This is the power control panel that acts as an interface between the solar cells, batteries, and home electrical circuits. Our photovoltaic array produces 24 VDC electricity, our batteries store 6 VDC electricity, and most conventional appliances require 120 VAC electricity. The power panel enables a smooth flow of energy from wherever it is produced to wherever it is most needed.

The charge controller is the interface between the photovoltaic array and batteries. It ensures the batteries remain in a “topped off” state, ready to supply backup during a power failure. If the charge controller senses the batteries are full, electricity is diverted to the inverter for use in-house or sale to the utility company..

The heart of the system is a Trace SW-4024 inverter. It interconverts the DC and AC electricity, and contains a computer that performs all necessary functions automatically. It may look imposing, but it runs itself with little need for human intervention. When the commercial power grid goes down, as it often does during our monsoon season, the Trace kicks in within microseconds … and my PC keeps purring along without so much as a flicker.

Our electrical system is “grid intertied.” This means we can use commercially generated electricity if we want, and save the batteries for power outages. We use the grid at night, while the solar cells sleep, and purchase this electricity at off-peak rates. During the day we use mostly solar electricity, avoiding the peak rates. On a sunny day when we’re not at home, we can produce more electricity than we use. This spins our electric meter backwards, generating credits that are applied to our electric bill.

The sub-panel is a standard 8-circuit breaker panel that connects critical house circuits to the inverter. By connecting only critical loads, we extend our backup time significantly. The Trace can output a total of 4000 watts AC, but if we used the batteries at that rate they’d be totally drained in about four hours. Running only critical loads (refrigerator, microwave, fans, lights, communication, and security systems) we extend our backup power to at least three days. Very few grid power outages last longer than that. Total cost of these components, including wiring and hardware, was about $4000.