I think there are a few ways of estimating this. It should be done at a household level as there is usaully on one member of the household that does the laundry.
If the problem is strictly at a residential level then demand would be a product of detergent used per wash by washes per week by 52 weeks by number of households.
If the demand was the entire amount of washing powder used it would be the sum of residential usage (defined above) and commercial usage. You would need to understand the nature of commercial use before formulating this.
The most important survey questions to ask are how many washes you do do per week? and how much detergent do you use per wash? There are many others depending on how much detail you require.
To forecast this you would grow out the number of households with the population data you have.
You could go into further detail with your survey by asking about different types of detergents and washing machines for more detail and maybe a more accurate estimate.