Solar PV Roof Mount  (photo voltaic) grid tied system, takes the sun's rays to create electricity. For every 100 square feet you can make 1,000 watts or 1kW. In Pennsylvania  the average solar exposure is 4 hrs a day all year round.

Pricing for roof mount solar PV systems
1.24kW : $3,968
3.1kW : $9,920
4.5kW : $14,400
8kW : $25,600
10kW : $32,000
50kW : $145,000*

Price to compare is $3.20 for roof mount solar PV systems. 
*50kW is priced at $2.90 a watt

Break it down:
If you have a 4.5kW solar PV system, the formula goes like this, 4500 x .80 efficiency =3,600. Take 3,600 x 4 hrs =14,400 watts or 14.4kW hrs will be generated a day. Take 14,400 x 30 = 432kW hrs a month would be generated. So if your electric utility bill is $110 a month, and it cost $.14 a kW. Take 110 / .14 = 785.71kW . The 5kW solar pv system would cut your utility bill in half.  You'r utility bill would go from $110 to $50 a month. 30% Federal tax credit is good til year 2020 and then will decrease to 25%. Estimated return on investment return would be 11 years.