This is the main array of photovoltaic cells powering Heimhenge. Each of the 12 individual panels are rated at 75 watts peak power output. The entire array has a peak output of 1200 watts — enough power to run a window AC on free solar energy in real time. Cool, huh? The two smaller (20 watt) panels power attic ventilation fans that activate automatically at sunrise.

The solar panels, AstroPower AP-7105’s, are rated for continuous performance in hot climates. Like most brands, they come with a full 10 year warranty. Industry experts say solar panels should last 80 years or more. This estimate is based on the fact that the first panels ever produced are still working fine after 50 years of use. The technology simply hasn’t been around long enough to generate statistically accurate failure data. Our panels are now (as of 2015) pushing 12 years old, and at last measurement were still producing 95% of rated output.

The 75 watt panels cost about $350 each. Given the rate of increase in commercial electricity costs, we estimate payback time for the system to be about 8 years. Heimhenge is intertied with the APS grid using their “time of use” pricing plan. Electricity purchased during peak-use hours costs $0.135/kWh, while off-peak cost is $0.043/kWh. Since peak-use hours (9am to 9pm) are when the Sun shines, any electricity we produce offsets the higher priced juice. Weekends, when we typically use more energy, are also off-peak from 9 pm Friday through 9 am Monday.

Included in this cost analysis is the value of what I like to call “energy insurance.” Imagine being secure in the knowledge that your cooling, refrigerator, microwave, communication, and security systems would continue to function during grid power outages. If you could buy that kind of insurance, what would it be worth to you? $10/month? $20? $30? These are typical responses when the question is asked, yet most cost analyses of solar energy systems neglect to account for this factor. Solar energy is good for the environment and increases your sense of personal independence.



By 2020 our original panels were pushing 17 years old, and still putting out nearly 90% of their rated power. But the technology had improved remarkably over that time, both in terms of solar panel efficiency and power density. Further, the cost-per-watt had dropped from $5.00 to $2.80. And the federal tax credit was soon going to drop from 30% to 25%, so we decided it was time to upgrade. You can see we’ve also added two solar water heating panels.

Using essentially the same amount of roof space we were able to increase the output from the original 1.2 kW to 3.5 kW — nearly a threefold gain! Of course, this necessitated change to some other components of the system including the power panel and battery storage. You can read more about that in their respective sections.

If you’re wondering where the old equipment went, it was all properly recycled. The solar panels (and rack) were sold to a buyer in northern Arizona, the inverter went to a buyer in Colorado. All the removed copper wiring and batteries were taken to a local recycling facility.