The best-known of these tools is probably the hand-held window squeegee, used to remove the cleaning fluid or water from a glass surface. A soapy solution acts as a lubricant and breaks up the dirt, then the squeegee is used to draw the now water-borne dirt off the glass leaving a perfectly clean surface.
Another way of estimating the cost of electricity would be the cost of an ordinary solar system, or $21,000 as shown in the example above. So the cost of a Tesla retrofit system would be in the range of $33,000 to $37,500.
The two-story 4,562-square-foot residence is located in a suburb of Houston, where the air conditioning might run 300 days per year. Tesla estimated the homeowner would need 2,779 square feet of roofing, and suggested half solar and half non-solar tiles. The cost of the roof came in at $73,600, and the calculator recommended two Powerwalls, for an additional $12,500.