Case Studies

9.15kW Ground Mounted, Grid-Tied Solar Power System

Job Report- N. Riggs, D.Tolson

Residents of Western Boyle County, Kentucky 

• Project Description: Wilderness Trace Solar installed a 9.15kW solar power system at a residence. The system consists of thirty 305 Watt Neo Solar Power polycrystalline panels mounted in landscape mode using Power Solar Solar Frames ballast mount kits. The output of the panels is inverted and optimized using the high efficiency SolarEdge inverter system. SolarEdge also offers online panel level monitoring for the system. In the 12-month period from May 1, 2017 to April 30, 2018, this system produced 13,937kWh. When quoted, a pvWatts production analysis estimated year 1 production would be 12,213kWh. Simulating this design in our new design tool, Aurora, gave an estimation of 13,250kWh in year one. Helioscope gave an expected production of 13,600kWh. These estimates lead to Helioscope having the lowest error margin for year one with -2.4%, Aurora had an error of -5.6% and pvWatts was at -14%. This comparison does not prove any one simulation software to be the best. This does suggest that, unless 2017-18 was an unusually productive year for solar, pvWatts is probably slightly conservative of a simulation. To properly assess the best simulation software, more years of data will need to be collected.

  •  First year economics – Installed May, 2017 – This system is already well on its way for paying for itself, both financially and environmentally. The initial cost of this system was $25,314. Assuming the owners of the system were able to fully utilize their 30% federal tax credit, they have saved $7,594.20 on their federal tax liability in 2017. As of October 25, 2018, this system has produced 22,230kWh, this has offset approximately $2,334.15 of the customer’s electric bill. To determine the true financial impact of the system, we would need to analyze each of the electric bills since this solar installation was commissioned. If the above assumption of $2,334.15 of electricity offset by solar production is true, the current effective system cost is $15,385.65, 60.8% of the initial cost. This means that on top of the 30% Federal tax credit, the system has produced 9.2% of its initial cost in the 18.5 months since commissioning the system. If production continues at the current rate, this system can be expected to pay for itself in January 2029, 11 years and 9 moths after installation.
  • Projected economics if same unit installed 2018 – 2019 – It is worth noting that since the installation of this system, component pricing has come down significantly. This system cost the consumer $2.76/Watt, a similar system installed today would more likely be in the range of $2.50-$2.60/Watt. Assuming a cost of $2.55/Watt, the system would have initially cost $23,332.50, the system cost would have been reduced to $16,332.75 by the 30% RETC (Renewable Energy Tax Credit). The savings realized by the production of solar would be just as much, if not greater due to utility rate escalation. The $2,334.15 in savings would make the effective cost of this system after 19 months be approximately $13,998.60. This would lead to a payback period of 10 years and 8 months after installation, which is 1 year and 1 month shorter than the payback period for the actual system.









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