Why UPS systems are rated in VAs and PSUs in Watts; Explaining Watts, VAr and VA

Sa mga naglibog, here is an article explaining everything:


INTRODUCTION

When shopping for an uninterruptible power supply (UPS), almost everyone would have noticed that they are either rated in VA's only or in both VA's and Watt's. This is not so with the vast majority of household equipment, including the very power supply unit (PSU) of the computer itself which comes rated in Watts only. As a result, this usually leads to confusion as most people either lack the technical knowledge to understand the difference between VA and Watt units or, even worse, do not even realize that there is a difference.

Of course, this is something natural because such knowledge is usually limited to electricians and electrical engineers, people who are educated and work on the AC power grid. To make matters worse, not only VAs (Volt-Amperes) and Watts are different but there is even a third "form of electrical power", Volt-Ampere reactive or VArs.

In this article, we will try and explain in terms comprehensible from the majority of users, who possess only basic skills and little to no knowledge on the matter, the difference between the three types of power.


Apparent Power, Real Power and Reactive Power

Not only people with little to no technical expertise at all are the ones confused by the difference between Watts, VAs and VArs. Most people who study computers, small-scale electronics and other similar subjects quite often believe that Volt-Amperes are equal to Watts, which is actually true but only when working under DC voltage / current. On the other hand, when making calculations on 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: P (Power in Watts) = V (Volts) * I (Amperes). 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 and DC current: Fundamental differences and a simple explanation) AC and DC current: Fundamental differences and a simple explanation.

For AC systems, a very similar definition is used to calculate Apparent power: S (Power in Volt-Amperes) = V (Volts RMS) * I (Amperes RMS). Apparent power has little to no meaning for residential and business users; however 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, which is why even the least adept of electricians who work on AC systems should be trained to be able to measure and calculate it.

Apparent power is of little concern to home users, 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: P (Power in Watts) = V (Volts RMS) * I (Amperes RMS) * cos(φ). Real power is all that residential and business users care about because that is the amount of power you purchase from the utility company; unless of course the cabling/safety systems are sized incorrectly and high Apparent Power caused them to fail even though Real Power is low enough.

Reactive power is something not widely known and rarely ever used because it usually only matters to electrical engineers designing and sizing electric power transmission and distribution systems or working in the industrial sector and on large electric motors/generators. 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 at all, which is why it is often referred to as the "imaginary" power; it is simply transferred from the utility company to you and from you back to the utility company, merely causing losses along the way. It can be calculated by using the following definition: Q (Power in VArs) = V (Volts RMS) * I (Amperes RMS) * sin(φ).

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 it will still be the cause of a higher, useless current. Even so, for the 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 at this point of time.


Explaining and calculating the Power Factor (PF)

As we mentioned in the previous page, 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.


Hypothetical ideal 160W load and real 160W loads with PF's of 94% and 90% respectively

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.

PF (power factor) = P (real power) / S (complex power) = cos(φ)

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

PF = 0.95 * (100%) = 95%




Sizing PSU and UPS units

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 500/0.81 =617W worth of AC power.

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 617W/0.97 = 636VA.

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.