GAGUA Pro

The size of the system depends on individual household energy usage, and whether you want to completely replace the local utility with a solar PV system.  Here is a quick way of assessing your system size:

Total up your kilowatt (kWh) usage for a year (see your utility bill).  For a small single family home the number would be approximately 4,000 kWh (annual usage)
Divide by the number of days in the year (365): 4,000/365 = 10.95 kWh (average daily usage)
Divide by the number of daily average sun hours for your location (5.5 in sunny locations): 10.95/5.5 = 2 kW (system requirement)
At $9.00 per watt installed the raw cost of the system would be $18,000 assuming no rebates or other incentives