Case Study - A Solar Jewel in Boulder

Read how design and energy-efficient features made an awkwardly-shaped building a net zero energy home.

Sizing Photovoltaics

Sizing a residential photovoltaic power system is not particularly complex. Sizing is based on the user’s projected needs (loads), goals and budget.

To optimize your system these areas need to be addresses: 
Siting The site should be clear of shade to increase the systems efficiency.
Orientation: The array orientation with respect to true south and proper inclination is important for maximizing annual photovoltaic output based on local climate conditions.
Mounting Options: The optimal mounting system can maximize gain either in summer, winter or both by the angle relative to horizontal.
Modules: PV modules should be selected according to the systems design parameters.
Wiring:System wiring should be designed to minimize voltage drop, meet safety codes, and provide protection from the environment.
Controllers:The controller must operate a system efficiently while meeting the needs of the user.
Battery Storage:The battery bank must be sized to the specific installation and time desired to be self-reliant.
Loads:The system loads determine the size of the system and should be minimized by intelligent planning.

Most photovoltaic dealers help designers size systems. Each module manufacturer has a design method based on their product specifications, ranging from simple analysis to full simulations. If needs are not critical, a more general sizing method may be adequate.

Calculating Solar Payback

Payback varies widely, but you can expect a simple payback of 4 to 8 years on a well-designed and properly installed solar water heater. (Simple payback is the length of time required to recover your investment through reduced or avoided energy costs). You can expect shorter paybacks in areas with higher energy costs. After the payback period, you accrue the savings over the life of the system, which ranges from 15 to 40 years, depending on the system and how well it is maintained.  You can use the Solar Water Heating Calculator from Texas State Energy Conservation office to explore the energy usage of your water heater, and to estimate whether a solar water heater could save you money.

Is a Solar Water Heater Practical for my Family?

As you might expect, solar water heating is generally sensible for families that use a lot of hot water. Most solar systems are designed to meet half to three quarters of a family's hot water needs, or all of their summer needs.  The remainder is supplied by a backup system. Your contractor will need to size the collectors, the storage tank, and the backup system. A rule of thumb is that an efficient collector in good weather will heat between one and two gallons per square foot of collector per day. So if your family uses 64 gallons of hot water per day, you would need between 16 and 32 square feet of collector to supply half of the household's needs.

Photovoltaic (PV) Energy

PV panels contain hundreds of small silicon cells that collect the sun's energy and change it into electricity that can be used in the home. PV cells are made mostly of silicon, an abundant semi-conductor material in the earth's crust that is also used in computer semi-conductors. one side of the material is electrically positive, the other negative. When light strikes the electrically positive side of the material, the negative electrons are activated in a way that produces an electric current - energy!

Groups of PV cells are electrically configured into modules and arrays, which can be used to charge batteries, operate motors, and to power any number of electrical loads. With the appropriate power conversion equipment (called an inverter), PV systems can produce alternating current (AC) compatible with any conventional appliances and operate in parallel with and interconnected to the utility grid.

There are different technologies available for making solar cells.

  • Silicon wafer-based solar (crystalline silicon) technology, which accounts for more than 86% of the solar cell commercial production today. 
  • Amorphous silicon, which is used for thin film technology. In a controlled environment, crystalline silicon is much more efficient than amorphous silicon, but if any part of a crystalline silicon panel is shaded, the whole system stops producing energy. This is not true for amorphous silicon. Amorphous silicon is cheaper and also more readily available.
  • Gallium is the rarest component of new photovoltaic compounds recently announced by South African researchers for use in solar panels as a more efficient alternative to crystalline silicon. 

How Efficient are Photovoltaic Panels?

A typical commercial solar cell has an efficiency of 15%—about one-seventh of the sunlight striking the cell generates electricity. The performance of a solar cell is measured in terms of its efficiency at turning sunlight into electricity. Only sunlight of certain wavelengths will work efficiently to create electricity, and much of it is reflected or absorbed by the material that make up the cell.  Low efficiencies mean that larger arrays are needed, and that means higher cost. Improving solar cell efficiencies while holding down the cost per cell is an important goal of the PV industry. Production of solar panels has risen sixfold since 2000 and is doubling every 2 years today.

Where should I put Photovoltaic Panels?

Determine which month has the least amount of sun on average. This is the month that you want to use if you are building a system that will be used year-round. (if you are only going to be using it for summer or winter, find month with least sun during months that you will use the system.)

