Hello. I have a 24" iMac and I have just bought an APC 500VA 300 watt UPS unit. I wasn't looking for long term battery backup, just surge protection and something to stop my computer and external drives from going down hard in an outage. The unit (much to my surprise) runs the iMac for about 20 minutes at lowest brightness. When it is on battery, however, the power supply on the iMac hums. I'm not sure what this means, but I suspect it is from insufficient power. I'm wondering if it could cause damage to the mac? How is one supposed to decide on a UPS based on the VA rating? If anyone knows a lot about UPS' and can give me some advice that would be great. The 24" iMac draws 220W max.