Tuesday, July 28, 2009

Solar Power - Installation Day 2

More mounting hardware is now on the roof, the brackets that hold the panels to the rails are now in place. Since the PG&E meter isn't mounted on the house itself, they also had to dig a trench. Tomorrow a county planning dept. inspector will come by to make sure everything is to code.

I also got an email from PG&E today, informing me that they know I have solar being installed and will schedule an appointment to do their own verification and turn the system on, once the hardware is in place.

One subtlety, since I also want to connect a generator for when we get power cuts (I'm on top of the Santa Cruz mountains) we had to decide what order to wire everything together. The sequence is

PG&E service meter - Solar Power system - Generator - House breaker panel

The generator is only wired into a subset of the house circuits, so it is looped into the house breaker panel. When installed, it will have a sensor that can tell whether there is any incoming power from PG&E, and if not, it fires up after 20 seconds or so.

The solar power system detects the PG&E power, synchronizes to the 60Hz phase and adds its own power by providing a few volts more than PG&E, to get the power to flow into the system. If PG&E goes away, the solar system also shuts down, then the generator detects no power, isolates the downstream components and takes over powering the house.

At one point I looked at having batteries rather than a generator, but it is a lot less efficient, they don't last very long and they are far more expensive. A 6KW battery system with inverter was around $10K, while I can get a 8KW propane generator for $2K, and I'm thinking of putting in a 14KW Generac 5503 that is currently available from Amazon for $3.3K delivered, since that can supply a 40A 220V circuit for the heat pump. I'm likely to need it for up to a week each year, and that will be the only thing that uses propane at that point.


  1. It sounds like you've really done your homework! Have you calculated the payoff period? (that is, how long before your investment pays for itself in lower power bills)

  2. The payoff calculation is very complicated, since we changed from Propane to electric appliances amongst other things. I'm looking at it in terms of raising the overall value of the house, and putting my money where my mouth is on advocating for reduction in CO2. The solarcity rep did a calculation based on some generic assumptions about our electric bill, but I don't remember the figures.


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