The difference between VA and watts

It's important to understand the relationship between watts and VA (or volt-amps) to correctly size an uninterruptible power system (UPS). We're here to help you understand power terminology.

Understanding the difference between VA versus watts.

Real power (measured in watts) is the portion of power flow that results in the consumption of energy. The energy consumed is related to the resistance in an electrical circuit. An example of consumed energy is the filament in a light bulb. Reactive power (measured in VAR or volt-amps reactive) is the portion of power flow due to stored energy. Stored energy is related to the presence of inductance and/or capacitance in an electrical circuit. An example of stored energy is a charged flash bulb in a camera


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Apparent Power (VA) vs Real Power (Watts)

Apparent power (measured in VA or volt-amps) is a mathematical combination of real power and reactive power.

The geometric relationship between apparent power, reactive power and real power is illustrated in the power triangle below:

Mathematically, real power (watts) is related to apparent power (VA) using a numerical ratio referred to as the power factor (PF), which is expressed in decimal format and always carries a value between 0 and 1.0. For many newer types of IT equipment, such as computer servers, the typical PF is 0.9 or greater. For legacy personal computers (PCs), this value can be 0.60 – 0.75.


Using one of the following formulas, a calculation can be made to determine the missing quantity:

Watts = VA * Power Factor or VA = Watts/Power Factor

Since many types of equipment are rated in watts, it's important to consider the PF when sizing a UPS. If you don't take PF into account, you may under size your UPS. As an example, a piece of equipment that's rated at 525 watts and has a power factor of 0.7 results in a 750 VA load.

750 VA = 525 Watts/0.7 PF

Sizing the UPS to operate at 75 percent capacity results in a UPS with a 1000 VA rating (750 VA/0.75 = 1000 VA).

Converting amps to VA

Single phase: Multiply amps by voltage (120 volts in the USA). 10A x 120V = 1200 VA.

Three phase: Amps x volts x 1.732 = VA.

Calculating the right size back-up power UPS

UPS battery back-up are given a power rating in volt-amperes (VA) that range from 300 VA to 5,000 kVA. This rating represents the maximum load that a UPS can support, but it shouldn’t match exactly the power load you have. To allow room for growth, the best practice is to choose a battery back-up with a VA rating that is 1.2x the total load you need it to support. If your UPS will be supporting motors, variable-speed drives, medical imaging devices or laser printers, add more VA capacity to your requirements to account for the high-power inrush that occurs when those devices start up.

Companies that are anticipating rapid growth should use a higher multiplier than 1.2x. Newer server hardware tends to have higher power requirements than older models, so factoring in additional VA will account for adding more and newer equipment.