a 400W HPS fixture uses significantly less energy than a small 6" space heater (those usually run between 1000-1500W). I don't think any eyebrows would be raised, but then again I don't live in a tight-knit community.

I think its more of a problem when people start running 2 or more 1000W systems that tips off authorities. Luckily I've never had to deal with that though, so again that is just my best guess.

good luck.