Key Considerations for Installing Solar
Mark D’Antonio, ERS, for Zondits
Last month we embarked on a multi-part series about solar photovoltaics (PV) and started with a discussion of five residential systems installed by ERS engineers in their homes. Several different selections were made related to system specifics as well as with economic approaches. The systems presented demonstrated a cross-section and diversity of choices. In this installment, we’ll look at the key components of a PV system and the types of considerations a purchaser is faced with.
But before we get into the details, here are a few nuggets about the trends in the market place related to PV installations and their rapid rise.
- Solar energy expansion in America is brisk. A new system is activated every 1.6 minutes (source: Solar Power International [SPI]).
- There are over 1 million solar systems installed on residential rooftops across America (source: Greentech Media [GTM]).
- The cost to install solar has dropped by more than 70% over the last 10 years and 18% from Q2 2015 to Q2 2016 (source: Solar Energy Industries Association [SEIA]).
- Installations increased 43% in Q2 2016 compared with Q2 2015 (GTM).
- In the first half of 2016, solar accounted for 26% of all new electric generating capacity brought online in the U.S. (GTM).
From the information above we can see that installations are up and prices are down, so perhaps the time is right and you are considering installing solar. If so, what do you need to know about the basic components of these systems and what are the primary characteristics that you’ll need to consider?
Here are the basics:
Panels – This is the heart of the system, where the conversion of the sun’s energy into electricity occurs. Entire books could be written (and have been) about the physics and particulars of mono- or poly-crystalline panels, but an average consumer does not need to be concerned with these details. What you do need to be concerned with is the ratings and performance of the system you intend to purchase and ultimately how it relates to your budget. There are many specifications that define the nature of a panel’s performance; the following should be considered at a minimum:
- Power rating (and power tolerance) – The nameplate power rating of a panel represents the dc output in watts that will be generated under standard test conditions (STC). Note that these conditions are fleeting in practice, thus the panels will typically generate at a lower output than stated on the nameplate. The actual output depends on temperature conditions and incident sunlight at any given moment. Typical panel ratings are in the 200‒350 W range. Additionally, there is a power tolerance (+/-%) band that specifies how the output might differ from the nameplate rating due to manufacturing variations.
- Efficiency – The efficiency rating of a panel is a metric of how well it will convert the sun’s radiation into electrical energy. Efficiency is already baked into the power rating of the panel, but a higher efficiency will mean a greater output for a given panel area. This is important if you have limited area on which to install panels. High efficiency commercial panels are available with ratings just above 20%, and standard efficiencies hover around 15%. There is a price premium for higher efficiencies, and so budget considerations, along with power requirements, often influence selection.
- Temperature performance – Although this may be surprising to hear, high temperatures actually reduce the performance of the photovoltaic conversion process. Thus, panel performance degrades as collectors really heat up. The temperature coefficient is a measure of how the panel will perform as the temperature varies from the standard test condition of 77˚ Typical panels have a coefficient ranging from 0.44% to 0.50% per degree. This often balances out in four-season climates, but if you live in a predominately hot or predominately cold climate you’ll need to do the math to understand how a rated panel will perform in your application.
- Cost – The cost of the panels has a primary influence on the overall cost of the system. Dramatic reductions have been seen in the past few years as the number of installations have risen. Generally speaking, more efficient panels cost more, but they also produce more. In some cases the cost of higher efficiency panels might make it sensible to obtain a greater power density if you have limited area, but in other cases it does not pay back. Each purchaser will need to assess their particular requirements and budget to determine which specific panels will provide the greatest value for the installation. The plot below (sources: GTM and SEIA) presents the dynamic nature of the combined PV market (residential/commercial/utility) for entire installations.
Inverters – These components convert the DC power generated by the solar cells in your panels into ac power that can be used directly in your home or business, or fed back onto the grid.
- String (centralized inverter) – This type of system uses a single inverter to control and perform the inversion process for all of the panels in your system. It is a cost-effective approach and works well in situations where all of the panels receive the same amount of sunlight. In systems where shading occurs on a portion of the panels in the array, string inverters become less efficient as they cannot optimize each panel independently, only as a group. Similarly, if one panel fails, it can cause a performance issue for the entire array.
