What’s the difference between AC and DC?

An off-the-grid novice asked us this question today: What’s the difference between AC and DC?

There are a few quick answers: AC (Alternating Current) looks like a wave, and DC (Direct Current) looks like a straight line. Batteries or dry cells use DC, while AC comes from your electrical wall outlet. However, if you’re using a portable computer, you’re actually using both AC and DC current.

What’s the story here? It’s all about the directional flow of electrons.

Think positive

A regular battery has two terminals – one is negative, the other is positive. The electrons in a battery flow at a constant rate from the negative to the positive terminal. This is Direct Current.

Alternating Current has a variable direction visualized as a wave-like pattern. The electrons in AC flow first in one direction, then reverse, or alternate. In most countries, current alternates at 50 or 60 times per second.[1] The measure of this cycle is called hertz – a unit of frequency that equals one cycle per second.

What’s the frequency of Direct Current?

Easy: the frequency of direct current is zero. This is important when it comes to electronics, as we’ll see below.

Why does current alternate?

In an AC system, a rotating magnet is used to create the phenomena of alternating current. As long as this alternator continues to generate current, the current will move along an electrical wire and away from the source. (See the Alternating Current page on Wikipedia for a more thorough explanation of this.)

Why do we use both AC and DC?

Although many devices use DC, AC is prevalent because it’s easier to transform and distribute. Its main advantage is that it can be effectively transferred across large distances over transmission lines. This is accomplished through the use of a transformer.

In simplified terms, a transformer converts AC to high or low voltage. AC is transformed to high voltage for easy transmission from the power plant to the distribution point, and the voltage is later decreased for use inside your house.

Electronics such as your portable computer require a steady, direct current with a single polarity. The reason is this: if the current fluctuated, a portable computer or similar device would be unable to accurately register ones and zeros. Other products such as LED lights also benefit from the steady flow of DC.

If you have a portable computer, you’ll notice a power brick attached to its cord. AC flows from your electric wall outlet and into this brick where the current is transformed to DC.

Those of us with solar photovoltaic panels are already creating DC. You can use an inverter to convert the energy to standard AC, but you’ll be wasting energy during the conversion. This conversion may be required if the system on the grid. One can also install a separate 12 or 24 volt system to power part or all of a home, depending on your environment and personal requirements.

Because of our lighting and electronics use, we’re quickly approaching a point where direct distribution of DC may become common. See the article Edison’s Revenge: The Rise of DC Power by MIT Technology Review for an interesting look at this.[2]

So…is it better to be off the grid and use DC power, or stay on the grid and use a split system, or invert everything, or what?

Personally, my take is this: simplify your life, get off the grid, and stay there – but that’s a question for another day.