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.
Tesla conveniently gives the national average cost of electricity ($0.11 per kWh), though actual cost varies widely by region and by season--even by time of day. But instead of national averages for premium gas prices, it lists an arbitrary average of $5 per gallon over three years.