How to Size a UPS for your Project: kVA, kW & Battery Runtime

Identify the Equipment the UPS Will Protect

Start with the critical equipment that must remain powered during an outage.

Typical UPS loads include:

• Servers
• Computers
• Network switches
• Routers
• CCTV and NVR systems
• Access control
• Telecommunications equipment
• POS systems
• Industrial controls

Whenever possible, use actual equipment wattage rather than estimates.

Check Both kW and kVA

A UPS may have both a kVA rating and a kW rating.

These are not always the same.

For example, one 10kVA UPS may support 10kW, while another older design may support only 9kW or 8kW.

Your selected UPS therefore needs to satisfy both the required kW and kVA.

For modern IT installations, this is particularly important because server and networking loads can have high power factors.

Choose the Correct Phase Configuration

UPS systems commonly come as:

Single-phase in / Single-phase out

Three-phase in / Single-phase out

Three-phase in / Three-phase out

A three-phase building does not automatically require every protected device to be three phase.

A large bank of single-phase servers, computers or network equipment can be distributed across the three output phases of a suitable three-phase UPS.

The key is good phase balancing.

Don’t Oversize for Every Short Power Peak

UPS inverters generally have some short-term overload capability.

That means a brief equipment startup peak does not always require a UPS whose continuous rating equals the peak.

For example, server power supplies may briefly demand more power during a simultaneous restart.

If that demand falls within the UPS manufacturer’s permitted overload level and duration, the UPS may carry it without requiring a larger continuous rating.

This can prevent unnecessary oversizing.

However, actual UPS overload curves and bypass behaviour must always be checked before final selection.

Battery Runtime Is a Separate Calculation

If you are sizing a three-phase generator, single-phase equipment should be distributed reasonably across: L1, L2 and L3.

You should not multiply every single-phase load by three, and you should not automatically assume all single-phase equipment is connected to one phase.

Good phase balancing allows better use of the generator’s available capacity.

Consider UPS and Battery-Charging Loads

A 10kVA UPS does not have a fixed backup time.

The same UPS could provide five minutes, thirty minutes or several hours depending on its battery system and actual load.

Battery sizing depends on: Load kW × required backup time with additional consideration for:

• UPS efficiency
• Battery chemistry
• Usable battery capacity
• Battery age
• Temperature
• Discharge rate

This is why UPS capacity and UPS battery runtime should be calculated separately.

Decide How Much Backup Time You Actually Need

Ask what the battery is expected to achieve.

If a generator normally starts within a few minutes, the UPS may only need enough battery capacity to bridge the transfer period.

If there is no generator, you may require significantly more battery storage.

For offices and server rooms, choosing the right runtime can make a major difference to system cost.

Leave Sensible Room for Growth

A UPS should not normally be selected to operate permanently at its absolute limit.

Some capacity should remain for:

• Additional equipment
• Short load changes
• Future growth
• Operating reserve

The answer, however, is not always to double the UPS size.

Good UPS sizing balances capacity, runtime, resilience and cost.