Power inverters is only since the 1960s when technology was readily available for producers to create the solid state inverter. For example, energy in a car is made by a battery which generates direct current. However, many drivers would prefer to power consumer devices like DVD players which run on alternating current. Power inverters manufactures are actually very common.
When there is an electrical outage and the power is being taken care of with an uninterruptible power supply (UPS), then the battery which runs using direct current will be converted within the UPS to generate alternating current. As an example, the typical 1000W car power inverter that's built to be used in an automobile is just strong enough to power two 60 watt bulbs or the equivalent. That is why it's very important to get an inverter that has sufficient capacity to get the job done.
Generally, they are very limited with the amount of current that can be produced. The cheaper power inverters that make a modified sine wave will be fine. Those running AC devices which are extremely responsive to the kind of AC current will have to buy much more expensive pure sine wave car power inverters that produce a pure sine wave. The price tag on power inverters can vary widely based on how much current can be handled.