I have not heard of one but you could contact Onset Technologies which create data loggers of all sorts. They may be able to supply you a light sensor that could be paired with a mechanism to turn off the lights when enough natural light was present.

I share your concern about energy waste. What is your goal? Do you someday want to build a commercial operation or are you looking for a hobby structure? We are in the process of building an energy efficient greenhouse and we considered artificial light to supplement the natural light but in the end did not really pursue it. We do have LED lights but just in normal fixtures and using typical warm white light. We were not interested in the blue and red LED lights because when we artificially light our greenhouse, it will mostly be for spending time in it. However, that might make sense for your application. No matter what artificial light you use, nothing comes close to the intensity of the sun. If you can use natural light for most -if not all- of your needs, you will be in good shape. For our GH, we are using windows strategically to get the most light and heat by putting them on the south side and a few on the east but none on the north. Everywhere else is R29 insulation which keeps things at a steady temperature.

Another thought, Logee's Greenhouse is an hour away from me in Connecticut and they sell tropical plants and trees. They use NO supplemental light and they are a commercial grower that sends their plants all over the country (Martha Stewart is a big fan). Their visiting hours are restricted in the winter because there is not enough light after 4pm for people to look around so they actually reduce hours when most stores are extending them for the holiday shopping season. They have a lemon tree that is over a century old that is producing HUGE fruit and many other fruiting trees that are pretty productive. Maybe you don't need supplemental light at all.

You can get daylight sensors, at least for screw in bulbs, at many hardware stores. I'm not sure if there are plug in versions or not and I'm not sure if they are adjustable or not. If they are too sensitive I suppose you could use a piece of tape to block part of the sensor??? Anyway it wouldn't check every hour or so, instead the timer would supply power and if there is sufficient sunlight during any period when the timer is on the light would be switched off. Hope that helps.

You just need a timer with a photo cell. I put a link to one below. There might be less expensive models out there.

Here is a link that might be useful: timer with photo cell

Supplemental lighting is often a "controversial" subject. I've looked into it as well, but there are only a handful of situations I've found where you REALLY need it:

Very high light plants (Citrus, bananas, & other tropical/subtropical fruits, but that's arguable, I've seen people argue both ways on this)
Photoperiod sensitive plants

I will say though, as a former Ohio resident, I can vouch that the sun makes relatively few appearances from November until February. So I understand the concern.

Another option is to just have a few smaller lights (250W or 400W) for a small area for the few plants that do need that extra light, but not lighting the whole GH.

That said, yes, such a thing would exist as was mentioned using a photocell system.

However, on a partly cloudy day, you might have the light switch on and off 30 times as clouds cover the sun then the sun comes out...burning out your expensive HID lamp in a matter of months!

I agree. Making back your electricity costs is very difficult. I've only seen supplemental lighting used on a large scale in crop research and university-funded facilities.

My greenhouses are poly tunnels and thus impossible to lock up. I wouldn't want to have expensive lights in a structure that anyone with a pocket knife can break into.

I have thought about building a little wooden shed inside the greenhouse, insulating it, and using the heat that the light gives off as heat for the plants through winter. That's the only way winter growing could work for me in zone 6. Poly tunnels don't hold in heat enough to heat them through the winter.