First off all, forget the HPS. I use HPS and it is fine for seed starting, but MH simply produces stronger plants. The HPS is for assistance when it comes time for flowering. It isn't a bad light at all and if you already had it I would say use it, it is simply that MH is better suited to vegetative growth than HPS is so if you are buying now, go with MH.

Next, to cover an 11x20 area there is no way a single light can do that. Your lumen calculations don't work out because lumens drop as distance from the light source increases. They drop rapidly.

You should be thinking of a 1000 watt light as being able to adequately cover a 6'x6' area. I use a 400 watt HPS in a 2'x5' area with appropriately shaped reflector and the plants at the edge of the 5' strip show that they are not getting as much light as those under it. Really my light only does a good job for high light plants in a 2'x3' area, but could probably handle 3'x3' with a different reflector.

Your 11x20 area could be covered with 6 1000w lights. Effectively giving you 12'x18' of coveraged if hung in 2 rows 3 in each row. Alternately you could go with fewer lights, but invest in an electric light mover.

Of course all this is just guidelines. A lot of it depends on how much light your specific plants really need to survive and whether you wish them to survive or thrive during the winter.

You might want to build the greenhouse and try it without supplemental lighting one year and see how things go before jumping into the cash outlay for lots of lights. You might be pleasantly surprised with the results.

I agree with username. I have a 400W MH with a very good reflector. Unless you are RIGHT under the center of the lamp and VERY close to it, I can only get about 800 to 1200 footcandles, even just about 2 feet out. It's enough to keep the hibiscus fairly green and bushy but doesn't add much in the way of flowering (maybe I should have gone for the HPS for blooms).

My current 1000W MH puts out 3200 fc 24 inches away and 1100 five feet away with Mylar on the walls.

It's amazing how little light a huge bulb like that puts out from a botanical perspective!

I dunno about not using the lights-I can try it. I have fruited oranges under the MH light but only if the majority of the plant gets at least 2000 FC.

There's no way that winter light levels will be near that naturally. Since citrus ripens in winter, I don't know if they'll be able to ripen fruit when it's cloudy almost every day from mid-October to early March.

But adding light later isn't all that hard.

BTW I was thinking HPS mainly because it has about 25-30% higher output per watt.

well, if your goal is to get ripe fruit under lights then HPS is the way to go. I wasn't clear on what you were trying to accomplish in my first post.

If you are able to measure the light levels from your light at the distance your plants would be from them then you are good to go and can plan accordingly.

Still, for an area the size you are thinking of you might still want to consider fewer lights and incorporate a motorized light mover.

I may do that.

Here's a question:

Is there a such thing as a "lumistat" that would turn on the lights, during pre-specified hours, ONLY IF ambient light levels in the GH drop below a certain level?

I realize that it couldn't be a constant sensor as you could have a single partly cloudy day burn out your lights as they turn ON when a cloud moves over the sun...then OFF as it moves away...then ON again...OFF again...etc...but maybe if it only took a reading say, once an hour, or maybe the average hourly input, once per hour?

I never heard of such a thing, but it certainly sounds like something that wouldn't take much for someone to make. Maybe you can ask on the greenhouse forum, if there is such a thing someone there probably knows about them.

Maybe I have an invention to work on!!

honestly it sounds like the kind of thing that should exist. If it doesn't then I agree you should get to work inventing it. Actually, just patent it ;-)

I remember reading someone who had a lamp hooked up to come on on cloudy days. Basically, it would be a photoelectric cell that turns the light on when it's dark, like they have for porch lights.