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Steven Strong's
The Solar Electric House
Steven Strong's Classic Book: The Solar Electric House

A consumer's guide to solar home design, by the pre-eminent authority on renewable energy systems.


questions and answers about renewable energy


solar power basics   photovoltaics


solar power basics

What is solar energy?

How is PV solar energy generated?

How much electricity does a PV system produce?

What is a utility-interactive system?

How is a site selected for building a PV-powered home?

Does a PV system produce power only on sunny days?

Do PV systems work in cold weather?

What are the PV modules made of?

What is solar energy?

Solar energy is any direct radiant energy from the sun. It also includes indirect forms of energy such as wind, falling or flowing water (hydropower), ocean thermal gradients, and biomass, which are produced when direct solar energy interacts with the earth.

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How is PV solar energy generated?

Photovoltaic solar energy is produced when the sun's energy, in the form of photons, strikes a photovoltaic cell and is transmitted to electrons within the semiconductor material in the PV cell, causing an electrical current. Each photovoltaic cell generates only a small amount of energy, so many cells are grouped together into a module; then several modules are combined to form a solar panel. Many solar panels place together, for example on a roof, is called an array.

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How much electricity does a PV system produce?

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What is a utility-interactive system?

A utility-interactive, or grid-interactive PV system, is connected to a conventional power grid. When the PV system is produces excess electricity, it is uploaded to the grid for use by the utility. Then, when the PV system is producing less energy than needed, the system can draw from the utility grid.

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How is a site selected for building a PV-powered home?

Solar designers look for sites that provide an optimal southern orientation, good exposure to the sun, and an adequate amount of structural support and space for solar panels to be placed.

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Can PV systems produce power on cloudy days?

Yes. Although a PV system may receive 80-90% less sunlight on a cloudy day, the modules can still generate electricity.

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Do PV systems work well in the cold?

Yes. In fact, photovoltaic systems generate more power when the temperature is lower. Because of the shorter hours of daylight and lower angles of the sun in winter, PV modules do generate less energy than in summer.

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What are solar modules made of?

The photovoltaic modules are made from very strong glass, and are able to tolerate extreme heat and cold, high winds up to 125 miles per hour, as well as hail and ice. Occasionally, however, the glass may break if subjected to a very strong impact.

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photovoltaics

Does the world have enough materials for PV to help address climate change?

What is the energy payback for PV?

How much land will PV need to supply our electricity?

Will we have enough materials for energy-significant PV production?


Does the world have enough materials for PV to help address climate change?

In the ongoing discussion of what needs to be done to stabilize atmospheric CO2 by mid-century (Hoffert 1998), one possible option would be to add about 10-20 terawatts (trillion watts, or TW) of photovoltaics (PV) in place of conventional energy sources. PV would help because, unlike burning fossil fuels, it produces no CO2. However, 10-20 TW is an enormous amount of energy. In peak watts, as PV installations are generally rated, it is about 50-100 TWpeak (TWp) of PV. We base our calculations on 75 TWp of PV by 2065. Would our planet have enough materials for this much PV? Although 75 TWp is a vast amount of energy, we think we have enough feedstock materials for PV to meet the "TW challenge."

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What is the energy payback for PV?

It takes energy to save energy; the term "energy payback" captures this idea. How long does a PV system have to operate to recover the energy-and the associated generation of pollution and CO2 that went into making the system? Energy paybacks for rooftop systems range from 1 to 4 years, depending on the system. With assumed life expectancies of 30 years, and taking into account the fossil-fuel-based energy used in manufacture, 87% to 97% of the energy that PV systems generate won't be plagued by pollution, greenhouse gases, and depletion of resources. Based on models and real data, the idea that PV cannot pay back its energy investment is simply a myth.

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How much land will PV need to supply our electricity?

A world relying on PV would offer a landscape almost indistinguishable from the landscape we know today. The impact of PV on the landscape would be low for three reasons. First, PV systems have siting advantages over other technologies; for example, PV can be put on roofs. Second, even ground-mounted PV collectors are efficient from the perspective of land use. Third, adequate sunlight is ubiquitous and present in predictable amounts almost everywhere. As we move away from fossil-fuel energy, PV use will be crucial because of=. its land-use advantages.

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Will we have enough materials for energy-significant PV production?

Producing PV modules and systems requires materials, and some people have wondered if we have enough of what we'll need. Within certain parameters, issues surrounding the availability of PV materials simply do not exist. Only indium and tellurium remotely approach becoming bottlenecks at the annual production rate we discuss, and simple strategies exist that would solve these problems.

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