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The demonstration used at shows produces a flow of 300 gallons per hour over 18". This means that the panel is capturing water and delivering it into the rain gutter at a flow of 16.8 gallons per hour per inch of gutter. In the example below of an average house, d = 180"; rain would have to fall at the rate of 21.4 inches in an hour to produce this flow.

Converting between inches of rain per hour and
gallons per inch per hour.

We need to know the horizontal distance from the eaves to the crown of the roof. For a typical house this will be half the width of the house. It is labeled d in the diagram. For the calculations below we should measure d in inches.

If x inches of rain fall, xd cubic inches will flow across an inch of gutter, on average. There are 231 cubic inches in one gallon. So if x inches of rain fall per hour there will be a flow of xd/231 gallons per hour across an inch of gutter. For example: If d is 15'=180", and the rain fall is 2" in an hour, then the flow over each inch of gutter will be 180 * 2/231=1.5 gallons per hour. Note that the demonstration model captures 16.8 gallons per hour.

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