How It Will Work

January 29, 2004 by
Filed under: Solar Energy 

First, I’m off to mail the check in a few minutes. After the installer receives it, he’ll be applying for the permits.

For our system, there will be 36 185-watt panels on the roof. Each panel produces a rated 24VDC. The panels will be connected in strings of 6, for a total voltage on any string of 144VDC. Our inverter wants a range of 110 – 350VDC as input, so we’re in the range. (The voltage can get as high as 36VDC per panel in ideal conditions.)

The 6 DC wire pairs will come down from the roof in a PVC conduit and enter the house near the electric meter. In the basement will be a DC disconnect switch that allows us to cut off power from the panels to the inverter if required. Then, each string will be connected to the inverters (3 strings per inverter).

The inverter converts the DC power from the panels into 120VAC 60Hz power frequency-synchronized with utility power. At night, the inverter is essentially asleep (much like the Mars Rovers). Once enough power is received early in the morning, the inverters will wake up and start providing power to the house/utility.

The inverters are connected to a new electrical sub-panel via AC wiring. There are 3 wires – two AC legs and ground. Each inverter’s connection goes through a 240VAC 20A dual breaker in the panel. There will be two of those in the panel.

The panel output runs outside of the house to an external AC disconnect switch. This switch allows the utility to disconnect our solar system from the utility and the house if required. From the switch, the power connection runs back into the house and into our main electrical panel (the one with the existing circuit breakers in it).

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The solar output is connected to our main panel by a 240VAC 40A dual breaker. At this point (conceptually) the power mixes with utility power.

From there, we have the existing connection to the utility. Our existing meter will apparently be replaced by a new meter that handles net metering. If the solar system is producing more power than the house needs (likely if the A/C isn’t on during the day in spring and fall), the meter will “spin backwards” and we’ll be credited for the power. If the house needs more power than the solar system is providing (or at night), the meter spins forward and we’re buying power from the utility.

There are security features that cause the inverters to shut down if utility power is lost. That would case a condition knows as “islanding”, where our power would be energizing power lines that the utility assumes are dead (not to mention trying to power our whole neighborhood!). There are also features in the inverters that shut down or reduce output for high temperatures (not likely – they’re going to be mounted in the basement and they handle up to 105F) or for other failures.

All of the electrical equipment is going to be mounted in the basement near the existing electrical panel, except for the panels, conduit, and external disconnect switch. The whole system is grounded through the electrical connections all the way from our existing main panel to the inverters.

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