I'm not quite sure how that works exactly, is it something to do with energy efficiency? Just wondering why it's called a 250w light if it uses 150w of electricity
How much would be good for a 60cm (2ft) x 60cm (2ft) x 160cm (5.2ft) without being overkill?
That's my grow space

Perhaps a 400/500/600w LED and ditch the HPS?