Note: The MOST IMPORTANT thing to consider when choosing a location for your Array is shading obstacles. A small amount of shade covering the panel can reduce the panel performance by 80%.

Some other points to consider:

  • Sun/Clouds: It is important to estimate sun availability and cloud cover. Sometimes you can obtain this information on the web ( if it is a large enough town.
  • Shade: You want to choose a location that is on or near the place where you loads will be. The array should be free of shade (during each month in use) from 9am to 3pm. This is the optimum time-frame a panel has to receive light and is called the “Solar Window.

Mounting a PV System

PV panels are often mounted on the roof, and that can be problematic. The primary job of a roof is to keep water out of the house and any time that barrier is penetrated the risk of a leak is increased.

Here are some options:

Standoffs - attached directly to rafter or roof trusses and then flashed with conventional roof jacks make more secure and weatherproof connections for photovoltaic panels than "L" brackets attached only to roof sheathing.

Integrating panels into a shade structure, such as a trellis, accomplishes two goals -  you'll get active solar electricity right along with passive solar shading.

Building-integrated PV array, which consists of a PV array that is incorporated into the roofing material itself. There are variations that look like standing-seam metal roofing, slate-like tiles, and three-tab shingle strips.

Ground-mounted photovoltaic arrays can be located in an area with good solar exposure and offer the option of adding a tracking mechanism for improved performance.

Solar Roof Shingles - PV Shingles are made with mono or polycrystalline solar cells directly integrated with regular asphalt shingles. They have a deep, dark, purplish-blue color, and look similar to other shingles. They are usually more expensive to install than typical PV panels, but the added value is in the aesthetics as they are not obviously solar collectors. Also, the solar shingles offset the cost of traditional shingles for the roof and are just as reliable. Some can be applied directly  intermixed with regular asphalt shingles while others may need special installation. The typical power output spans several watts (roof shingles) to about 50 watts (roof tiles with crystalline solar cells). They are manufactured by only a few companies worldwide including SunPower Corporation, Solar Components Corporation, and Atlantis Energy Systems.


PV Advantages

Independence:Many homeowners feel that energy independence from utilities is their primary motivation for adopting PV.
Reliability:Even in harsh conditions PV systems are reliable and sturdy. They also prevent costly power failures when constant power is essential. They are durable and work for at least 20 years.
Low Maintenance Cost:Systems don’t need much attention once installed since there are no moving parts. They usually run without needed tune-ups like other energy systems.
Safety:PV systems do not require the use of combustible fuels and are very safe when properly designed and installed 
No Fuel Cost:There is no costs associated with storing, purchasing or transporting fuel, because no source is required.
Reduced Sound Pollution: This system operates silently and with minimal movement. 


PV Disadvantages 

Initial Cost:An array can be a sizable investment, but the ROI (return on investment) is getting shorter and the cost of energy from the grid continues to rise and incentives are provided.
Solar Orientation:In order to function as their best, panels need direct sunlight. Shadows from trees or other objects can greatly reduce the systems efficiency. Unlike solar hot water, PV panels can face other directons—East or West—and still proved over 80% of the performance of south-facing collectors.
Energy Storage:Some PV systems need batteries if they are to provide power when the utility grid is down, to store power generated during the day. This can increase the size, cost and complexity of a system.
Efficiency Improvements:In order to use the new power generated as efficiently as possible, electrical loads need to be reduced through efficiency and old outdated appliances need to be replaced.
Manufacturing: Panels have a high embodied energy and can made from some toxic materials. There are health and safety issues only involving in the manufacturing process. Typically PV panels will produce enough energy to offset their embodied energy in 7-10 years. 


Residential Renewable Energy Tax Credits

Consumers who install solar electric systems can receive a 30% tax credit for systems placed in service from January 1, 2006 through December 31, 2016; the previous tax credit cap of $2,000 no longer applies.

For more information, go to:

There are additional state and local rebates available for photovoltaic installation. Contact you local energy provider to see what is available in your area.

Power Storage

  • You may want a way to store your power:
  • Batteries allow you to store energy directly from the energy generated by the PV Array.
  • Batteries store DC energy and allow you to utilize the energy during the night, or when there is a blackout (if you are connected to the grid).
  • Batteries are an extremely important power supply for critical electrical loads that consistently require usage. If you are wishing to power a load only during the day, a battery may not be required, i.e. to power a fan on sunny days inside of a greenhouse. Utility grid-connected PV systems do not require the use of batteries, though they can be used as an emergency backup power supply.