By Rich Varlinsky
If your energy audit is complete and improvements have been made, we can proceed to the next step of the “going solar” process. Read Part 1 in the Jan/Feb/Mar 2016 issue of Lotus Guide or online at www.LotusGuide.com.
Solar PV System Types
Solar PV (photovoltaic) systems are divided into two types, grid tied and off grid. We will be exploring grid-tied systems, as off-grid systems rely on battery backup and are considerably more expensive, requiring specialized components and considerations.
Solar can be roof or ground mounted. Our focus is on roof-mounted systems. A ground-mount system uses the same panels and inverters but costs about 30-50 percent more because of the engineering, pipe, concrete, and labor involved. The permitting process for ground mounts is generally longer.
The first step in the system-sizing process is to collect your electric usage for the past year. This information will be critical for sizing your solar system and almost all solar contractors will ask for this data. You can obtain your usage information directly from your utility company, via setting up your online account with it. You can usually download your usage history in a file format that can be opened in Excel. Be sure to export only the bill totals. You want a full year of electric usage, including kilowatts used and dollars spent.
Set up a tracking sheet, listing all 12 months, kilowatts used per month, and dollars spent per month, rounding the kilowatts and dollars up to whole numbers. Also, write down your billing rate schedule. You can call your utility company to obtain rate-schedule information if you cannot find it on your bill.
Total the kilowatts and dollars spent. This is your yearly usage and will be the basis for sizing your solar system. If possible, look at several years of your power usage.
Has your use increased or decreased? Which months have the highest consumption? Have you recently replaced anything such as heating and air-conditioning systems, which will alter use? If you have upgraded insulation, heating and air conditioning, windows, or major appliances, you need to consider these changes when sizing your solar system. For example, if you replaced your AC system, and your old unit was a 10 SEER (seasonal energy efficiency ratio) and your new one a 16 SEER system, you will save approximately 38 percent in electricity use. This means your solar system size could be reduced by 10-20 percent. If you expect a reduction or increase in household size, this needs to be factored into system size.
Calculating System Size
The reason for doing your own calculations is to have an idea of what system size you will need. Solar contractors can do these calculations as a part of the proposal process, but it is helpful to have an understanding of how solar size is determined and of your particular needs.
California restricts the amount of sunlight used by solar contractors in solar calculations to 5.5 hours per day, even though we receive considerably more. Because of this restriction, when sizing your system, size it to approximately 80 percent of yearly usage. The system will usually produce 15-20 percent more energy because of the extra sunlight we receive. Sizing the system to 100 percent is excessive and not cost effective, as most utilities will pay you only wholesale for each kilowatt produced. Following is a simple formula to calculate approximate system size.
Solar array size (kilowatts) = (annual kilowatt usage) / (365 days/year) / (solar hours/day) / (0.86 derate factor)
Example: If you use 900 kilowatts/month (900 x 12 = 10,800 kilowatts/year)
(10,800 kilowatts/year) / (365 days/year) / (5.5 sunlight/day) / (0.86 degradation*) = 6.25 kilowatt array (10,800 / 365 / 5.5 / 0.86= 6.2556)
*Because of real-world efficiency losses (irradiance, dust, temperature, inverter, panel degradation, and wiring), you should expect your system power output (AC power) to be about 86 percent of the system (DC power) size. This is the 0.86 efficiency constant included.
If you will be buying 315-watt solar panels (current technology), you will need 20 panels to produce about 80 percent of the power you consume. This assumes you do not have any shading problems. If you have significant shade, you will need to account for this loss of sunlight. If you suspect shading issues, look at vegetation removal or trimming, if possible. When you begin to receive solar quotes, ask the contractor to conduct a shading analysis. This shows any shading issues and gives a year-round picture of the effect shading will have on your system.
Solar Panel Size Requirement, Direction, and Tilt
Examine where you intend to install the solar panels. If you will be installing the 315-watt panels from our example, consider that each panel will take up about 21 square feet (77” x 39”). The 20 panels from our example above will require 420 square feet, not including required setbacks from the city or county. Most solar panels range in size from 18 to 22 square feet per panel. You can go to the manufacturer’s site and obtain size and other data about panels you are interested in using.
Many people are concerned about the weight of a solar system on their roof. The average weight of a solar system, including panels, wire, and racking is approximately 2.38 pounds per square foot. Most residential roofs can support a solar system without problems.
It is prudent to have a roof inspection, correcting any problems before installing your solar system. If your roof is showing wear and will require replacing in a few years, it is best to replace it before installing a solar system. When having your roof inspected, ask the inspector to tell you the size of the rafters and their spacing. This will be useful information when shopping for a system.
The placement direction for solar panels is critical to achieve maximum production. In California, the most popular direction to face the panels is south, west is next best, and then east. The system production will vary based upon direction. If we assume our calculation example is in Chico, following are the production differences:
West: 10-12 percent less overall production, but better in summer because of late-afternoon sun. A study by Pecan Street Research Institute in 2014 demonstrated that solar systems produce 54 percent more power during peak demand periods (3pm to 7pm) than south-facing panels.
East: 13-15 percent less overall
All solar-production numbers are estimates and can vary 5-20 percent depending upon weather, dust, and equipment performance. Solar systems will also lose production power as they age, about 0.7 percent per year. This is why it is important to constantly practice energy-conservation techniques. Solar and conservation must work hand in hand to save money and energy.
The tilt and air gap for the panels is also important. In the Sacramento Valley a tilt of approximately 18 degrees will provide balanced solar production. The air gap between the roof and the solar panels is critical for a number of reasons. If the panels are too close to the roof surface, less than a 3” air gap, a number of problems can occur.
Insufficient air gap will shorten the life of the roof through excess heat buildup. This can also drop the production of the solar panels by up to 15 percent. The generally recommended air gap between panels and roof surface is 6”. This allows for good air circulation and prevents heat buildup under the panels.
In the next segment we will look at the parts of the solar system, panels, inverters, racking, and monitoring.
Important Update on Net Metering Agreement
The current Net Metering agreement for solar customers with PG&E is scheduled to reach its caps in August 2016. After twenty-two months of work, the CPUC (California Public Utilities Commission) approved a new Net Metering Agreement on January 28, 2016
Under this new agreement existing solar customers will be “grandfathered” in for a period of 20 years. The agreement will keep solar as a viable option for homeowners and allow the spread of residential solar.
However, the utility companies are not pleased with the new plan and the three biggest in the state have issued a legal challenge to the new net metering agreement. To quote a solar industry insider “this would effectively prevent customers from installing solar after the current agreement expires.”
If the Utility companies are successful in having their version of Net metering passed, the changes would make solar inaccessible to a majority of customers. This would be a severe blow to clean energy and devastate the solar industry in California.
For further information, www.calseia.org/news/