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106 ⏐⏐⏐ OHM’S LAW, POWER, AND ENERGY
Energy input
W i
Since P � W/t, we have the following:
P i � P o � P lost or stored
(W) (4.16)
The efficiency (h) of the system is then determined by the following
equation:
power output
Efficiency ���
power input
and h � � (decimal number) (4.17)
Po
�
P
i
where h (lowercase Greek letter eta) is a decimal number. Expressed as
a percentage,
h% � � Po
� � 100%
P
i
(percent) (4.18)
In terms of the input and output energy, the efficiency in percent is
given by
h% � � Wo
� � 100%
W
i
(percent) (4.19)
The maximum possible efficiency is 100%, which occurs when P o �
P i, or when the power lost or stored in the system is zero. Obviously,
the greater the internal losses of the system in generating the necessary
output power or energy, the lower the net efficiency.
EXAMPLE 4.10 A 2-hp motor operates at an efficiency of 75%. What
is the power input in watts? If the applied voltage is 220 V, what is the
input current?
Solution:
h% � �100%
0.75 �
System
Po �
Pi
Energy
stored
Energy
lost
Wlost or stored
FIG. 4.18
Energy flow through a system.
(2 hp)(746 W/hp)
��
Pi
Energy output
W o
V
I R