DC/AC inverters are necessary in order to utilise a DC power supply, such as from a battery or vehicle electrical system, to operate AC equipment. The output voltage usually equates to a domestic mains supply, with the input often being 12V or 24V, but other voltages are available for both input and output. Although many early inverters had moving parts, most are now electronic devices employing switching technology. When choosing an inverter, several things should be taken into consideration. The first and most obvious is the power output and this should be determined by looking at the potential load and by giving consideration to the DC power source. Another important factor is the wave form. Basic inverters initially produce a square wave, which is then electronically modified to an approximation of a sine wave. Such inverters are usually described as modified, approximate or quasi sine wave. More advanced inverters produce a sine wave that manufacturers usually call a pure or true sine wave. Some basic equipment will happily run on a modified sine wave but for delicate electronic or audio equipment a true sine wave is necessary. A useful rule of thumb is that if a device uses microprocessors, it usually needs a true sine wave supply.