Dan & Liz's 2nd Solar Retrofit: this time it's about carbon

Last time we went solar the 8kW system cut our net electricity use by 80%-90%... but that house still uses about 800 therms of natural gas per year, generating about 4 tons of co2 ( about average for a duplex in Southern California).

Since then, we moved, and at the new house, we put in a 8.9kW system, big enough to handle future improvements like adding A/C or an electric car. Thanks to the march of technology, it's only 29 panels instead of 46 like last time.

As the new house is a city-designated historic landmark, the panels can't be visible from the street; all the panels have to go on the back of the house.

LA Solar Group had the best quote, and was willing to install two panels on the north side of the roof using reverse tilt mounts. Price was roughly $3.50 per kW before subsidies, $2.20 per kW after subsidies, and payback time should be about 7-8 years. We had to reroof first, which kills the payback time, but hey, it needed it anyway.

Because there are trees in the area, there is some shading morning and evening, so we use a SolarEdge inverter (which optimizes each panel individually) to reduce power lost from shading.

Reverse tilt bifacial panels
Flush-mounted bifacial panels
The system is 25 LG310 310 watt panels, plus, as an experiment, four bifacial GxB 300W 300 watt panels.

Two of the bifacial panels are mounted normally, and two are mounted on the north slope with a reverse tilt (to point at the sun, and to allow indirect light to hit the back surface). The GxB300 data sheet suggests there will be "up to" a 10% or 20% boost from the back side in this application, depending on albedo of the surrounding surfaces. (And their sales literature even claims a 20% boost in total daily energy.) It will be interesting to see how that pans out in practice.


Live total power output graphs are online at our pvoutput.org page.

The SolarEdge monitoring system provides a nice map showing output of each solar panel during the course of the day; see a screencast of a day's output from dawn to dusk. You can see that the west-facing panels light up later in the day; you can also see the morning and evening shading at work. (Panels 1.1.1, 1.1.2, 1.1.3, and 1.1.4 are the GxB300's.)

The system went online at the end of March, 2016. Here's our energy usage history from LADWP from three different eras:

Usage before
Usage as efficiency upgrade kicked in, but before solar
Usage after solar installed (ignore zeroes before April 2016)

And here's another view of net energy usage... once the solar was installed, the definition of the tiers change from low, medium, and high usage to night, morn/evening, and day, and tiers with net overproduction are shown as below zero. Bit confusing, what?

Bifacial Back Generation

See for a note on the performance of the bifacial panels.

Last Change 9 Sept 2017
(C) Dan Kegel

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