What size UPS does a server need? The sizing maths, with a worked example

Step 1 — the energy the load actually consumes

Multiply the load in watts by the hours you need to ride through.

600 W × 2.5 h = 1.5 kWh usable energy.

Measure the load rather than reading the nameplate: a server's power supply is rated for its maximum, not its typical draw. A 750 W-rated 2U server with two CPUs and eight drives typically idles at 150–250 W and works at 300–450 W. A managed switch adds 20–60 W without PoE and much more with it; a small firewall adds 10–30 W.

Step 2 — the battery you have to buy

Batteries do not give you everything they store, and inverters lose some of it on the way out.

1.5 kWh ÷ 0.9 depth of discharge ÷ 0.9 inverter efficiency = 1.85 kWh → specify 2.4 kWh.

Lithium (LiFePO4) batteries are comfortable at 90% depth of discharge; lead-acid should be sized at 50%, which doubles the battery you need for the same ride-through — the main reason lithium wins on total cost for anything beyond an hour. Round up to a real battery size; 2.4 kWh is a common lithium module.

Step 3 — the inverter

The inverter must carry the load continuously, with headroom for start-up currents.

600 W ÷ (0.8 power factor × 0.9 efficiency) = 0.83 kVA → 1 kVA minimum.

We specify 2 kVA for a rack like this: server power supplies draw a brief inrush when they start, and the next server always arrives. UPS and inverter capacity is sold in kVA, which is why the power factor sits in the calculation.

Step 4 — the bill nobody budgets

A server never switches off, which makes it one of the largest single electricity consumers in a small office.

600 W × 24 h × 30.4 days = 438 kWh a month = R 1 226 at R2.80/kWh — R 44 126 over three years.

Put that number next to the purchase price. It changes the answer to "should we go to the cloud?" more often than the hardware price does. The load-shedding calculator runs all four steps for your own load and hours.

UPS prices in South Africa by capacity (September 2026)

UPS capacity	Live listings	Low	Median	High
Under 1 kVA	2	R 720	R 810	R 900
1–1.5 kVA	7	R 1 410	R 4 410	R 8 500
2–3 kVA	13	R 2 280	R 7 480	R 12 560
5–6 kVA	3	R 16 300	R 16 450	R 17 780
8–10 kVA	2	R 18 390	R 20 185	R 21 980

Source: Server Hub price index, live published prices, VAT-inclusive, 10 September 2026. The 1–1.5 kVA and 2–3 kVA bands span line-interactive units at the low end and lithium inverter trolleys at the high end, which is why they are wide. Inverters and portable power stations sit in their own band with a median of R 11 660. The full range is in solar, inverters and backup power.

Which UPS is best for a server?

• Line-interactive (the R 720 – R 4 000 units) is for desktops and a router. It switches to battery in a few milliseconds and corrects voltage roughly. Fine for a workstation; not for a server that runs a database.
• Online double-conversion rebuilds the mains waveform continuously and switches with zero transfer time. It costs more and runs warmer, and it is what a server wants. The 2 kVA online rackmount unit at R 7 480 is the typical spec for a single-server rack.
• A lithium inverter trolley behind the UPS is the answer to four hours or more. The UPS handles the transfer; the trolley carries the hours. The lithium 2 kVA trolley with a 2.4 kWh battery sits at the top of the 2–3 kVA band.
• A generator is the answer to all-day outages — with the UPS still in front of it, because generators produce dirty power while they stabilise.