If you are thinking of generating your

own electricity, you should consider a

photovoltaic (PV) system—a way to generate electricity by using energy from the

sun. These systems have several advantages: they are cost-effective alternatives in

areas where extending a utility power line

is very expensive; they have no moving

parts and require little maintenance; and

they produce electricity without polluting

the environment.

This publication will introduce you to the

basic design principles and components of

PV systems. It will also help you discuss

these systems knowledgeably with an

equipment supplier or system installer.

Because this publication is not intended to

cover everything about designing and

installing a PV system, a list of additional

PV resources is provided at the end. 

Introduction to PV Technology

Single PV cells (also known as “solar

cells”) are connected electrically to form

PV modules, which are the building blocks

of PV systems. The module is the smallest

PV unit that can be used to generate substantial amounts of PV power. Although

individual PV cells produce only small

amounts of electricity, PV modules are

manufactured with varying electrical outputs ranging from a few watts to more

than 100 watts of direct current (DC) electricity. The modules can be connected into

PV arrays for powering a wide variety of

electrical equipment.

Two primary types of PV technologies

available commercially are crystalline silicon and thin film. In crystalline-silicon

technologies, individual PV cells are cut

from large single crystals or from ingots of

crystalline silicon. In thinfilm PV technologies, the

PV material is deposited

on glass or thin metal

that mechanically supports the cell or module.

Thin-film-based modules

are produced in sheets

that are sized for specified electrical outputs.

In addition to PV modules, the components

needed to complete a PV

system may include a

battery charge controller,

batteries, an inverter or

power control unit (for

alternating-current

loads), safety disconnects

and fuses, a grounding

circuit, and wiring. (See

Balance-of-System Equipment section.)

Photovoltaics: Basic Design

C

Principles and Components

LEARI

NGHOUSE

ENERGY

EFFICIENCY AND

RENEWABLE

ENERGY

T OF ENERGY

DEPARTMEN

U

E

NITED STAT S OF A

ERICA

M

This stand-alone PV system consists of four modules, each with 

36 cells. It provides power for lights, radios, televisions, and other

loads at remote homes in New Mexico. 

This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national laboratory.

The document was produced by the Information Services Program, under the DOE Office of Energy Efficiency and Renewable Energy. The Energy Efficiency

and Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc., for NREL / DOE. The statements contained herein are based on

information known to EREC and NREL at the time of printing. No recommendation or endorsement of any product or service is implied if mentioned by EREC.

Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 20% postconsumer waste

DOE/GO-10097-377

FS 231

March 1997

Jim Yost / PIX01809
PV System Applications

Many people are familiar with PV-powered calculators and watches, the most

common small-scale applications of PV.

However, there are numerous large-scale,

cost-effective PV applications, including:

• Water pumping for small-scale remote

irrigation, stock watering, residential

uses, remote villages, and marine sump

pumps;

• Lighting for residential needs, billboards, security, highway signs, streets

and parking lots, pathways, recreational

vehicles, remote villages and schools,

and marine navigational buoys;

• Communications by remote relay stations, emergency radios, orbiting satellites, and cellular telephones;

• Refrigeration for medical and recreational uses;

• Corrosion protection for pipelines and

docks, petroleum and water wells, and

underground tanks;

• Utility grids that produce utility- or

commercial-scale electricity; and

• Household appliances such as ventilation fans, swamp coolers, televisions,

blenders, stereos, and other appliances.

The decreasing cost of PV systems and the

increasing number of manufacturers and

dealers for PV equipment have contributed to widespread use of the technology. In PV’s early days, do-it-yourselfers

had to search for companies that manufactured PV modules and often had to adapt

or reconfigure components from other

non-PV systems. Today, dealers offer

ready-to-use systems and state-of-the-art

equipment designed specifically for PV

systems. Many dealers have computer

software that helps to design systems and

specify appropriate components. As PV

markets expand, dealers are gaining

greater experience with PV applications,

making it cheaper and easier to purchase

PV systems.

