W: the two ratings on the label
Every UPS shows two values, for example 500 VA / 300 W. VA (volt-amperes) is the apparent power the unit can pass through. W (watts) is the real power it can deliver to your devices. The watt figure is the one to size with.
The ratio between them is the power factor: W = VA × power factor. On the Eaton 5E500I (500 VA / 300 W) it is 0.6. On the Eaton 5E 900 (900 VA / 480 W) it is about 0.53. Two units with the same VA rating can deliver different watts, so always compare watts.
A common mistake: buying a 1000 VA unit for a PC with a 650 W power supply and assuming there is headroom. VA cannot be compared with watts, and an entry-level 1000 VA unit often delivers around 500 to 600 W.
Adding up the load
List everything you will plug into the UPS and note what each device draws. Three sources, from most to least reliable:
- A plug-in power meter between the wall socket and the device, in real use. This is the most accurate figure.
- The power adapter: multiply output voltage by current. A 12 V 1.5 A adapter supplies 18 W at most, and real draw is usually lower.
- The rating plate on the device, which gives a maximum.
Watch out with PCs: a 500 W power supply does not draw 500 W. That is its maximum capacity. An office PC without a dedicated graphics card draws far less, a gaming PC under load far more. Measure if you can.
Two worked examples
Office desk (example values, replace with yours): PC 150 W, monitor 25 W, internet box 18 W. Total 193 W. Add 20 to 30 % headroom so the UPS does not run at full load: about 240 W. The Eaton 5E500I (300 W) covers it, but the 480 W model from the same range gives more runtime on the same load.
CCTV: a DVR on a 12 V 2 A adapter (24 W max), four cameras on 12 V 1 A adapters (12 W each, 48 W in all) and the internet box (18 W). Total 90 W at most. The load is small, so runtime rather than power drives the choice.
Runtime: what the quoted minutes mean
Runtime depends on load. Eaton quotes 6 minutes for the 5E500I at half load, which is 150 W. At full load it will be much shorter. With a lighter load runtime grows more than proportionally, because a lead-acid battery gives back more energy when little is asked of it.
An office UPS is there to ride through short cuts or to let you save your work and shut the PC down cleanly. It is not a generator. If you want longer runtime, buy more power than you need: on the same load, a bigger unit lasts longer. On a model with a USB port and software, the PC can shut itself down before the battery runs flat.
Off-line, line-interactive or on-line
The three technologies differ in how they handle mains power.
| Technology | How it works | Unstable voltage | Where it wins |
|---|---|---|---|
| Off-line (standby) | Mains goes straight to the outlets. The battery takes over on a cut, after a short transfer time. | Little or no correction: the unit switches to battery. | Internet box, small low-sensitivity devices. |
| Line-interactive | Mains passes through a regulator (AVR) that boosts or trims voltage. Switches to battery on a cut. | Corrected without using the battery. | Desktop PC, workstation, DVR, small network. |
| On-line (double conversion) | Power is rectified and rebuilt continuously by the inverter. No transfer time. | Fully filtered. | Servers, critical equipment, very disturbed mains. |
For home or office use, line-interactive is the sensible middle ground. All four Eaton 5E models in the shop use it.
Why AVR matters
AVR (automatic voltage regulation) corrects voltage dips and surges without going to battery. Without it, every dip drains the battery a little, which wears it faster and can leave it half empty when a real outage comes. If your area sees low voltage in the evening, this is the first feature to check.
The 5E 900, 1200 and 1600 models also let you set the output voltage to 220, 230 or 240 V, according to the manufacturer’s sheet.
Output sockets: IEC C13 or wall-type
Many UPS units, including the whole Eaton 5E range, have IEC C13 outlets on the back rather than standard wall sockets. C13 is the female connector found at the end of a PC or monitor power cord.
- For a PC or monitor with an IEC C14 inlet: use a C14 to C13 cable.
- For an internet box, a DVR or anything with a plug-in adapter: you need a C14 to female wall-socket adapter.
Count your devices first: the 5E500I and 5E 900 have 4 outlets, the Eaton 5E 1200 (660 W) and the Eaton 5E 1600 (900 W) have 6. Avoid adding a power strip behind the UPS to get more: it is an easy way to exceed the rated power without noticing.
What not to plug in
- Laser printer: its fuser draws a heavy current while heating, far above its average consumption. It overloads the UPS and makes it trip or switch to battery. Plug it into the wall.
- Heater, kettle, hair dryer, air conditioner: their power far exceeds an office UPS.
- Another UPS or a surge-protected strip in cascade: the protections interfere with each other.
An inkjet printer draws little and can stay on the UPS if the power budget allows.
The battery: lifespan and replacement
Office UPS units use sealed lead-acid batteries. They age even without outages, and faster in a warm room. Expect 3 to 5 years in general.
Signs of wear: runtime shrinks, the unit beeps or reports a battery fault, or it cuts out at the first short outage. Replace it with a battery of the same voltage and capacity, and take the old one to recycling. Keep the UPS somewhere dry, away from heat, with air around the vents.
Our advice by use
- Internet box and router only: the load is light, and the Eaton 5E500I keeps them up well beyond its 6 minutes at 150 W.
- Desktop PC with monitor and box: aim for at least 300 W, or 480 W for more runtime and headroom.
- CCTV kit: the power needed is modest, so choose by the minutes you want and the number of outlets. Plan C14 adapters for the plug-in power supplies.
- Two workstations, or one powerful PC: step up to 660 W or 900 W, with 6 outlets.
Do the sum in watts, add your headroom, then compare the product sheets in the UPS category.
