Performance monitor for a photovoltaic supply

A method and apparatus is disclosed for monitoring the performance of a solar powered electrical supply where the supply comprises an array of photovoltaic cells that are mounted on a building. In one embodiment, the apparatus comprises: a device for producing a signal representative of solar irradiance; circuitry for computing a running performance signal by using at least the irradiance signal and a measure of the electrical power supplied to the load from the array; a radio for broadcasting the performance signal; and a portable unit for receiving the performance signal from the radio and for visually displaying the performance signal, power production, and electrical utility usage.