again when it rises. This requires considerable amounts of energy storage in a small package. The lowest-cost option for the required amount of storage is the electrolytic capacitor, but these have relatively short lifetimes normally measured in years, and those lifetimes are shorter when operated hot, like on a rooftop solar panel. This has led to considerable development effort on the part of microinverter developers, who have introduced a variety of conversion topologies with lowered storage requirements, some using the much less capable but far longer lived thin film capacitors where possible. Three-phase electric power represents another solution to the problem. In a three-phase circuit, the power does not vary between (say) +120 to -120 V between two lines, but instead varies between 60 and +120 or -60 and -120 V, and the periods of variation are much shorter. Inverters designed to operate on three phase systems require much less storage. A three-phase micro using zero-voltage switching can also offer higher circuit density and lower cost components, while improving conversion efficiency to over 98%, better than the typical one-phase peak around 96%. Three-phase systems, however, are generally only seen in industrial and commercial settings. These markets normally install larger arrays, where price sensitivity is the highest. Uptake of three-phase micros, in spite of any theoretical advantages, appears to be very low. Protection Microinverters protection usually include anti-islanding; short circuit; reverse polarity; low voltage; over-voltage and over temperature. Portable uses Foldable solar panel with AC microinverters can be used to recharge laptops and some electric vehicles. History The microinverter concept has been in the solar industry since its inception. However, flat costs in manufacturing, like the cost of the transformer or enclosure, scaled favorably with size, and meant that larger devices were inherently less expensive in terms of price per watt. Small inverters were available from companies like ExelTech and others, but these were simply small versions of larger designs with poor price performance, and were aimed at niche markets. Early examples In 1991 the US company Ascension Technology started work on what was essentially a shrunken version of a traditional inverter, intended to be mounted on a panel to form an AC panel. This design was based on the conventional linear regulator, which is not particularly efficient and dissipates considerable heat. In 1994 they sent an example to Sandia Labs for testing. In 1997, Ascension partnered with US panel company