Why companies rate UPS in VAs and PSUs in Watts

1. Introduction2. Apparent Power, Real Power and Reactive Power 3. Explaining and calculating the power factor (PF)4. Sizing PSU and UPS units5. View All Pages

Almost everyone who has went to buy power equipment such as a power supply and UPS has assuredly noticed that while power supply units are rated in Watts, UPS systems are rated in VA or in both VA and Watts. This usually leads to confusion as most people not only lack the technical knowledge to understand the difference between VA and Watt units but few might even realize that there is a difference.

Even less realize that not only VAs and Watts are different but there is even a third “type” of power, VA reactive or VArs. In this article we will try and explain in layman's terms the difference between the three types of power so people with basic skills and knowledge can understand.

People with little or no technical expertise are not the only ones to fall victim as many who studied computers, electronics and other related subject often believe that VAs are equal to Watts; this is actually true, but only for DC circuits. When talking about any AC circuit, there are three types of power to consider : Apparent (or Complex) power, Real (or True or Effective) power and Reactive (or Magnetic) power.

For DC circuits power is calculated by using the definition:

This definition is not valid for AC circuits because the vast majority of power loads will cause a phase shift between voltage and current (related reading : AC-DC Voltage; What’s the difference?).

A very similar definition is used to calculate Apparent power :

Apparent power has little to no meaning for residential and business users, but it is absolutely necessary for sizing any and all AC power equipment ranging from your household safety fuses and simple UPS systems to immense transformers and power generators.

So what about real power? Real power is, much like the name suggests, the actual amount of power used by your equipment and it is commonly used to calculate the thermal loading generated by the equipment.

For AC circuits real power is calculated by using the following definition :

Real power is usually all that residential and business users care about because that is the amount of power you purchase from the utility company.

Reactive power is something not widely known and rarely ever used because it usually only matters to engineers designing and sizing electric power transmission and distribution systems. Any inductive and/or capacitive load which will cause a phase shift between the current and voltage waveforms will cause reactive power to be drawn by the equipment, even though the equipment will not actually use it. Reactive power moves no energy, which is why it is often referred to as the “imaginary” power.

It can be calculated by using the following definition :

To summarize, apparent power is the total amount of power that will move through your equipment and therefore it is critical to size all wiring, circuit breakers and any other equipment according to it, yet residential and business users will not be charged based on their apparent power but by their real power consumption. Real power is the effective power used by your equipment and moves energy. Reactive power moves no energy but will still be the cause of a higher, useless current.

For most part of the world only large businesses and industrial consumers are being penalized if reactive power exceeds a certain portion of their total power consumption.

As we mentioned before, the vast majority of power loads will cause a phase shift between voltage and current and will draw in more current than they will actually use. For a load which will consume a certain amount of real power, apparent power increases the larger the phase shift is. The following vectors diagram can be used to explain how increasing the phase shift angle φ will increase the apparent and reactive power while real power remains unchanged.

The angle φ of this phase shift can be used to calculate the power factor, which is usually defined as the ratio between the real power P and apparent power S and/or as the cosine of the angle φ. Being the result of a cosine number it cannot ever be lower than 0 or greater than 1, which is verified by simple reason; it is impossible for real power to surpass the complex power under any circumstances.

Power factor is commonly presented as a percentage, e.g.:

For power supply units specifications are rather straightforward; the manufacturer specifies how much power under specific circumstances their product can output. For example, a 500W power supply can (supposedly) continuously output up to 500W worth of DC power. However output power and input power differ significantly.

Input power relies on the unit's efficiency and efficiency is not constant across its entire load range. It is not wise to try and calculate the input power based on the manufacturer's ratings; if the power supply is 80Plus certified you should take into account the 80Plus program specification limits at 100% load, otherwise you should consider the efficiency to be as low as 60%. Let us speculate that our 500W power supply carries an 80Plus Bronze certification which implies an efficiency of at least 81% at maximum load. At maximum load our power supply will output 500W worth of DC power but will consume

So now we have the Watts rating of our power supply. But UPS units are also rated in VA. Most computer PSUs nowadays feature active power factor correction (APFC) which allows them to have a power factor of up to 99%, however if a manufacturer states that their unit features APFC but not a specific rating you should consider it to be as low as 90%. Passive PFC power supplies have a power factor of 70-75% and units without any form of power factor correction (which are nearly extinct) have a power factor as low as 50%. We will assume that our 500W 80Plus Bronze certified power supply has an APFC rating of 0.97.

Complex power now becomes

Most UPS products from reputable manufacturers have both a VAs and a Watts rating. Neither of the two ratings may be exceeded, which is the most common cause of sizing errors. For example, an APC RS 1200VA/720W UPS can output either 1200VA or 720W of power and will cease to operate if either rating is exceeded. Here are the examples of three hypothetical loads:

As you can see, it is very easy for a low quality 400W switching PSU to overload a rather powerful UPS. At the same time, the same UPS can power a significantly more powerful high quality PSU.

But what about other computer loads such as monitors, printers and speakers? Manufacturers always display the AC Watt specifications of their monitors and all TFT monitors can be safely assumed to have a power factor of over 95%; therefore it is easy to calculate the exact VAs consumption or even safely assume that the VAs and Watts are almost equal. Laser printers and powerful speakers should not be connected to the battery backup outlets of the UPS because they draw immense amounts of power but the surge protection outlets can be used. Low power speakers and most inkjet printers can be safely connected to battery backup outlets because their power consumption is very low and their power factors are high, however you might want to avoid connecting non-essential devices to the battery backup because the battery runtime decreases exponentially as the UPS load increases.

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