I recently switched from Radium 20Ks on 250W HQI (M80) ballasts with Iwasaki 15Ks on 175 W electronic ballasts. The measured electrical usage is literally half (~360W/ballast vs ~180W/ballast).
OLD (250 HQI Radiums):
NEW (175 electronic Iwasaki) - I've done some re-aquascaping since the last readings:
Not bad and on par (sorry for the pun) with Sanjay Joshi's results.