There is one thumb rule we can say about in terms of area requirement for solar panel installation which is that, if your efficiency is 15% then a solar module will produce 150 watt/m2 that is 10 times more than your efficiency.
For Requirement of Home 1 KW = 1000 watts
15 % Efficiency = 150 watt/m2
Total solar panel area = 1000/150 = 6.66m2 = 71.7 square feet
( 1 square meter = 10.76 square feet )
I.e. for 1KW = 72 square feet area is required.
And for 2 KW = 144 square feet area is required.
With this little efficiency we can fulfil all our requirements which are lot of power that we can use. And the overall requirement for the roof area is 70 – 100 square feet which is really not much.