Will a porch light keep bats away?

A porch light may have some effect on bats but it will not reliably keep bats away and can have opposite effects depending on the situation. Bats are nocturnal and generally prefer darkness for roosting and hunting, so a bright porch light shining directly on a roost entry point or attic vent may discourage bats from emerging or roosting there because it makes them feel exposed to predators, and some homeowners use lights in attics as part of humane harassment to encourage bats to leave. However, many insect-eating bats are actually attracted to porch lights at night because lights attract moths, mosquitoes, and other night-flying insects which are their food, so you may see bats hunting around your porch light at dusk catching insects, which is beneficial for pest control. So a porch light may keep bats out of a specific dark roost if it illuminates the entry, but it will attract them to hunt in your yard. Porch lights alone are not an effective or humane method to exclude bats that are living inside your house, because bats will tolerate light if the roost is warm and safe. The most effective and legal way to get rid of bats inside is to use one-way exclusion devices that let bats exit but not re-enter, seal all entry holes larger than a quarter inch with caulk and hardware cloth after confirming all bats are out and no babies are trapped inside, and do this outside maternity season.