A 3-4kW solar system is beneficial for a household of 1-2 people or those with low energy usage (around 4-8kWh per day). This is an approximation and will depend on individual circumstances.
You can work out your average daily usage on your electricity bill by dividing the peak usage by the number of billing days. Your average daily usage may also be stated on your bill. Getting a proposal for a system this size also depends on whether or not it will fit on your roof as well as other variables.