500W is equivalent to several desktop PCs running under load. Make sure it's not some "out of sight, out of mind" device. Could be a refrigerator/freezer in defrost when you thought it was not running.
As for the DVR, it's surprisingly simple to build your own with MythTV. I have one running on an Atom D2700 and it uses about 20W most of the time, jumping up to 32W when the main hard drive is spinning. (And it does a lot more than just a DVR.) Could possibly go even lower with a Raspberry Pi or something, though it won't be able to do as much.
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To my surprise, shortly after Naomi Wu gave me a bit of fame for making good use of solar power, Allie Moore got really jealous of her...
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