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| Fig. 1: Globally weighted average total installed costs, capacity factors, and LCOE (2010-2021). [1] (Source: M. Irgau) |
By the end of 2021, 843 GW of solar photovoltaic (pv) systems had been installed across the world. Relative to 2010, this quantity is a jump by a factor of 21. [1] Some related trends like drop in cost can be seen in Fig. 1. As photovoltaic cells with greater efficiencies are developed for less of a cost, we are seeing this trend continue. Homeowners account for a proportion of these installations, seeking to reduce dependence on externally generated energy and potentially save on electric bills. In this report, we will investigate residential solar generated energy to get a sense of savings homeowners in California can expect.
We will consider solar panels manufactured by SunPower, a solar energy company based in San Jose. They boast one of the most efficient modules, in terms of its cells. Specifically, the National Renewable Energy Laboratory measured it at 22.08%. [2]
On average, the solar radiance level in Riverside, California, is 5 kilowatt hours per square meter per day, and we'll assume a roof size of 200 m2. [3] We will also assume a performance ratio loss of 75%; this is due to shadings as well as inverter, temperature, dc cable, and ac cable losses etc. We can calculate the electrical energy successfully converted from the solar panels given the total solar power area A, the solar power efficiency η, the average solar irradiance Φ, and the performance ratio (PR):
| E | = | η A Φ × PR | = | 0.2208 × 200 m2 × 5 kWh m-2 d-1 × 0.75 |
| = | 165.6 kWh d-1 | |||
As seen above, our conditions generate 165 kilowatt hours a day. Let's see how much it would cost to draw this much power externally from the grid through a utility. Let's take one of the more notable utility companies: Pacific Gas and Electric Company (PG&E). We can approximate the price of energy $0.23 per kilowatt hour; this is the rate PG&E charged in Northern California in 2018. [4] We can calculate the associated cost below:
| Cost | = | 165.6 kWh d-1× $0.23 kWh-1 | = | $38.09 d-1 |
In other words, the value of energy generated per day is $38.09. However, this does not mean homeowners can expect this exact figure in savings.
It's important to note a couple of things. Initially, homeowners will have to pay off the initial upfront costs of their solar system, which can jump to the tens of thousands of dollars; for this reason, most gain will be realized in the long term. Also, 165 kWh is higher than the average household consumes per day. Without ways to store or resell the energy, this energy has no value and the system is thus definitely not worth it for homeowners to undertake. For this reason, it's common for homeowners to install battery packs tied to the grid. Furthermore, the batteries may store the excess energy and during. In addition, in California, depending on one's utility, it is possible to receive credits on electricity bills as compensation for providing the grid with more energy.
© Medhanie Irgau. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Renewable Power Generation Costs in 2021,", International Renewable Energy Agency, 2022.
[2] M. A. Green et al., "Solar Cell Efficiency Tables (version 47)," Prog. Photovolt: Res. Appl. 24, 3 (2016).
[3] O. VanGeet and G. Mosey, "Feasibility Study of Economics and Performance of Solar Photovoltaics at the Stringfellow Superfund Site in Riverside, California," U.S. National Renewable Energy Laboratory, NREL/TP-6A20-48770, December 2010.
[4] C. Hay and M. Chhabra, "The Impact of Wildfires and Beneficial Electrification on Electricity Rates in PG&E's Service Territory," Electr. J. 33, 106710 (2020).