UPS Sizing and Runtime Calculator

Size a UPS for your load and redundancy, then find the battery for a target runtime or the runtime of the battery you have.

Inputs

The load

Total load

Measured draw from the PDUs or the existing UPS is best. Leave it blank to size from the device list below, or use both: the devices are added to it.

Devices (optional)

Watts per device, from measured draw or the vendor's power calculator. Up to 6 rows.

Load power factor

Converts between kW and kVA for the load. 0.9 is a common planning value for IT loads.

Growth

Maximum UPS loading

Growth is the load you expect to add over the UPS's life. Keeping the UPS at or below 80% leaves room for inrush and new equipment; both are planning choices.

The UPS

UPS output power factor

The kW rating divided by the kVA rating on the UPS data sheet. Some recent models are rated at 1.0.

Output voltage

For three-phase, the line-to-line voltage.

Phase

Single-phase Three-phase

Redundancy

N N+1 2N

N: just enough capacity. N+1: one spare module, so any one can fail. 2N: two complete, independent systems, each able to carry the whole load (usually on separate A and B feeds).

Modules that carry the load (N)

1 for a single UPS. For a modular UPS, the number of power modules the load needs; the rating is split across them.

Runtime

Calculate

Battery for a target runtime Runtime of a battery

Target runtime

At the load including growth, with the battery at end of life.

DC bus voltage (optional for kWh)

The nominal battery voltage from the UPS data sheet, e.g. 20 × 12 V blocks = 240 V. Used to convert between kWh and Ah.

kWh per battery string or cabinet (optional)

Battery model

Chemistry

Peukert exponent

Rated hour rate

Lead-acid only. The hour rate is the discharge time the capacity is rated at (20 for a C20 rating). The exponent default, 1.15, is the top of the 1.05 to 1.15 range commonly given for VRLA AGM batteries.

Inverter efficiency

End-of-life capacity

Efficiency on battery, from the data sheet (94% is the default). Batteries are commonly considered at end of life at 80% of rated capacity, so the runtime is sized to still be met then.

Result

Enter the load and a target runtime to see the UPS size and the battery needed.

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How the calculation works

Load. The load in kW is the total you enter (kVA × the load power factor if you enter kVA) plus the device rows. Load kVA = kW ÷ load power factor.

UPS rating.

Load with growth = load × (1 + growth %)
Required kW = load kW with growth ÷ maximum loading
UPS kVA = the larger of (load kVA with growth ÷ maximum loading) and (required kW ÷ UPS power factor), so the load fits both the kVA and the kW rating
UPS kW = UPS kVA × UPS power factor
Output current = kVA × 1,000 ÷ voltage (÷ √3 more for three-phase, with the line-to-line voltage)

Redundancy. With N modules carrying the load, each module is rated UPS kVA ÷ N. N installs N modules, N+1 installs N + 1, and 2N installs two independent systems of N modules each.

Runtime. The battery has to supply the load plus the inverter’s losses: DC power = load kW ÷ inverter efficiency.

Ideal hours = battery kWh × end-of-life capacity ÷ DC power
Lithium-ion: runtime = ideal hours (no rate adjustment, a simplification)
Lead-acid: runtime = H × (ideal hours ÷ H)^k, from Peukert’s law (t = H × (C ÷ IH)^k), where H is the hour rate the capacity is rated at and k is the Peukert exponent. The calculator assumes a constant DC bus voltage, so the current is proportional to the DC power.
Battery for a target runtime: the same formula solved for the battery, sized at the load including growth, so the target is still met when the battery reaches end of life. Ah = kWh × 1,000 ÷ DC bus voltage.
The table shows the runtime at 25%, 50%, 75% and 100% of the UPS kW rating, with the battery at end of life.

Defaults and limits. Load power factor 0.9 and UPS power factor 0.9 (check your UPS data sheet; some recent models are rated at 1.0). Growth 20% and maximum loading 80% are planning choices. Inverter efficiency 94% and end of life at 80% of rated capacity, a common replacement point. The Peukert exponent defaults to 1.15, the top of the 1.05 to 1.15 range given for VRLA AGM batteries (lead-acid generally runs 1.1 to 1.3); the hour rate defaults to 20. This is a simplified planning model: at UPS discharge rates, minutes against a 20-hour rating, Peukert’s law is used well outside the range it is usually measured over. The battery maker’s constant-power discharge table for your model and temperature is the authority. Battery ratings are usually given at 25 °C (77 °F); a hotter room shortens battery life and a colder one reduces the capacity available.

