RAB makes a good quality 12V motion sensor. I think it is called the Stealth. There a bit pricey ($75 I paid for IIRC),but work well and hold up. I've got one that must be 10-years old, and now use two of them. There are cheaper versions, but you get what you pay for on this stuff. You can adjust both sensitively and delay.
We have one on the garage/shop that controls power to the overhead door outlets - three of them - and turns on spot lights, all via a SS relay, if its dark. A close-nipple photocell controls the dusk/dawn part of it. This my be silly, but it keeps the inverter off all day long when were not home.
~kitestrings