- Microinverters (inverter per panel) – With this type of system, there is an inverter installed at each panel (and inversion occurs at the panel) in order to maximize every panel’s production. This approach is beneficial when partial shading situations occur or if multiple panel orientations exist on different rooflines. Because of the greater number of inverters, this approach typically has a higher initial cost than a centralized string inverter system.
- Hybrid power optimizers – As the name suggests, these systems are a hybrid of string and microinverter systems. As with microinverters, power optimizers are installed at each panel, but instead of converting the dc electricity at the panels, the dc power is optimized before sending it to a central inverter. This facilitates individual panel optimization at a lower cost than microinverter systems, but a higher cost than string inverters.
Mounting systems – Regardless of what panels you select, they’ll need to be securely mounted in a suitable location. The mounting system will dictate whether your panels remain fixed in one position (most common) or are allowed to track the sun as it traverses the sky throughout the day (and even the season). Fixed-mount systems are the least expensive and are the standard for rooftop locations; however, they can also be used for ground installations. Tracking systems that follow the sun are typically ground mounted, are more costly, and require maintenance, but can increase output up to 30% or so and serve as an option to maximize production.
Key System Considerations:
- Size – A critical consideration a consumer faces is the size of the system they want to purchase. The initial response to this question is often that the consumer wants to generate all of the power that they currently use – and maybe even more! This may or may not be feasible, but you’ll need to first understand how much electricity in kilowatt hours (kWh) you do use by evaluating your electricity bills. Once you’ve established the amount of kWh you use, you can then determine (often with an installer’s help) what might be possible given your rooftop or yard area and the amount of sun exposure at your location. Most residential installations end up serving 75%–90% of the annual usage, but anything is possible. In cases where you have the ability to generate more than you use, a key consideration will be whether you can sell the excess generation back to the grid cost-effectively (we’ll discuss this later in the series).
- Orientation – One of the first considerations to address is whether your site has the proper orientation to effectively install a solar PV system. Solar panels are most effective when they face directly south, are free of shading throughout the day, and are at the correct pitch for your latitude (that is, approximately equal to your latitude). Many systems however are not installed with the perfect orientation, but produce plenty of power in a cost-effective manner. Some systems are even placed on west-facing rooftops to capture the afternoon sun. A good installer will be able to accurately predict the power generation for your installation by taking various sun and shading measurements and calculating the power output of the proposed system.
- Mounting location – An obvious decision point you’ll need to determine is where you will locate your system – on your rooftop or on the ground. The decision comes down to whether your roof is structurally suitable and if you have the space as well as a favorable enough orientation to the sun. If not, ground mounting may be an alternative and may be best for optimizing the exposure. The cost is comparable for rooftop or ground-mount fixed installations. As mentioned above, if you plan to use a system that tracks the sun, you’ll also likely look at a ground-mount option.
- Battery storage – There is considerable interest in battery storage these days for many applications in the energy sector, and solar is no exception. Battery technologies are evolving quickly and big-name players are entering into the market. That said, most systems that are grid tied do not need storage, as shortages or excesses in production can be augmented or absorbed by the grid, respectively. Although it is costly, adding battery storage to your system may make sense if you have frequent outages or are off-grid entirely and need nighttime ride-through capability.
- Installer selection – As with any construction project, selecting the right contractor is incredibly important to the success and outcome of the installation. There is a growing contingency of contractors out there and you’ll need to do your homework and due diligence to find the right one for you. A good contractor will be an invaluable resource in helping you define the right system to select for your site and clearly supply you with the details you’ll need to understand the cost and benefits of the proposed system.
As with all purchases, there are many elements to consider. We’ve touched a few of the fundamental system components and some key considerations of installing solar, but these need to be aligned with your budget and ultimately the overall cost of ownership. In our next installment we will bring forward the discussion about economic attributes of different ownership models, respective revenue streams, tax credits, and other details you’ll need to conduct a financial assessment of going solar.