The first things that you will need to know are your local net metering policies and average hours of solar irradiance.
For you net metering policy, you will need to go to your local power company website and search for something called "net metering agreement".
If the net metering policy does something besides give dollar for dollar credit on power that you send back and use you will want to get a thorough understanding on what the policy is and factor that in to your equation.
If you want to simplify this even more you can skip this step.
For the solar irradiance hours you can go to the NREL interactive map and find your location.
https://www.nrel.gov/gis/solar.html
Now that you have that info you can do this simple equation and get a good idea of how big of a solar system you would need.
Get the average KWH usage from the past 12 months and average them together to get the average monthly kWh usage.
Divide the monthly kWh by 30.42 to get the daily usage, if you already have the avg daily usage you can skip these steps
Find 82% of your solar irradiance hours
Divide your daily average kWh usage by the number you just got from your irradiance hours.
That number will be what you can use for rough estimations for DC kilowatts.
Example:
Average monthly kWh usage is 1,438
Divided by 30.42 is 47.27 kWh per day
Solar irradiance hours are 5.78 hours per day multiplied by 0.82 = 4.74
47.27 / 4.74 = 9.97
9.97 kilowatts DC
So now I know that including regular solar inefficiencies such as DC to AC conversion, cloudy days, rainy days etc. That this systems will need to be around 10 kilowatts.
This wont include factors such as shading, abnormal system inefficiencies, suboptimal azimuth or tilt, or how much solar you can actually fit on the roof. This isn’t a final design its just a base line to start with.
The formula:
Daily kWh usage / solar irradiance hours X 0.82 = DC kilowatts needed
If you want more info on solar energy leave some comments and I will post more stuff on here.