This thread is reassuring. Generally speaking, electricity wise this household is fairly frugal and the running of a 400w hps actually doubles the useage. Wouldn't the timer, especially switching on at night on a regular basis, look a tad suspect? All my damn bulbs are cfl as it is so I don't know where I could feasibly cut back.
Like Rolla8 said... it's non-issue... But, I know that worried feeling you have all too well. If you want a little peace of mind... time your light to run during the period when your Central Heat A/C unit is running the least. that will even things out and it doesn't have to be timed exactly perfect. Another way is to compare the power consumption from 30 minutes before you wake up to 30 minutes after you wake up. You'll find a spot to put it in... and I'll be that comparison will represent a much bigger spike than a 400w HPS.
But, again... like Rolla8 said, it's non-issue. I've never really seen anyone worry much about a spike when the light come one with anything less than a few thousand watt bulbs... and even then, all that did was to stagger the on times about five minutes a bulb.
Oh, one other note on this issue... There was a thread a while back where someone was discussing how the power company had sent an electrician out to their house to make sure everything was alright... It took him using 6000w worth of lights plus fans, heaters, water pumps and air filtration for that to happen and even then, I think he completely explained it away by telling the guy he had been doing some welding.