There are a series of good qualifying questions for sizing a storage system.
The first two-part question for homeowners:
What do they want to run during a power outage and what do they really need?
When people truly think about it, there is a big difference between the two.
Additionally, I recommend asking these follow-on questions to further assist the customer in making an informed decision:
1] What is your average daily kWh use in winter? in summer?
This question tells you a lot about the rest of the questions - do the math for them. i.e. How long will a 10kWh battery last over the course of ONE day?
2] When your power goes out, how long is it out for?
3] What is your minimum need for the desired period of time?
4] What is the weather like when the power goes out?
This addresses the ability for solar to recharge the battery during the day - is it winter and cloudy?, or hot and sunny?
5] What is the homeowner's budget for storage to meet their needs?
This makes them re-think about question 3] and how much back-up they really need
6] After addressing all of the above - How many batteries do you need?
Storage will rarely "take you off-grid" for the typical home. Energy demand for the entire home normally exceeds battery capacity.