Alternating current (AC) is electrical current that reverses its direction at a standard frequency of 60 Hz (cycles per second, or 50 Hz in South America and Europe). Conventional AC power is produced by rotating machines (alternators) that produce a smooth alternation, like that of a pendulum. It is described mathematically as a "sine wave". It is the ideal waveform for the transfer of AC power.
An inverter is an electronic device that converts DC to AC through a switching process. Thus it produces a sort of "synthesized" AC. There are two types of waveforms available from high-quality inverters. These are the so-called "modified sine wave" and the "true sine wave".
The "modified sine wave" is not really a sine wave at all. It is a stepped wave, like a pendulum that is being hit back and forth by soft hammers. It achieves voltage regulation by varying in width according to the battery voltage and the load. Thus, the wave is not as smooth as a sine wave. The quality of "mod sine" inverters should not be underestimated, however. They are highly capable, and (by narrowing the waveform) they save energy when running only small loads, as happens during most of the day in a typical home. They also cost half the price of sine wave inverters!
The disadvantages of modified sine inverters are (1) additional electrical noise may be produced, showing up as a buzz in some audio equipment and from some transformers, (2) some electric motors and transformers run hotter and draw a bit more power, (3) digital clock and timing circuits can be fooled, sometimes counting double-time and (4) in rare cases, power supplies in sensitive electronic equipment can be damaged. In spite of these occasional problems, mod-sine inverters have been successful in many thousands of remote home, RV and marine systems since 1986.
True sine wave inverters are more efficient for running motors, including AC pumps. They are less likely to draw complaints from people who enjoy high quality audio, or who simply have lots of electronic gadgets. If a mod-sine user has a problem with one or two small applications, here is a solution. Add a second inverter to the system, a small sine wave unit, to handle the problem circuits. Sine wave inverters in the 125-1000 watt range are made by Exceltech and Statpower and are available from Affordable Solar.
If an an inverter is not a true sine wave type (a so-called "modified sine wave" inverter), a simple voltmeter or multimeter will not read accurately. Typically it will show around 95-105V. However, the normal amount of power WILL be transfered to the load. This DOES NOT indicate a problem, but we frequently hear from customers and electricians who notice this discrepency.
In order to measure a non-sine wave or a distorted waveform, a more expensive "true RMS" meter is required. (RMS is root-mean-square, a sort of exponential averaging.) Prices of RMS multitestors begin at around $100 in USA. If you don't have one and you want to confirm proper inverter output, simply connect a lamp with an incandescent light bulb. If it looks like its normal bright white, running on any modern inverter, you can safely assume that the RMS voltage is in its normal range.