kVA vs kW: The Difference, How to Convert and How to Size a UPS

UPS Systems · Power Basics · Sizing Guide

If you have ever compared UPS systems or generators, you have seen two ratings side by side: kVA and kW. They look similar, they are often used as if they mean the same thing, and getting them confused is one of the most common reasons a UPS ends up undersized. A 3 kVA UPS is not necessarily a 3,000 watt UPS, and the difference matters when the load is a rack of servers that must stay up during an outage.

This guide explains what kVA and kW mean, how power factor connects them, how to convert kVA to kW and amps, and how to use both numbers to choose the right size UPS or generator for your site.

What does kVA stand for?

kVA stands for kilovolt-ampere, which is 1,000 volt-amperes (VA). It is calculated by multiplying voltage by current: a device drawing 10 amps at 230 volts draws 2,300 VA, or 2.3 kVA. This is called apparent power because it is the total power the supply has to deliver, including power that flows back and forth without doing useful work.

kW stands for kilowatt, which is 1,000 watts. It measures real power, the energy actually converted into computing, heat, light or motion. Your electricity bill is based on kWh (kilowatt-hours), so kW is the figure that reflects what your equipment really consumes.

Are kVA and kW the same?

No. kVA and kW are only equal when the power factor is exactly 1.0. In almost every real installation, kW is lower than kVA. A simple way to picture it is a glass of beer: the whole glass is kVA, the beer is kW, and the froth on top is reactive power. You pay to fill the whole glass, but only the beer is useful.

The ratio between the two is the power factor (PF), a number between 0 and 1. Equipment with motors, transformers or older power supplies has a lower power factor. Modern IT equipment with active power factor correction runs close to 0.95 to 0.99.

How to convert kVA to kW

The conversion is simple once you know the power factor:

For example, a 10 kVA UPS with an output power factor of 0.9 delivers 10 × 0.9 = 9 kW. The same 10 kVA rating on an older unit with a 0.8 power factor delivers only 8 kW. Generators in Australia are usually rated at a 0.8 power factor, so a 100 kVA generator is typically an 80 kW generator.

So, what is 3 kVA in watts? It depends on the power factor: 2,400 W at 0.8, 2,700 W at 0.9, or 3,000 W at 1.0. Always check the kW or watt rating on the UPS datasheet rather than assuming.

How to convert kVA to amps

Converting kVA to amps tells you what circuit, breaker and PDU a UPS or load needs. Australian supply is nominally 230 V single-phase and 400 V three-phase.

• Single-phase: Amps = kVA × 1,000 ÷ 230
• Three-phase: Amps per phase = kVA × 1,000 ÷ (1.732 × 400), or roughly kVA × 1.44

Amps shown are at full rated load. UPS input current is slightly higher than output current because of conversion losses and battery charging, so circuit and breaker sizing should always be confirmed by a licensed electrician against the manufacturer's installation manual and AS/NZS 3000.

Why UPS systems have both a kVA and a kW rating

A UPS has two limits, and your load must stay within both. The kVA rating is the maximum current the inverter can push. The kW rating is the maximum real power it can deliver. If your servers draw 2.9 kW and you buy a 3 kVA UPS rated at 2.7 kW, the UPS will overload even though 2.9 is less than 3. This is the single most common sizing mistake we see.

Older UPS designs had an output power factor of 0.7 or 0.8, which matched older IT power supplies. Modern servers have power factors close to 1.0, so current-generation online UPS systems such as the Eaton 9PX and APC Smart-UPS On-Line are rated at a 0.9 or 1.0 output power factor. That means a modern 6 kVA UPS can carry noticeably more real load than an older 6 kVA unit.

What size UPS do I need?

Follow these five steps to size a UPS correctly:

1. List every device the UPS must protect: servers, switches, firewalls, storage, monitors and phone systems.
2. Find the real power draw in watts from the nameplate, datasheet or, better, measured readings from a metered PDU. Nameplate figures are maximums and usually overstate actual use.
3. Add the watts together to get your total load in kW.
4. Add 20 to 30% headroom for growth, start-up surges and to keep the UPS running at 80% or less of its rating, which improves efficiency and battery runtime.
5. Choose a UPS whose kW rating (not just kVA) exceeds that figure, then confirm the runtime you need with the right battery configuration.

Worked example: a small server room with two servers at 450 W, a storage array at 300 W, two switches at 100 W and a firewall at 50 W totals 1,450 W. Add 25% headroom and you need about 1.8 kW. A 2 kVA / 1.8 kW UPS would be at its limit, so a 3 kVA / 2.7 kW model is the safer choice. Our free UPS runtime and battery sizing calculator will confirm how long that UPS will hold the load on battery.

kVA and generators: sizing for a whole site

Generators are rated in kVA at a 0.8 power factor, so divide your kW load by 0.8 to find the kVA needed. When a generator backs up a UPS, the generator should generally be sized at 1.5 to 2 times the UPS rating so it can handle the UPS rectifier and battery recharge current without voltage or frequency problems. A three-phase UPS with low input harmonic distortion reduces the oversizing needed. If you are pairing a UPS with a generator and an automatic transfer switch, size all three together.

UPS sizing and supply across Australia

Indigi Power and Cooling sizes and supplies single-phase and three-phase UPS systems nationally from our Brisbane, Sydney and Melbourne hubs, with service coverage across QLD, NSW, ACT, VIC, SA, WA, NT and Tasmania. We carry Eaton UPS systems and APC UPS systems and can load-test your existing setup to confirm your real kW figure before you buy.

Frequently asked questions

What is the difference between kVA and kW?

kVA is apparent power, the total power drawn from the supply (volts times amps). kW is real power, the part that does useful work. They are linked by the power factor: kW equals kVA multiplied by the power factor, so kW is always equal to or lower than kVA.

How do I convert kVA to kW?

Multiply kVA by the power factor. A 10 kVA UPS with a 0.9 power factor delivers 9 kW. Generators are usually rated at a 0.8 power factor, so a 100 kVA generator delivers about 80 kW. To go the other way, divide kW by the power factor.

What is 3 kVA in watts?

3 kVA equals 2,400 watts at a 0.8 power factor, 2,700 watts at 0.9, or 3,000 watts at 1.0. Most modern 3 kVA UPS systems deliver 2,700 W. Check the watt rating on the datasheet to be sure.

How do I convert kVA to amps?

For Australian single-phase supply, amps equal kVA times 1,000 divided by 230, so 1 kVA is about 4.3 A. For three-phase 400 V supply, amps per phase equal kVA times 1,000 divided by 692.8, or roughly kVA times 1.44.

What size UPS do I need?

Add up the real power draw in watts of every device the UPS will protect, add 20 to 30% headroom, then choose a UPS whose kW rating, not just its kVA rating, is higher than that total. Then select batteries to give the runtime you need.

Related services and products

• UPS runtime and battery sizing calculator
• UPS buying guide: how to choose the right UPS
• Eaton UPS systems
• APC UPS systems
• Automatic transfer switches
• UPS maintenance