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 cost of a solar electric system is measured in dollars per watt. The average cost for a residential system is currently $3-5 per watt. That means the average 5-kW residential system will cost $15,000-$25,000, prior to tax credits or incentives.