The difference in AC current and DC current is that rather than being delivered up in a smooth, continuous flow like DC electricity, AC current is switched on and off at a high rate of speed usually 50 to 60 times per second. In order to do any work, electricity requires at least two wires. In DC electricity we call these Positive (+) and Negative (-) but in AC electricity they are called Hot and Neutral. The 50 or 60 pulses per second (called hertz or Hz) of AC electricity are alternately sent down the Hot and Neutral wires to the load. Since only one or the other of the wires is charged at any given moment this alternation of current came to be called Alternating Current or AC. Clever, huh? If you have ever had an electrical shock from a wall socket, the vibrating effect is the result of the 60 Hz of 120 volt electricity traveling through your body.

If you followed the previous section on DC current you discovered that you can store DC electricity in batteries for use later. Unfortunately, AC power such as we are accustomed to coming out of our wall sockets is not so easy to store. Small amounts of AC electricity can be stored in electronic devices called capacitors but they are not practical for storing electricity for home use.

Since most home appliances in North America are designed to operate at 120 volts AC and not 12 volts DC we have to have some method of converting our stored renewable energy from 12 volts DC to 120 volts AC (note the 1 to 10 ratio). In earlier times, we could connect a motor which operated off of 12 volt DC to a 120 volt generator and run the 12 volt motor off of our battery to produce 120 volts AC. This was all very good but very inefficient. Approximately 60% of our energy was lost as we moved from one electrical system to the other. Over the years since electricity has been in wide-spread use, other, more efficient devices have come about to make this conversion more efficient. Transformers (a metal core wound with wire) and electronic circuitry or a combination of the two have pretty much replaced the old fashioned rotary inverter. Inverters today have an efficiency rate that can exceed 90% (less than 10% of the power lost in conversion) when properly sized for the application.

No discussion of AC electricity in relation to renewable energy is complete without mentioning that there are two basic types of inverters commonly in use today. The modified sine wave and the full sine wave inverter vary only slightly in their ability to run common household appliances. While the modified sine wave inverter is less expensive due to the simplicity of its design there are a few loads such as digital clocks, fluorescent lights with magnetic ballasts, ceiling fans, variable speed devices and electronics which employ silicon controlled rectifiers which do not perform well from a modified sine wave inverter. To operate these devices at their full potential, an inverter with a full sine wave output is required.

So what’s the difference in the two? If you are familiar with a device called an oscilloscope which lets you look at electricity you will know that conventional electricity from the wall plug looks like a regular series of ocean waves that are all identical. In a 60 Hz system such as we have here in North America, there are exactly 60 identical crests and 60 identical valleys per second in the wave pattern. With a modified sine wave inverter, we will still see 60 identical crests and valleys per second, however they are not smooth like ocean waves but rather stepped, much like an Aztec pyramid with little flat tops and bottoms. Some electrical motors and fluorescent lights buzz and some small battery chargers (the type used in portable tools) do not even recognize this type of waveform as electricity at all. However, most appliances will never know the difference so you can see why the modified sine wave inverter is still popular today considering it costs less to buy than the full sine wave inverter. For more on inverters, see our section on Inverters in the Sunelco market place or order a copy of our Sunelco Planning Guide and Product Catalog.