wrote:

(one of these days I am going to put a light sensor on that thing
so it turns off during the day).

A motion detector with incandescent bulbs might use less energy.

That would totally drive the horses bonkers...

It doesn't bother ours. Then again, they are used to low-flying airplanes.

What I need is a photocell, but it MUST be outside to work...

You might look into X10 motion detector fixtures. Ours can send
a signal at dawn and dusk. Perhaps some gadget can integrate that
to inhibit an indoor X10 light during the day.

Nick