Power System Fundamentals 29
- There are 1000 W in a kilowatt (1000 W = 1 kW).
- Multiply the kW that the heater has used by the hours of use:
kW × 200 h = kilowatt-hours (kWh)
0.72 × 200 h = 144 kWh- Multiply the kWh by the cost:
kWh × cost = 1.44 KWh × 0.05 = $7.20
Some simple electrical circuit examples have been discussed in this
chapter. They become easy to understand after practice with each type of
circuit. It is very important to understand the characteristics of series, par-
allel, and combination circuits.MAXIMUM POWER TRANSFER
An important consideration in relation to electrical circuits is maxi-
mum power transfer. Maximum power is transferred from a voltage source
to a load when the load resistance (RL) is equal to the internal resistance
of the source (RS). The source resistance limits the amount of power that
can be applied to a load. Electrical sources and loads may be considered
as diagrammed in Figure 2-8.
For example, as a flashlight battery gets older, its internal resistance
increases. This increase in the internal resistance causes the battery to
supply less power to the lamp load. Thus, the light output of the flash-
light is reduced.
Figure 2-9 shows an example that illustrates maximum power
transfer. The source is a 100 V battery with an internal resistance of 5
Ω The values of I. L, Vout, and power output (Pout) are calculated as fol-
lows:VTIL = ————; Vout = IL × R (^) L; Pout = IL × V (^) out
RL + RS
Notice the graph shown in Figure 2-9. This graph shows that maxi-
mum power is transferred from the source to the load when RL = RS. This
is an important circuit design consideration for power sources.