How Do I Select a PV Dealer?

Choosing a PV professional will be one of

your most important decisions. If you

choose a competent dealer, you won’t

need to know all the details of designing,

purchasing, and installing your PV system. Instead, you can rely on the dealer’s

expertise to design and install a system

that meets your needs. However, just like

buying a car or a television, you must

have confidence in the dealer’s products

and services and be an informed consumer. With the growth of the PV industry, the number of regional dealers,

mail-order businesses, and local distributors has expanded rapidly. Many telephone directories contain listings for PV

dealers under the “Solar” heading.

Professional credentials are one indication

of a PV dealer’s knowledge and qualifications. Ask dealers what PV-related courses

they have taken, certifications they have

earned, and licenses they have received.

A second consideration is the dealer’s

experience in the field. How long has the

company been in business? The local Better Business Bureau can advise you

whether any customers have registered

complaints about the dealer. You should

also ask the dealer how many systems like

yours he or she has designed and

installed. Ask to see installations, and talk

with owners of systems similar to the one

you want to purchase. 

A third consideration in selecting a system

installer is the variety and quality of products offered for each component of the

system. Because PV systems are often

designed for a specific site, one company’s

products may not be appropriate for all

applications. Competent dealers will stock

components manufactured by several

companies. A variety of product options

will help ensure that the most appropriate

components are available for your system.

When a dealer recommends a product, ask

what the recommendation is based on,

whether there are consumer or independent testing facility reports you can read,

and whether the products are listed with

Underwriters Laboratories (UL).

Fourth, consider the service agreements

and performance guarantees the dealer

provides and the warranties given by the

product manufacturers. No system is

maintenance-free, nor will all components

function flawlessly forever. When problems emerge with your system, what services will the dealer provide? What

2

Hundreds of costeffective applications

for PV systems have

been developed.
warranties do the manufacturers provide?

What costs should you expect to pay, and

which costs will be assumed by the dealer

and/or the manufacturer?

Finally, you should compare prices from

different dealers. Because distribution

channels and dealer networks have

expanded dramatically, the opportunity to

“shop around” is much greater today. If

possible, approach more than one dealer

about a draft design and cost estimate for

your system.

When Are PV Systems 

Appropriate?

People select PV systems for a variety of

reasons. Some common reasons for selecting a PV system include:

• Cost—When the cost is high for extending the utility power line or using

another electricity-generating system in a

remote location, a PV system is often the

most cost-effective source of electricity.

• Reliability—PV modules have no moving parts and require little maintenance 

compared to other electricity-generating

systems.

• Modularity—PV systems can be

expanded to meet increased power

requirements by adding more modules

to an existing system. 

• Environment—PV systems generate

electricity without polluting the environment and without creating noise.

• Ability to combine systems—PV systems can be combined with other types

of electric generators (wind, hydro, and

diesel, for example) to charge batteries

and provide power on demand.

PV systems are not cost-effective for all

applications. The following discussion

gives some general guidelines to consider

when deciding whether a PV system is

appropriate for your situation.

First, if your site is already connected to a

utility grid, or within one-quarter mile of

the grid, a PV system will probably not be

cost-effective. Each utility company

spreads the cost of its power plants and

fuel costs among all its customers. Most

utilities can provide electricity to consumers for about 6 cents to 14 cents per

kilowatt-hour. When you install a PV system, you are essentially installing your own

mini-utility system. You pay all the costs of

generating the electricity you consume.

Although the sun’s energy is free, the PV

equipment is not free. The electricity generated by PV systems at current module and

balance-of-system prices can cost 20 cents

to 40 cents per kilowatt-hour, depending

on the installation cost and intensity and

duration of the sunlight at the site. 

Second, small PV systems are not practical

for powering space-heating systems,

water heaters, air conditioners, electric

stoves, or electric clothes dryers. These

loads require a large amount of energy to

operate, which will increase the size and

cost of your PV system. Therefore, select

the most energy-efficient loads available.

