More domestic batteries reduces the electricity demand at night, meaning power companies need to buy less natural gas and coal powered electricity from producer

More domestic batteries reduces the electricity demand at night, meaning power companies need to buy less natural gas and coal powered electricity from producers. They lose some solar generation during the day that is now going into charging the batteries, but they have too much of that already. Net result a lower proportion of (more expensive) fossil fuels in the overall mix, meaning total cost of power generation comes down, and retail prices come down.