How to size a UPS for your server room or office

In most Nigerian offices, a UPS is not there for the rare outage. It carries your equipment through frequent grid cuts, the gap before the generator starts, and voltage swings that quietly damage hardware. Get the size wrong and you either waste money or watch the UPS shut down when you need it. Here's how to size one properly.

Step 1: List what the UPS must protect

Be selective. A UPS should keep critical equipment running, not the whole office. Typical candidates:

• Servers, NAS and storage
• Firewall, router, switches and Wi-Fi access points
• Internet modem or fibre terminal
• CCTV recorder (NVR) and PoE switch
• Key desktop PCs and monitors

Leave out laser printers, kettles, air conditioners and space heaters. Laser printers in particular draw large surges that can overload a UPS.

Step 2: Add up the real load in watts

Find each device's power draw on its label or datasheet. Use the actual or typical consumption where available, rather than the power supply's maximum rating, which can overstate the load considerably. If you can, measure with a plug-in power meter.

These figures are illustrative. Always use your own equipment's numbers.

Step 3: Understand VA vs watts

UPS units are often advertised in VA (volt-amperes), but what matters for sizing is the watt rating. The ratio between them is the power factor. A "2000 VA" UPS might be rated anywhere from about 1200 W to 1800 W depending on the model. Always check the watt figure on the datasheet.

Step 4: Add headroom

Don't run a UPS at 100%. Aim for your load to be around 60–75% of the UPS's watt rating. That leaves room for growth, start-up surges and battery ageing.

For the example above: 890 W ÷ 0.7 ≈ 1,270 W, so you would look for a UPS rated at roughly 1,500 W or more.

Step 5: Decide how much runtime you need

Runtime is about batteries, not the UPS rating. Ask: what must happen during an outage?

• Bridge to generator: 10–15 minutes may be enough if a generator with automatic changeover reliably starts.
• Safe shutdown: enough time for servers to shut down cleanly, often 10–20 minutes, ideally automated with the UPS's shutdown software.
• Keep working through outages: several hours, which usually means an external battery bank, or an inverter and battery system for the wider office.

A rough estimate of runtime is:

Runtime (hours) ≈ battery voltage × amp-hours × usable depth of discharge × inverter efficiency ÷ load (W)

Example: a 48 V, 100 Ah lithium (LiFePO4) bank at 80% usable capacity and 90% efficiency, powering 890 W: 48 × 100 × 0.8 × 0.9 ÷ 890 ≈ 3.9 hours. The same bank in lead-acid, where you should typically use only about 50% to protect battery life, gives roughly half that.

Step 6: Choose the right UPS type

• Online (double-conversion): equipment always runs from the inverter, with no transfer gap and full protection from voltage swings and generator fluctuations. The right choice for servers and core network gear.
• Line-interactive: regulates voltage and switches to battery during outages. Good for desktops and small network cabinets.
• Offline / standby: basic and cheapest. Fine for a single PC, not recommended for servers.

Look for pure sine wave output, especially for servers and modern power supplies.

Common mistakes we see

• Sizing on VA instead of watts.
• Plugging printers, ACs or extension boards full of chargers into the UPS.
• No battery replacement plan. Lead-acid UPS batteries often need replacing every 2–4 years, sooner in hot rooms.
• Poor earthing and no surge protection on incoming power and network lines.
• Never testing. Run a controlled test every few months to confirm the runtime is still there.