The difference between VA and watts

The engineering answer: To correctly size a UPS, it’s important to understand the relationship between watts and VA. However, we must first have a brief discussion about power terminology. 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 voltamps 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.

Apparent power (measured in VA or voltamps) 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 U.S.). 10A x 120V = 1200 VA.
Three phase: Amps x volts x 1.732 = VA.

In simple terms, imagine UPS power capacity as the amount of soda in a glass. The top of the glass represents the limit of apparent power the UPS will accept, or the VA. The actual liquid in the glass represents the real power of the UPS, or the Watts. Unless you desire a frothy glass of foam, you’ll want to maximize the actual amount of liquid in the glass. This—the Watts rating—is what you’re actually paying for.

When you’re buying a UPS and sizing your equipment, pay attention to both the VA and Wattage ratings. This ensures you will get the maximum amount of power for your investment.