1 · Build your load
Add each device. Presets are typical figures — replace the watts with your nameplate or measured power for an accurate result.
2 · Power factor & margins
VA = W ÷ PF. Modern online UPS are 0.9–1.0; legacy line-interactive ~0.8.
3 · Runtime target
Exact runtime depends on the model, battery age and any extended battery modules — Servnet confirms it for your build.
Advanced — battery runtime estimate & N+1 redundancy
We don't publish prices — a 24-year UK reseller, Servnet confirms the exact UPS, batteries and pricing on quote.
Watts, VA and kVA — and why they differ
A UPS is rated in VA (apparent power) but your equipment is measured in watts (real power). They are linked by the power factor: VA = W ÷ PF. Size too small on either and the UPS overloads; that is why this tool checks both the kVA and the kW limit and rounds up to a real rating.
It also reports the heat output of your load — almost all the power drawn becomes heat — so you can size cooling with the cooling calculator, and the load current so you can check the supply circuit. Runtime is a Peukert-corrected estimate, because a battery gives far less than its rated capacity under heavy load.
Plan the whole rack
UPS sizing — common questions
What size UPS do I need?
Add up the wattage of everything the UPS will protect, divide by the UPS output power factor (typically 0.9) to get VA, then add headroom (20–25%) and any growth allowance, and round up to the next standard rating. This calculator does all of that automatically: enter your equipment and it returns the recommended kVA/kW, so a 3,000 W load at 0.9 PF with 25% headroom needs roughly 4,200 VA — a 5 kVA UPS.
What is the difference between VA, watts and kVA on a UPS?
Watts (W) is real power — the actual work done and the heat produced. VA (volt-amps) is apparent power, which is what the UPS is rated on. They are linked by the power factor: VA = W ÷ PF. A 5 kVA UPS with a 0.9 output power factor can deliver 4.5 kW. The calculator shows both your watts and VA, and sizes the UPS against both limits so neither is exceeded.
How accurate is the runtime figure?
Runtime is shown as a clearly-labelled estimate. Battery runtime is non-linear under load (Peukert’s law) — a battery delivers far less of its rated capacity at high discharge rates — and the real figure also depends on the exact UPS model, battery age and whether extended battery modules are fitted. We apply a Peukert correction (so it is realistic, not optimistic) and badge it as an estimate. For the exact runtime of a specific model and battery configuration, we confirm it on quote.
How much cooling does my IT load need?
Almost all the electrical power a server draws is converted to heat, so a 3 kW IT load produces about 10,236 BTU/hr (3 kW × 3,412 BTU/hr per kW), or roughly 0.85 tons of cooling. The calculator shows the heat output of your load in BTU/hr and cooling tons so you can size air-conditioning or CRAC capacity alongside the UPS.
Should I size the UPS for future growth?
Yes. UPS run most efficiently at 40–80% load, and most rooms add equipment over their life, so a growth allowance (we default to 15%) plus operating headroom avoids an early forklift upgrade. The calculator separates your current load, the headroom and the growth allowance so you can see exactly how the recommended kVA is built up.
Does it work for single-phase and three-phase?
Both. Most server rooms are single-phase (UK 230 V); larger installations and data centres use three-phase (400 V line-to-line in the UK). Switch the supply phase and the load current is calculated with the correct formula — single-phase is A = W ÷ (V × PF), three-phase is A = W ÷ (√3 × V × PF). The recommended kVA is the same either way, because the VA requirement is independent of the phase count.
Does it show pricing?
No — we do not publish prices. The calculator gives you the full sizing picture for planning; for pricing, enter your email and a Servnet power specialist sends the matched Eaton, APC, Vertiv or Riello model, the battery runtime for your target and itemised pricing within one business day.