For example, if your PV system will

power lights, look for the most energyefficient light bulbs. If your system will

pump water for toilets and showers, look

for the most water-conserving fixtures.

3

In the United States,

PV systems must

have unobstructed

southern exposure.

Sun’s Path in the Sky

June 21

December 21

West

South East

North

PV modules

The sun’s noontime height above the horizon changes seasonally. This is 

important to consider when siting and positioning a PV array.
Is My Site Adequate for PV?

A PV system designer can conduct a

detailed site assessment for you. To save

the dealer time (and possibly save yourself

some money), you can conduct a preliminary assessment to determine whether your

site has potential for a PV system. Contact

the Energy Efficiency and Renewable

Energy Clearinghouse (EREC—see Source

List) for more information on conducting a

detailed site feasibility assessment.

There are three factors to consider when

determining whether your site is appropriate. 

First, systems installed in the United States

must have a southern exposure. For maximum daily power output, PV modules

should be exposed to the sun for as much

of the day as possible, especially during

the peak sun hours of 10 a.m. to 3 p.m.

Second, the southern exposure must be

free of obstructions such as trees, mountains, and buildings that might shade the

modules. Consider both summer and 

winter paths of the sun, as well as the

growth of trees and future construction

that may cause shading problems.

Finally, the unobstructed southern exposure must also have appropriate terrain

and sufficient space to install the PV system. A flat, grassy site is appropriate terrain, whereas a steep, rocky hillside is not.

How Does Weather Affect PV

Module Output?

Unlike utility power plants, which produce electricity constantly despite the time

of day and year or the weather, the output

of PV modules is directly related to these

two factors.

Where you live will affect the number of

PV modules you will need for power,

because different geographic regions experience different weather patterns. Seasonal

variations affect the amount of sunlight

available to power a PV system. The above

map shows annual average “peak sun

hours” for regions in the United States. 

4

Where you live will

determine the number

of PV modules your

system will need.

Make sure you have

selected the most

energy-efficient loads

possible.

Annual Average Daily Peak Sun Hours

4

4

5

6

5

5

3

4

4

4.7

4.7

4.5

4.5

This diagram illustrates the annual average daily peak sun hours for the United States. 
Module temperature also affects output.

The conversion efficiency of crystallinesilicon modules falls significantly at 

elevated module temperatures. 

When designing a PV system, be sure your

PV installer obtains data specific to your

area, rather than relying on general data.

The National Oceanic and Atmospheric

Administration began collecting solar data

nearly 20 years ago. The National Renewable Energy Laboratory’s Renewable

Resource Data Center (see Source List) can

provide solar radiation information, as can

EREC. Some state energy offices also have

solar data-collection programs to assist

solar designers. Finally, books are available that contain solar data on most major

cities in the United States, and a few of

these are listed in the Reading List.

Sizing the System to Meet 

Your Needs

After you have assessed the appropriateness of your site, you need to determine

how much electricity your PV system

must generate. This depends on how

much electricity your loads require. Again,

your dealer can help you with sizing a system that will meet your needs. The sidebar

illustrates the steps involved in sizing a

PV system. You can also contact EREC for

more specific information on sizing.

Balance-of-System Equipment

In addition to the PV modules, you must

purchase balance-of-system (BOS) equipment. This includes battery charge controllers, batteries, inverters (for loads

requiring alternating current), wires, conduit, a grounding circuit, fuses, safety disconnects, outlets, metal structures for

supporting the modules, and any additional components that are part of the PV

system. Below, we’ll discuss PV and BOS

configurations first for loads requiring

direct current, then for loads needing

alternating current. 

Note that, in many systems, the cost of

BOS equipment can equal or exceed the

cost of the PV modules. When examining

the costs of PV modules, remember that

these costs do not include the cost of BOS

equipment. Ask your PV dealer about the

BOS equipment required by your system.

