The temperature needs to be consistent at 37.5ºC (99ºF) in an incubator which has a fan, higher (38 - 39ºC, 102ºF) in a still air incubator.  If it drops below that for a while it's not the end of the world – the chicks are just likely to hatch a little later. If you're unlucky enough to have an electricity cut while you're incubating just keep the incubator closed, wrap it in something warm such as a blanket or quilt, and hope the power comes back fairly quickly. If it does, the chances are your chicks will be fine. If the temperature rises, it's much more serious. The embryos won't be able to survive a raised temperature.