The back side of my house faces east. In the mornings the sun beats down pretty hard on my sliding glass door and bay windows. Inside the house, I have black out curtains on the glass door and blinds on the bay windows. They do a good job of blocking out most of the sunlight. Not completely though.
What I want to do is put up a tarp or something outside that provides shade leading up to the glass door and bay windows.
I’m wondering if it will be worth the time and effort and will it make a dent in my electric bill.
The blinds and curtains do help to block the heat by reflecting some light outside, but they themselves get warm and may be radiating heat into your house. A cover or something outside will help prevent the light from getting in the windows in the first place.
One thing you can try as an experiment is to tape aluminum foil on the windows and see how it changes your electric bill. That’s not necessarily a permanent solution, but the foil will reflect a large part of the heat back outside and will give you an idea of how much a patio cover will help.
But another issue is that the walls are also facing the sun and will be leaking some heat into the house. Planting trees on that side will block a lot of the sun’s rays and help keep that side cooler in general.
Hanging something outdoors is very effective, as @filmore explains, it keeps the heat outdoors. Reflective material applied to the windows like aluminized Mylar would be pretty effective also, it will reflect 90% of the light hitting your windows. UV blockers on your window won’t help.
Are the back of your black-out curtains white? If they’re not, maybe getting a cheap white curtain to put behind the black one (either like a ‘liner’ or just a sheet thumb tacked over the window for now). It may reflect a good amount of that light back out. You’ll probably be able to tell fairly quickly by how much cooler (or not) the black out curtain is when the sun is hitting it.
If the sunlight gets past the glass, then the heat is inside, and trying to reflect it back thru the glass is minimally useful. I hang those cheapo plastic roll-up blinds, the ones that resemble mats of flattened soda straws, from screw hooks over the outside of the critical windows.
When I moved into my present house, I basically ran two small AC units upstairs and big one down, from late AM 'til bed time, and still had to run an upstairs unit most nights. Now I run one upstairs about 18 hrs, with a small desk-type fan on the floor pushing cool air from that room to the rest of the house, and the downstairs unit might run 6-8 hours, with similar small fans circulating the cooled air; I consider moving air almost as important as chilling it.
Back to the blinds - these are insanely cheap, but they are also fragile; It pays to watch the weather, especially wind speed and direction, until you are familiar with how these “sails” behave. I wind 'em up at night, and don’t bother to roll 'em down on overcast days. I’ve been here five years and haven’t had to replace any. I give them a modest mount of attention and the reward is lot more comfort with a lot less machinery noise.