The best way to explain these terms is to compare them to water.
Voltage (V) is the potential for energy to move and is
equivalent to water pressure. Current (I) is a rate
of flow and is measured in amps. Ohms (r) is a measure of resistance and
is equivalent to the water
pipe size.
These three terms are related to each other with a simple
formula that reads, current is equal to the
voltage divided by the resistance. I=V/r
Imagine you have a tank of water with a hose connected
to the bottom of this tank.. What
happens if you increase the pressure inside of this tank? The amount of
water flowing out of the hose
will also increase. The same is true when you increase voltage, more current
will flow.
What happens if you connect a larger diameter hose to
this tank? The flow rate will also increase
because the resistance dropped. The same is true if you use a large gauge
wire when moving current.
The larger the wire the more current you can move through it with damaging
the wire.
Watts is a measure of power. If you want to increase the
speed of a turning water wheel you can do this
in two different ways. The first way is to increase the water pressure.
This will cause the water to hit
the wheel with more force. The second is to increase the amount of water
hitting the wheel by
using a larger amount of water. The larger amount of water will weigh
more and cause the water wheel
to spin faster.
There is also a formula for power. It reads, power equals
voltage multiplied by current.
P=VI
A lot of times this is also displayed as W=VA or watts equals volts multiplied by amps.
Lets re-order this formula for an example:
W=VA
V=W/A
A=W/V
This example will show why a higher DC voltage is best in large solar systems.
Lets say you have 1000 watts of loads to run.
This is equal to:
83.3 amps at 12 volts
41.6 amps at 24 volts
20.8 amps at 48 volts
8.3 amps at 120 volts
4.1 amps at 240 volts
Knowing how much current is flowing to your load is very
important in selecting the correct wire. We
take the distance into consideration to calculate the voltage loss. Ideally
we don't want to exceed a 3%
voltage loss. The other half of this calculation is the current. You need
a larger wire to move more
current. If you have a choice the higher voltage is best.
These formulas are also useful in calculating AC wattage
to determine an inverter size. All appliances
have a face plate which contains all of its electrical data. Lets suppose
you have a microwave oven.
Often, the manufacturer will list an amp requirement on the electrical
data face plate. If the
rating is 8.3 amps. To find the wattage multiply this by the homes voltage
of 120 volts. This equals 996
watts.
To take this one step further. Lets see how much power
the microwave will use in one day. If you use
the microwave for 2 hours a day. We multiply the hours per day by the
watts to get watt-hours per
day. 996*2=1992 watt-hours per day. When sizing a system this formula
is very important in
determining the total power you use per day.
I hope this helps clear things up.
