Rooftop Solar could generate half of Miami-Dade County's Electricity

Solar panels on the roof power both house and electric car

The solar train is leaving the station (with or without FPL).

Here are some back-of-the-envelope calculations.

The total area of roofs in Miami Dade County is:  116 million square meters. Or 1.244 billion square feet.  Or 44.6 square miles. Thanks to Jeffrey Onsted, an FIU colleague, who wrestled this figure out of the Miami Dade County Property Assessors map of 2011.

Electricity use in Miami Dade county in 2008 was about 27.3 billion KWh.  Here's the source: Greenprint
I’m sure it’s more now, but that’s a figure I could find.

A solar panel in Miami generates about 216 KWh per square meter per year, more or less. 
This figure comes from from the universal formula for solar yield:  E = A * r * H * PR

E = Energy (kWh)
A = Total solar panel Area (m²)
r = solar panel yield (%)  (usually about 15%)
H = Annual average solar radiation on tilted panels (shadings not included) (Miami is about 5.25 kWh/ sq m/day, or 1916kWh/square meter/year)
PR = Performance ratio, coefficient for losses (range between 0.5 and 0.9, default value = 0.75)

Therefore, take the total square footage of the roofs, 116 million sq. meters then reduce it by about 40% (I’m guessing) because panels must be 3’ back from the edge and roofs are populated with air conditioners and stuff.  That comes to about 69 million sq. meters

Multiply by 216 KWh/sq.m/yr.  Equals 14.9 billion KWh.

Therefore, Miami Dade County could generate about half of its electricity use from rooftop solar.  Hmmm.

I did a similar calculation for all of Florida’s energy use, including electricity, gas, oil, jet fuel etc, to figure out the area of solar panels needed.  It came out to about 2,200 square miles (47 miles per side of a square)  Here’s the map:

Area of solar panels needed to power all of Florida at current energy usage