That must be so frustrating! I'd love to have some bats living in our area, but not quite like that!

They might be willing to relocate easily if you put up a spiffy bat house for them. Check out the Bat Conservation Society website for info and ideas -- I'll attach a link. The people are really nice and very helpful, and can help you find ways to discourage the bats from roosting there. (Since you're in MI, they might even be local.)

As for where to get a bat house, you can buy them in various home-improvement stores like Home Depot, Lowes, and so forth. The conservation group sells them too.

Good luck!

Here is a link that might be useful: Bat Conservation Society site

Consider yourself lucky! There are many people who would love to have bats and try as they might, they just can't get them. They eat a ton of bugs every night. Just be careful if you are sweeping the bat poop (guano) You don't want to be breathing that in. Guano is however, a wonderful fertilizer so place your plants underneath. You also should know that even though it's not in a real convenient spot, it is still against the law to disturb a bat nesting site so get them out of there before they really get going. You can place a rubber snake or any other natural enemy close by, place a bat house in a more suitable position with similar environment. Face the house the same direction, at the same height, etc. Place some of the bat poop on the ground under the bat house and hope for the best. Just be careful messing with that stuff it's way not good for you. You can purchase a bat house for anywhere between $50-$100 or build one yourself for around $20.

We have one little bat that also hangs out at night on our front porch under the eave...I love'm!
He really keeps the bug population under control.
If you don't want'm...I'll take'm! :)

Make or buy some bat houses and screen off their ports
of entry. I lived in Texas for several years and New
Mexico, and this helps. ALSO you can SAVE the droppings
in piles outside after you clean out the area, let them
season over for 6 months. It makes a SUPER FERTILIZING
soil additive!
Jewel

Try smaller lightbulbs, to hold down the cost. You may not have to leave them on forever, just until they relocate. good luck

What about a porch ceiling fan type of thing. Turn it on before they come by the whirring blades should disturb them enough and you can turn it off when they find someplace else to sleep.