In terms of cost and benefit, solar panels are among the best upgrades available for buildings. High-quality solar panels have a service life of over 25 years, and their payback period is typically below 5 years. If you use a loan to install a solar power system, the savings achieved can often pay off the loan by themselves.
Depending on where you live, solar panels may be eligible for financial incentives. As of 2020, there is a 26% federal tax credit for all solar power installations in the US, and some states also have local tax credits. You may also find cash rebates that are offered through government programs or local utility companies.
Does your new roof meet Local Law 97 of 2019?
One of the main design decisions when going solar is the power generation capacity. The capacity of a PV system is normally measured in kilowatts, but megawatts are used for large commercial and industrial projects, and also for utility-scale projects.
In the case of New York City, new roofs and existing roof extensions are legally required to have a sustainable roof system (Local Law 97 of 2019). Installing solar panels is one option, and the other alternative is a green roof.
Size Difference Between 60-Cell and 72-Cell Solar Panels
Solar panels are manufactured in various sizes, but most installations use either 60-cell or 72-cell modules. Typically, 60-cell panels are described as residential, while 72-cell panels are described as commercial. However, this does not limit their applications: 60-cell panels work normally in a commercial building, and 72-cell panels work in a home.
For a given system capacity, you will normally need less 72-cell panels because each of them has 12 additional solar cells. For the same reason, 72-cell panels typically have a higher wattage.
For example, if a 60-cell panel has a capacity of 300W, each cell produces 5W.
Using these same cells, a 72-panel would have a capacity of 360W (20% higher).
Assuming you need to reach 360 kW of capacity with these power ratings, the installation will use 1,200 of the 60-cell panels, but only 1,000 of the 72-cell panels.
The dimensions of solar panels vary slightly depending on the product model and manufacturer. However, 60-cell panels normally measure 65x39 in, while 72-cell panels normally measure 77x39 in. In other words, 72-cell panels are taller than 60-cell panels, but they have the same width. The following table summarizes the typical area covered by each type of solar panel, in square inches and square feet:
Solar Panel Type
65 in x 39 in (5.42 ft x 3.25 ft)
2,535 in2 (17.6 ft2)
77 in x 39 in (6.42 ft x 3.25 ft)
3,003 in2 (20.85 ft2)
The space required for a solar power system will depend on how many kilowatts you want to add, and also the technical specifications of the specific model of solar panel. Using the 360kW example again, the estimated area covered by solar panels would be the following:
21,215 sq.ft. with 60-cell modules (1,200).
20,854 sq.ft with 72-cell modules (1,000).
The larger 72-cell modules save on racking and installation costs, since the total number of panels is reduced by 200. The space usage of the larger modules is also slightly more efficient.
How Solar Panel Efficiency Affects Space Requirements
The space needed by a solar power system will also depend on the photovoltaic material used, which affects efficiency. Solar panels with a higher efficiency condense more watts of capacity per square foot, and this reduces the total area required. Monocrystalline silicon solar panels are the most efficient, followed by polycrystalline panels. Thin-film panels have the lowest efficiency, but they can be cost-effective when plenty of space is available.
If your goal is maximizing the electricity production with the available space, monocrystalline solar panels offer the highest output per square foot. When space is not a limitation, you can compare several solar panel options to determine which one offers the highest return on investment.
For roofs that are subject to LL97 in New York City, using monocrystalline solar panels is recommended. In most cases, the law requires at least 4 kW of capacity, or otherwise a green roof must be used. Solar panels are a better option from a financial standpoint, and high-efficiency monocrystalline panels can reach 4 kW more easily when space is limited.
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