Real or Reactive? The 2 Kinds of Electric Power

June 13, 2017 meriah.jamieson



At 
Schneider Electric, we regularly host client workshops to help them better understand energy and how they use it. This new series-  A Beginners Guide to Electrical Engineering focuses on highlights from these sessions.

There are two (2) types of electric power

  1. Real power measured in “kilowatts” abbreviated [kW], and
  2. Reactive power measured in “kilovolt-ampere reactive” [kVAR]

All electric loads use a little of both types of power.  Electric heaters and lights use mostly real power and a small amount of reactive power; motors can use a significant amount of both types. Electricity providers typically charge for;

  • real energy [kWh] called the “energy charge”
  • real power [kW] – the maximum rate at which real energy was used [kWh per hour] or simply [kW] which is called a “demand charge”
  • reactive power [kVAR] – some reactive power may be offered as part of the other charges above, and excessive reactive power requirements may be provided for a fee, usually called a “power factor charge”

Why should you care about the two types of power?  Consumers that confuse or misunderstand the difference might;

  1. Make an unwise investment in on-site
    reactive power generation when such power is inexpensive to buy.
  2. Waste time and money on products “guaranteed”
    to reduce energy costs that will surely not deliver.
  3. Fall victim to the false claim that lower electric
    current “proves” savings.
     

3 + 4 = 5

All electric loads use at least some real power and some reactive power.  These two parts are added together to express the “total” or “apparent” power of the load. Total or Apparent power is measured in kilovolt-amperes [kVA].

Real and reactive power must be added in a special way (as vectors) not as a simple sum.  Consider the triangle shown below.  It is a fact that A2 + B2 = C2.    This relationship was discovered by a Greek mathematician named Pythagoras of Samos and is called the Pythagorean Theorem.

1

For example, if A is 4 feet long and B is 3 feet long, the diagonal side (C) will be 5 feet long because 32 + 42 = 52 (9 + 16 = 25). Electric power vectors (arrows) are added in exactly the same way.

2
If real power is 4 kW and reactive power is 3 kVAR, apparent power is 5 kVA because 32 + 42 = 52 (9 + 16 = 25).

3

 

What is Power Factor ?

The ratio of real power to apparent power, is called, “the Power Factor”. It tells us how much of the total (or apparent) power is real power.

Power Factor = Real Power [kW] / Apparent Power [kVA]

In the example above, the power factor is 0.8 or 80% because 4 kW/5 kVA = 0.8

 

“Make or Buy” Reactive Power

So you need reactive power – should you buy it from your energy supplier, or generate your own?  This is mostly an economic decision.  Although many suppliers describe the charge as a power factor “penalty,” it is simply the cost of the reactive power and often a good deal. The most common way to generate your own reactive power is to install a capacitor bank.  A reasonable budget estimate of the cost is US$100-$125 per kVAR installed (2016 dollars).

The alternative is to let your electric distribution company provide the reactive power, most likely by installing capacitor banks along the path from the generating station to your facility.  Prices from suppliers for reactive power vary over an extraordinarily wide range, from free (no charge) to over US$25 per kVAR per month!
 

Should you Buy Reactive Power, or Make Your Own?

In February 2017, a client installed their own capacitor bank to avoid $1,360 per monthly in power factor “penalties” charged for reactive power by their local distribution company.  This required a capital investment of $495,000 to purchase and install a 4,500-kVAR capacitor bank. With annual savings of $16,320 this investment has a simple payback of 30 years!
 

Recommended Action:
Find out how much reactive power your facility requires and what you are paying for that power.


Never Rely on Current (Ampere) Measurements to “Prove” Savings

Are you using less water and saving money when your water meter slows down?  Yes, the meter is measuring water volume (gallons, cubic meters, cubic feet, etc.) so you are using less volume and paying less. The same is true for natural gas; the gas meter is measuring mass (or adjusted volume), so you are using less and paying less when the meter slows down.

Are you using less electricity and saving money when electric current (amperes or amps) is reduced?  Not necessarily, that’s right – amps can go DOWN while real power (and therefore cost) go UP!  The trick is to get you to confuse the two types of electricity.  A reduction in current may be entirely the result of a reduction in reactive power (a simple and inexpensive capacitor), not real power.  Equating a reduction in current to a reduction in real power is a very common mistake that will lead you to expect savings when there will actually be none.
 

Recommended Action: Always base potential electricity savings on measuring real electric power and do not fall for the ole “use an ammeter” trick.


Products That Guarantee Savings

One of our clients purchased a “black box” from a vendor that guaranteed 20-30% energy (and cost) savings when installed on a 40-hp induction motor driving a chilled water pump. The vendor offered solid “proof” of savings using with actual current measurements before and after installation.

  • Electric current before = 32.1 amps
  • Electric current after = 25.5 amps
  • Reduction in electric current = 6.6 amps or 21% “savings!” yes?   NO!

The reduction in current was entirely due to a reduction in reactive power for which the client was paying nothing.  There was actually a small increase in real power; the device actually caused an increase in cost.

Had the client insisted on the vendor measuring real power [kW], the results would have been as follows;

  • Real electric power before = 4.94 kW
  • Real electric power after = 5.01 kW
  • Increase in real electric power = 74 Watts or 1.6% increase!
Recommended Action: Always base electricity savings on real power measurements.


Beware Misleading Claims- How to Protect Yourself 

When considering a product or solution that is intended to reduce energy cost – focus only on
real power, measured in kilowatts [kW].  If it does not reduce real power, it does not produce savings.

Terms such as voltage, current (amperes), phase angle, harmonic distortion, power factor, neutral current, zero-sequence and others are only a distraction.


3 Points to Remember

1.  If a product will reduce reactive power consumption, base savings estimates on the cost of reactive power [kVAR]. Don't value the reactive power reduction using the rate paid for real power [kW].

2. Lower current (amperes) is not the same as lower real power – a product that reduces current may or may not produce savings – measure real power directly [kW] and ignore claims to “prove” anything based on current [amps] measurements.

3. Power factor “penalties” are just charges for reactive power that might be a good deal, it is a “make or buy” decision.

Contributed by: Paul Stiller, Principle Engineer, Schneider Electric

 

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