An interesting question arose yesterday about why a certain APC UPS would not work with a DR Inverter/Charger. That UPS model did not recognize the modified sine wave of the DR inverter – similar to the erroneous voltage reading you will receive from a DR with a non-true RMS Multimeter. The installer wanted to put in a pure sine inverter, but the real question was why use a UPS at all? The load was servers and PCs, so the transfer time of the DR was not an issue. I think the only other reason to use a UPS would be for the “power chute” feature which shuts down the computer operating system on a low battery signal and saves data before cutting power to the computer. In this case, however, the customer loved his UPS, and so the installer quoted him a sine wave inverter as a replacement to his DR at three times the cost.