Worked example

A 12 kW server room, power factor 0.9, with 20% growth, the UPS kept at or below 80%, a UPS power factor of 0.9, N+1 with one module carrying the load, 208 V three-phase. The target is 10 minutes on a lead-acid battery with a 240 V DC bus:

Step	Result
Load today	12 kW · 13.33 kVA
With 20% growth	14.4 kW · 16 kVA
UPS rating: larger of 16 ÷ 0.8 and 14.4 ÷ 0.8 ÷ 0.9	20 kVA / 18 kW
N+1	2 modules of 20 kVA; today’s load is 33.3% of the 40 kVA installed
Output current at 208 V three-phase	55.51 A per phase
DC power from the battery: 14.4 ÷ 0.94	15.32 kW
Ideal hours: 20 × ((10 ÷ 60) ÷ 20)^1/1.15	0.3112 h
Battery at end of life: 0.3112 × 15.32	4.77 kWh
Battery to buy: 4.77 ÷ 0.8	5.96 kWh (24.8 Ah at 240 V DC)

The new battery runs the 14.4 kW load for about 12.9 minutes; it falls to 10 minutes as it ages to 80% capacity. Without the Peukert adjustment, 10 minutes would need only 15.32 × (10 ÷ 60) ÷ 0.8 = 3.19 kWh; the difference is why lead-acid UPS batteries are sized from the maker’s discharge tables rather than from their amp-hour rating.

Generator ride-through or graceful shutdown

With a standby generator, the UPS only has to carry the load until the generator starts and the transfer switch moves the load across. Size for that time from your generator and transfer switch data sheets, plus a margin for a failed first start, and size the generator for the UPS’s input, which includes battery recharging. Without a generator, size the runtime for a clean shutdown of every system that matters, and test that the shutdown actually runs. The UPS’s losses also add heat to the room; the server room cooling calculator includes them. For the circuits that feed the UPS and the racks behind it, see the colocation power calculator.

Frequently Asked Questions

How do I size a UPS?

Start with the load in kW, convert it to kVA with the load's power factor, add the growth you expect, and divide by the maximum loading you want to run at (80% is a common planning choice). The UPS must meet both figures: its kVA rating and its kW rating, which is the kVA rating times the UPS's output power factor. Then choose the redundancy and size the battery for the runtime you need.

How long will my UPS run on battery?

Divide the battery's usable energy by the power it has to deliver: the load divided by the inverter efficiency. A 6 kWh lithium-ion battery at 80% of its rated capacity, feeding 12 kW through a 94% efficient inverter, lasts about 22.6 minutes. Lead-acid batteries deliver less of their rated capacity at the high discharge rates of a UPS, so the calculator adjusts their runtime with Peukert's law. The battery maker's runtime table for your exact model is the authority.

What is the difference between N+1 and 2N for a UPS?

N+1 adds one spare module to the modules the load needs, so the system keeps running if any one module fails; the modules usually share one battery and one output. 2N is two complete, independent UPS systems, each able to carry the whole load, typically feeding the A and B power supplies of dual-corded equipment. 2N also covers maintenance and failures upstream of the UPS, and needs twice the UPS capacity.

How much UPS runtime do I need if I have a generator?

Enough to cover the time the generator takes to start, come up to speed and for the transfer switch to move the load, plus a margin for a failed first start. Take those times from your generator and transfer switch data sheets or the last load test. Without a generator, size the runtime for a graceful shutdown: the time your systems need to shut down cleanly, plus a margin.

Why does a lead-acid battery give less runtime at high load?

A lead-acid battery's usable capacity falls as the discharge current rises, an effect Peukert's law describes. Halving the load more than doubles the runtime. Lithium-ion batteries are much less affected, so the calculator treats their capacity as fixed, which is a simplification. Both chemistries lose capacity with age, which is why the runtime is sized for end of life.

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UPS Sizing and Runtime Calculator

Inputs

The load

The UPS

Runtime

Battery model

Result

Email me this result

Related Tools

How the calculation works

Worked example

Generator ride-through or graceful shutdown

Frequently Asked Questions

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Related Terms

Uninterruptible Power Supply (UPS)

N+1 Redundancy

High Availability (HA)

Planning backup power for a server room or colocation cage?