Buy a decent sized whole house unit not too big of one then put some small point of use ones where there is a lot of apliances. That way you will never run out. You may also have to add low flow aerators to your faucets and shower 2 gal min or less. With a gas unit you could get a Myson Thermar ond and then put electric ones where needed. Or you could go all electric. The unit size depends on your usage in all areas of the home. Tell me your usage in all areas of the home and I will try to help you.