The Double Conversion online UPS systems are the most common type of UPS above 10kVA and are ideal for applications where electrical isolation is necessary or for very electrically sensitive equipment. Such UPS systems a few years ago were reserved for business installations only and even though several companies made low power models available to home consumers during the past few years, the cost remains quite restrictive for the average technology enthusiast. It is an expensive type of UPS, mainly reserved for environments with very low utility power quality (industrial applications, data centers, systems running off several different and / or low quality power generators, e.t.c.).
The Double Conversion online UPS is not much different than a simple Standby UPS in terms of technology. The core design is very similar or even identical to that of a Standby UPS; however the rectifier no longer acts as a battery charger only but is designed to run continuously. This means that not only the rectifier is tremendously larger but the inverter is larger as well, since both are designed to work 24/7, and they also require improved cooling systems.
This type of UPS is called "Double Conversion" because the rectifier continuously powers the inverter, isolating the load from the utility power source even while the UPS is normally powered from it. Because of the design, a failure of the input AC load will not trigger a transfer switch since the batteries are always connected to the inverter. During a power loss the rectifier will cease to work and the batteries will keep the power feed to the inverter steady and unchanged, in no transfer time and without any switching transients. After the power is restored, the rectifier will get back to work and resume powering the inverted, as well as slowly charging the batteries at the same time.
Even though this design provides nearly ideal electrical performance and exceptional isolation (many people call Double Conversion UPS systems "electrical firewalls", with a reason), it has several disadvantages as well. Because of the presence double conversion, where the power the load needs gets converted to DC and then back to AC again, the electrical efficiency of the design is relatively low, increasing the overall power consumption of the system and definitely increases heat generation. It is also considered to be an unreliable design compared to others because the constant wear of the power components may cause equipment failures. High reliability Double Conversion systems do exist, however the cost is extremely high and entirely out of the reach of home consumers.