Producing electricity with photovoltaics (PV) emits no pollution, produces no greenhouse gases, and uses no finite fossilfuel resources. The environmental benefits of PV are great.

But just as we say that it takes money to make money, it

also 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 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 multicrystalline-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 energy paybacks of 1 to 4 years and assumed life

expectancies of 30 years, 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. Indeed, researchers Dones

and Frischknecht found that PV-systems fabrication and fossilfuel energy production have similar energy payback periods

(including costs for mining, transportation, refining, and 

construction).

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-cell

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 encapsulation), and overhead energy use for the manufacturing

facilities.

Today’s PV industry generally recrystallizes any of several

types of “off-grade” silicon from the microelectronics 

industry, and estimates for the energy used to purify and

crystallize 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.

To calculate payback, Dutch researcher Alsema reviewed 

previous energy analyses and did not include the energy

that originally went into crystallizing microelectronics

scrap. His best estimates of electricity used to make nearfuture, frameless PV were 600 kWh/m2 for single-crystalsilicon modules and 420 kWh/m2 for multicrystalline 

silicon. Assuming 12% conversion efficiency (standard 

conditions) and 1,700 kWh/m2 per year of available sunlight energy (the U.S. average is 1,800), Alsema calculated 

a payback of about 4 years for current multicrystallinesilicon PV systems. Projecting 10 years into the future, he

assumes a solar-grade silicon feedstock and 14% efficiency,

dropping energy payback to about 2 years.

Other recent calculations support Alsema’s figures. Based 

on a solar-grade feedstock, Japanese researchers Kato et al. 

calculated a multicrystalline payback of about 2 years

(adjusted for the U.S. solar resource). Palz and Zibetta also

calculated an energy payback of about 2 years for current

multicrystalline-silicon PV. For single-crystal silicon, which

Alsema did not calculate, Kato calculated a payback of 

3 years when he did not charge for off-grade feedstock.

Knapp and Jester studied an actual manufacturing facility

and found that, for single-crystal-silicon modules, the actual

energy payback time is 3.3 years. This includes the energy 

to make the aluminum frame and the energy to purify and

crystallize the silicon.

What is the energy payback for PV?

U.S. Department of Energy

Energy Efficiency and Renewable Energy

Bringing you a prosperous future where energy is clean, abundant, reliable, and affordable

Reaping the environmental benefits of solar energy requires spending energy

to make the PV system. But as this graphic shows, the investment is small.

Assuming 30-year system life, PV systems will provide a net gain of 26 to 

29 years of pollution-free and greenhouse-gas-free electrical generation.

Figure 1. Energy Payback for Rooftop PV Systems

Years

03548901

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Multicrystalline, current

Thin-film, current

Multicrystalline, anticipated

Thin-film,

anticipated

Technology (current and anticipated)

System Components

 Balance of system

 Frame

 Module
What is the Energy Payback for Thin-Film 

PV Systems?

Thin-film PV modules use very little semiconductor 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 requirements, 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 rooftopmounted, grid-connected system. Assuming 6%

conversion efficiency (standard conditions) and

1,700 kWh/m2 per year of available sunlight

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 to 2 years.

Deleting the frame, reducing use of aluminum

in the support structure, assuming a conservative increase to 9% efficiency, and factoring in

other improvements, Alsema projected the payback for thin-film PV that would drop to just 

1 year by 2009.

CuInSe2 and CdTe modules are already being

sold in the 9%–12% 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 provide 30 years or more of clean energy. However, 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 a 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 (see Figure 2).

PV is clearly a wise energy investment that

affords impressive environmental benefits.

References

E. Alsema, “Energy Requirements and CO2

Mitigation Potential of PV Systems,” Photovoltaics and the Environment, Keystone, CO.

Workshop Proceedings, July 1998.

R. Dones; R. Frischknecht, “Life Cycle Assessment of Photovoltaic Systems: Results of Swiss

Studies on Energy Chains.” Appendix B-9.

Environmental Aspects of PV Power Systems.

Utrecht, The Netherlands: Utrecht University,

Report Number 97072, 1997.

K. Kato; A. Murata; K. Sakuta, “Energy Payback

Time and Life-Cycle CO2 Emission of Residential PV Power System with Silicon PV Module.”

Appendix B-8. Environmental Aspects of PV Power

Systems. Utrecht, The Netherlands: Utrecht

University, Report Number 97072, 1997.

K. Knapp; T.L. Jester, “An Empirical Perspective

on the Energy Payback Time for PV Modules.”

Solar 2000 Conference, Madison, WI, June

16–21, 2000.

W. Palz.; H. Zibetta, “Energy Payback Time of

Photovoltaic Modules.” International Journal 

of Solar Energy. Volume 10, Number 3-4, 

pp. 211–216, 1991.

S O L A R E N E R G Y T E C H N O L O G I E S P R O G R A M

For more information

on PV, please read the

other PV FAQs in this

series. You can order

hard copies of the

FAQs from the

National Center for

Photovoltaics, or 

visit our Web site at

www.nrel.gov/ncpv.

A Strong Energy Portfolio

for a Strong America 

Energy efficiency and

clean, renewable energy

will mean a stronger

economy, cleaner 

environment, and greater 

energy independence for 

America. Working with 

a wide array of state, 

community, industry, 

and university partners,

the U.S. Department of

Energy’s Office of Energy

Efficiency and Renewable

Energy invests in a

diverse portfolio of 

energy technologies.

The National Renewable

Energy Laboratory, a DOE

national laboratory,

produced PV FAQs for:

U.S. Department of Energy

Office of Energy Efficiency

and Renewable Energy

1000 Independence Ave., S.W.

Washington, D.C. 20585

DOE/GO-102004-1847

January 2004

Printed with a renewable-source

ink on paper containing at least

50% wastepaper, including 10%

postconsumer waste

PV systems can repay their energy investment in about 

2 years. During its 28 remaining years of assumed operation, a PV system that meets half of an average household’s

electrical use would eliminate half a ton of sulfur dioxide

and one-third of a ton of nitrogen-oxides pollution. The 

carbon-dioxide emissions avoided would offset the operation of two cars for those 28 years.

Figure 2. Cumulative Net Clean Energy Payoff

140

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-20

20

60

100

0 2 5 10 15 20 25 30

Megawatt-hours clean energy

Investment Return

Cumulative PV energy production

Manufacture energy

Years