UPS Sizing (Data Center) Calculator

N+1 finds a module size where the load needs four or fewer units and adds exactly one spare module of that same size, so if a single module fails the remaining units still cover the full load. 2N instead builds two completely independent, fully-rated UPS systems — each one alone sized to carry the entire load — so it always needs at least double the total capacity of an equivalent N configuration, trading much higher cost for protection against an entire system failure, not just a single module.

The raw energy needed is IT load × runtime hours, but a battery can't actually deliver 100% of its rated capacity without over-discharging and damaging it, so the depth-of-discharge percentage inflates the required capacity to compensate. A fixed 95% round-trip efficiency further inflates it slightly to account for conversion losses between the battery and the load, so the reported battery capacity in kWh is always somewhat larger than the raw energy-times-runtime figure.

Those two outputs use flat density assumptions of 25 kg per kWh and 3 square feet per kWh, which are typical figures for VRLA (sealed lead-acid) batteries specifically. Lithium-ion batteries are substantially lighter and more compact per kWh of storage, so if you're actually planning to install Li-ion, both the weight and footprint figures here will overstate your real requirements — check your battery vendor's datasheet for accurate numbers.

The UPS has to draw more power from its input than it delivers to the IT load, because some energy is lost as heat during AC-to-DC-to-AC conversion. That loss is calculated as IT load × (1/efficiency − 1), so a UPS running at 94% efficiency loses about 6.4% of the IT load as heat that a data center's cooling system also has to remove.