Step #3: Consider how much power you actually use and how and when you use it.
Realistically, you will probably look to supplement your power needs via solar (“gridtie” as previously described) rather than use a Battery StandAlone system. If you want more information about battery back up and true system sizing, please let us know – we have an overview you can use to determine your full power needs. However, if you simply want an understanding of how much electricity you use and how much a solar system will produce, follow the steps below.
One way to look at the math:

Solar systems are generally sized in 1 kW – 6 kW (and larger) systems. A typical residential size is 3 kW.

3 kW, or 3 kilowatts = 3,000 watts

A 3 kW system will generate around 3,000 DC watts per hour

Multiply the per hour generation by 5.4 which is an average number of sun hours in a day (3,000 x 5.4 = 16,200)

Multiply the new total by the average number of days in a month (16,200 x 30.5 = 487,620)

Multiply the new total by .77. This is the “derating” factor, or the amount of energy lost when DC current is turned into AC current. (487,620 x .77 = 375,467)

So, a 3 kW system will generate about 375,467 watthours per month, or about 375 kWh.

Now compare this number with the kWh usage noted in your electric bill. How many kWh do you use in a typical month? Twice this amount? Then you would save roughly ½ your electric bill if you installed a 3 kW system.
 Consider how much money you save per month to figure out how long it will take to pay off your system.
Another way to look at the math – in reverse:
If you want to get all of your energy needs met through solar power (and get a “0” bill from your electric company) calculate how large a system you will need by following the steps below. Before you start, choose an average electric bill. Look for how many “kilowatt hours” you consumed. This is generally expressed as “kWh”.
Direction  Example  YOUR info 
Note the average number of kWh you use per month  550  
kWh X 1000 = total AC Watts used per month  550,000  
Total AC Watts / 30.5 (days in a month) = AC Watts used per day  18,033  
AC Watts used per day / Sun Hours per day (Central Texas = 5.4)  3339  
AC Watts needed per hour per day X 1.29 (AC to DC conversion factor)  4307  
Solar array in DC Watts to reach a Zero electric bill  4307  
Solar array in kilowatts, or kW  4.3 