Your employer (reasonably) has an expectation that you will give them 100% during at least 9-5 hours and also be available for overtime periods on occasion.
If you are building a product or doing consulting on the side for something with real customers (especially if they are businesses) they also probably have a reasonable expectation of being able to reach you within business hours to resolve any serious problems.
You may be able to find customers who are happy with limited evenings and weekends only support, but these are probably only going to have a small intersection with "serious" customers who are happy to drop enough cash on your product/service to make it a viable full time thing one day.
What is the best way to balance this?