Cops will leave their engines running because they may have to go into service at a moment notice.

mmmm.. I don't think this is the crux. They need power for their devices - the computer and radio gear foremost, but there's a lot of other bits and gadgets. Our local PD has been deploying hybrids and phev's for this reason - they can sit all day and still have juice to do pursuit etc. one thing they don't like is the auto-cutoff that's present in a lot of ICE vehicles nowadays because that lag is problematic for their electronic systems and ability to quickly pursue.