I've been looking into this as well to power just my 1000W HPS HID light. At first I thought I would need 1000 Watts in solar power in order to power it, but I was wrong. Don't look at it in watts, but look at it in Amps.
You could buy a $200 120 Watt solar panel system. Then add your necessary hardware of a charger and a monitor, Inverter, and your batteries.
A 120 Watt system will supply about 10 amp hours of power. A 1000 Watt HID light on 120V power grid will draw 8.33 amps.
If you have those solar panels in direct sunlight for 10 hours a day then you will be collecting 100 amp hours of power per day.
my 1000 Watt HID draws 99 amp hours per day.
Now what you want to do is run 6V deep cycle marine batteries. And if you run two of them in series you should have 400 amp hours available.
If you are keeping track just the batteries could run my light for 4 days without being charged.
So in a perfect world you would be running off of the batteries, and your solar panel will fully charge your batteries every day.
Of course in my example 120 Watts is technically not enough. You don't want to gamble with exact numbers. I would probably be safe if I bump up to a 200 Watt system. That should give me about 16 amp hours of charge.
What I've come up with so far is for a $500 investment I would be saving $60/mo. It would pay for itself in less than a year.
Edit: Also if you want to really be efficient you could do a grid tie in, so that if your batteries are fully charged, and are being charged at a rate that is faster than your current draw, then you could bleed off any overages back into the power grid. By doing this you will reduce your overall electric bill on a monthly basis.
Oh and did I mention that if you live in the US, this will qualify for a 30% tax credit on your federal taxes? And in the state I live in it will also qualify me for a property tax exemption. That alone pays for the system and then some.
Well your math is sound but problem is it just doesn't work that way .my buddy runs 250 watt solar array with a 6 battery system ,his 600 watt hps will only run about 6 hours a day before the voltage drops to the point of his invertor shuts off which is around 10 volts this protects your elec. also very few places get 10 hours of uninterupted light a day,then add in the cloudy and rain days. well think your figures are a little off but you are right about not looking at in watts but in amperage hours. i truly wish it was that easy to run a 175 watt panel but fact is it will take much more to run a 1000 watt hps.