Solar Frequently Asked Questions

What is the energy payback for PV?
Producing electricity with photovoltaics (PV) emits no pollu¬tion, produces no greenhouse gases, and uses no finite fossil-fuel re s o u rces. The environmental benefits are great. But just as we say that it takes money to make money, it also takes e n e rgy to save energ y. The term “energy payback” captures this idea. How long does a PV system have to operate to recover the energy—and associated generation of pollution and CO2— that went into making the system, in the first place.

Energy payback estimates for rooftop PV systems are 4, 3, 2, and 1 years: 4 years for systems using current multi-cry s t a l l i n e ¬silicon PV modules, 3 years for current thin-film modules, 2 years for anticipated multicrystalline modules, and 1 year for anticipated thin-film modules (see Figure 1). With energ y paybacks of 1 to 4 years and assumed life expectancies of 30 years, 87% to 97% of the energy that PV systems gener¬ate 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. In fact, Swiss researchers Dones and Frischknecht found that the small greenhouse-gas emissions required to make PV systems are comparable to non-power-plant energy requirements for fossil-fuel electricity such as mining, transporting, and refining.

What is the Energy Payback for Crystalline-Silicon PV Systems?
Most solar cells and modules sold today are crystalline silicon. Both single-crystal and multicrystalline silicon use large wafers of purified silicon. Purifying and crystallizing the silicon are the most energy-intensive parts of the solar- c e l l manufacturing process. Other aspects of silicon-cell and module processing that add to the energy input include: cutting the silicon into wafers, processing the wafers into cells, assembling the cells into modules (including encap¬sulation), and overhead energy use for the manufacturing facilities.

Today’s PV industry generally recrystallizes any of several types of “off-grade” silicon from the micro e l e c t ronics indus¬t ry, and estimates for the energy used to purify and cry s t a l l i z e silicon vary widely. Because of these factors, energy payback calculations are not straightforward. Until the PV industry begins to make its own silicon, which it could do in the near future, calculating payback for crystalline PV requires that we make certain assumptions.

What is the Energy Payback for Thin-Film PV Systems?
Thin-film PV modules use very little semicon¬ductor material. The major energy costs for manufacturing are the substrate on which the thin films are deposited, the film-deposition process, and facility operation. Because PV technologies all have similar energy require¬ments, we’ll use amorphous silicon as our representative technology.

Alsema estimated that it takes 120 kWh/m2 to make near-future, frameless, amorphous-silicon PV modules. He added another 120 kWh/m2 for a frame and support structure for a roof¬top-mounted, grid-connected system. Assuming 6% conversion efficiency (standard conditions) and 1,700 kWh/m2 per year of available sun¬light energy, Alsema calculated a payback of about 3 years for current thin-film PV systems. Kato and Palz calculated shorter paybacks for amorphous silicon, each ranging from 1–2 years.

Deleting the frame, reducing use of aluminum in the support structure, assuming a conserva¬tive increase to 9% efficiency, and factoring in other improvements, Alsema projected the pay¬back for thin-film PV that by 2009, would dro p to just 1 year.

CuInSe2 and CdTe modules are already being sold in the 8%–11% efficiency range, so their energy payback may be less than a year, depending on design details such as frames and mounting.

For an investment of 1 to 4 years-worth of energy output, rooftop PV systems can pro¬vide 30 years or more of clean energy. How¬ever, support structures for ground-mounted systems, which might be more advantageous for utility generation, would add about another year to the payback period.

How Much CO2 and Pollution Does PV Avoid?
An average U.S. household uses 830 kWh of electricity per month. On average, producing 1,000 kWh of electricity with solar power reduces emissions by nearly 8 pounds of sulfur dioxide, 5 pounds of nitrogen oxides, and more than 1,400 pounds of carbon dioxide. During its projected 28 years of clean energy production, a rooftop system with 2-year energy payback and meeting half of a household’s electricity use would avoid conventional electrical-plant emissions of more than half a ton of sulfur dioxide, one-third a ton of nitrogen oxides, and 100 tons of carbon dioxide. PV is clearly a wise energy investment that affords impres¬sive environmental benefits.