Following the Minute Lecture Four analogies to explain reactive power, we received many reactions. Several writers suggested further analogies. This inspired us for making a sequel to this minute lecture, with four more analogies worked out. When explaining reactive power to lay persons, you can pick any analogy of your choice, according to your target audience.

1. Minute Lectures
Analogies to explain reactive power (part 2)
Following the Minute Lecture ‘Four analogies to explain
reactive power’, we received many reactions.
Several writers suggested further analogies
This inspired us for making a sequel to this minute
lecture, with four more analogies worked out
When explaining reactive power to lay persons, you
can pick any analogy of your choice, according to
your target audience

2. Minute Lectures
Analogies to explain reactive power (part 2)
What does the analogy needs to explain?
Remember: these analogies are didactic tools; none of
them are 100% correct
They elucidate the difference between active power (W),
reactive power (VAR), and apparent power (VA)
Some of them also explain why reactive power limits the
capacity of the lines in the grid, why reactive power
increases energy losses in the grid, or other related
phenomena

3. Minute Lectures
Analogies to explain reactive power (part 2)
I. The beer analogy (1/2)
Suppose you want to quench your
thirst by a cool beer. This beer is
the active power (W)
Along with your ale comes a bit of
foam, which doesn’t quench your
thirst. This is the reactive power
(VAR)

4. Minute Lectures
Analogies to explain reactive power (part 2)
I. The beer analogy (2/2)
The total content of the glas, is the
apparent power (VA)
The more foam, the bigger glas you
need (= you need a line with a
higher capacity)

5. Minute Lectures
Analogies to explain reactive power (part 2)
II. The marble-in-a-tube analogy (1/3)
Suppose your goal is to push marbles out of one end of a
horizontal pipe
You accomplish this by pushing marbles into the pipe

6. Minute Lectures
Analogies to explain reactive power (part 2)
II. The marble-in-a-tube analogy (2/3)
The first ten marbles will give you no result, they will
just fill the tube
Only the 11th marble will result in a marble coming
out of the other end of the pipe

7. Minute Lectures
Analogies to explain reactive power (part 2)
II. The marble-in-a-tube analogy (3/3)
The ‘marbles in’ can be compared to apparent power
(VA), the ‘marbles out’ to active power (W) and the
length of the pipe to reactive power (VAR)
The longer the length of the pipe (more reactive
power), the more marbles are needed (apparent
power) for the same result (active power)

8. Minute Lectures
Analogies to explain reactive power (part 2)
III. The cash flow analogy (1/2)
Suppose you are running a business. You have to
spend an amount of money C (cost) on one day,
for earning a larger amount of money E (earnings)
one month later. Your profit will be P = E - C
C is not lost money. However, without spending C,
you will not be able to make the profit P
The profit P can be compared to the active power.
The earnings E are the equivalent of the apparent
power. The cost C is the reactive power

9. Minute Lectures
Analogies to explain reactive power (part 2)
III. The cash flow analogy (2/2)
To bridge the time between the spending of C and the
earning of E, you will need some money. You could
borrow this money (take reactive power from the
power grid). That will however cost interest
(reactive power penalty)
A better solution would be to have some cash money
available at your business (= a capacitor bank at
your site).

10. Minute Lectures
Analogies to explain reactive power (part 2)
IV. The water tower (1/7)
Suppose a water tower
and men that have
to bring water to the
tank by climbing
ladders. After
emptying their
bucket, they return
by another ladder.

11. Minute Lectures
Analogies to explain reactive power (part 2)
IV. The water tower (2/7)
The climbing ladder = the electricity line
The number of men on the ladder = the voltage
The amount of water each man carries = the current
The amount of water added to the tank per time
interval = the frequency

12. Minute Lectures
Analogies to explain reactive power (part 2)
IV. The water tower (3/7)
Energy = voltage x current
The active power = the height of the tower.
If the tower is high (high power), there are two
solutions to keep the frequency: using more men
(higher voltage) or increasing the amount of water
each man carries (higher current)

13. Minute Lectures
Analogies to explain reactive power (part 2)
IV. The water tower (4/7)
There are three ladders
= three phases

14. Minute Lectures
Analogies to explain reactive power (part 2)
IV. The water tower (5/7)
Width of the tower = reactive power
Consequence: the length of the ladder (apparent
power/voltage) depends on the height of the tower
(active power) and the width of the tower (reactive
power).
The higher the reactive power, the longer the ladder.
So if the same voltage is kept (same amount of
men on the ladder), each man has to carry more
water to keep up the same frequency.

15. Minute Lectures
Analogies to explain reactive power (part 2)
IV. The water tower (6/7)

16. Minute Lectures
Analogies to explain reactive power (part 2)
IV. The water tower (7/7)
If a man is carrying more water (higher current), he
will lose some along the way. So more reactive
power means more losses.
If the reactive power is so high that the men have to
carry really a lot of water, one of the men could
collapse  the other men have to carry even more
water  if no support from new men is given, the
system risks to experience a complete breakdown

17. Minute Lectures
Analogies to explain reactive power (part 2)
Round-up
Another four analogies represent the idea of active and
reactive power in an electric system:
• The beer analogy
• The marble-in-a-tube analogy
• The cash flow analogy
• The water tower analogy
Some people prefer one or another analogy
We hope they will increase the reader’s insight in the
phenomenon, or help explaining the phenomenon to
others

18. Minute Lectures
Analogies to explain reactive power (part 2)
Links and references
• P Sauer, What is Reactive Power?,
PSERC, Sep 2003
• Minute Lecture: 4 analogies to explain
reactive power