5

How to Size Your PV System

To size your PV system, you must first know your energy needs, which you

figure by listing all your daily loads. A load includes anything that uses electricity from your power source, such as lights, televisions, radios, or batteries.

Some loads need electricity all the time, such as refrigerators, whereas others

use electricity less often, such as power saws. To determine your total energy

consumption, multiply the wattage of the appliance by the number of hours it

is used in a day. Some appliances do not give the wattage, so you may have to

calculate the wattage by multiplying the amperes times the volts. After adding

the totals for each appliance, you can decide what power output you need for

your PV system.

Example

Daily Use Total Energy

Load (hrs) Wattage Consumption (watt-hrs)

Radio 2 x 25 = 50

Lamps 3 x 27 = 81

(fluorescent)

VCR 0.5 x 30 = 15

Television 6 x 60 = 360

Total Daily Energy Consumption 506 watt-hrs

For the items listed above, you would need a system that produces an average

daily energy output of 506 watt-hours. Obviously, different parts of the country receive varying amounts of sunlight. Because sunlight is the source of

power for PV, you must determine the daily amount of sunlight in your

region. Remember that PV systems are rated by peak watt, which is the

amount of power produced when the module receives 1,000 watts per square

meter of exposure to the sun (insolation). 

Let’s examine two locations: Albuquerque, New Mexico, and Pittsburgh, Pennsylvania. Albuquerque is a fairly sunny area. In Albuquerque, for each peak

watt that a PV module is rated, it will produce a yearly average of 6.2 watthrs* of electricity daily. In Pittsburgh, a cloudier area, the same module will

produce an average of 2.4 watt-hrs* of electricity daily. 

If you wanted to use a PV system in Albuquerque for the appliances listed in

the table, you would divide 506 watt-hrs by 6.2, divide that by 0.8 to account

for inefficiency of the batteries and, finally, multiply by 1.2 to cover anything

that may have been overlooked. You find that you would need a PV system

rated at 124 peak watts. If you were buying 50-watt modules, you would need

three modules, because you round up to the next highest number.

506 ÷ 6.2 = 82

82 ÷ 0.8 = 103

103 x 1.2 = 124

124 ÷ 50 = 3 modules

For Pittsburgh, you would divide 506 watt-hrs by 2.4, divide by 0.8, and multiply by 1.2, which yields 317 peak watts, or seven modules at 50 watts each.

506 ÷ 2.4 = 211

211 ÷ 0.8 = 264

264 x 1.2 = 317

317 ÷ 50 = 7 modules

Determining your daily energy consumption can be done through simple calculations like the example above or with the aid of sophisticated computer

programs. If you are seriously considering purchasing a PV system, there are

also other factors to consider. You may want to refer to other sources (see

Source List) for more precise ways to make your calculations.

*This is based on the winter average. For more precise calculations, consult month-bymonth averages and use the lowest monthly average.
Direct-Current System Equipment

Charge Controller. The charge controller

regulates the flow of electricity from the

PV modules to the battery and the load.

The controller keeps the battery fully

charged without overcharging it. When

the load is drawing power, the controller

allows charge to flow from the modules

into the battery, the load, or both. When

the controller senses that the battery is

fully charged, it stops the flow of charge

from the modules. Many controllers will

also sense when loads have taken too

much electricity from batteries and will

stop the flow until sufficient charge is

restored to the batteries. This last feature

can greatly extend the battery’s lifetime.

Controllers generally cost between $20

and $400, depending on the ampere capacity at which your PV system will operate

and the monitoring features you want.

When selecting a controller, make sure it

has the features you need; cost should be a

secondary consideration.

Battery. The battery stores electricity for

use at night or for meeting loads during

the day when the modules are not generating sufficient power to meet load

requirements. To provide electricity over

long periods, PV systems require deepcycle batteries. These batteries, usually 

lead-acid, are designed to gradually discharge and recharge 80% of their capacity

hundreds of times. Automotive batteries

are shallow-cycle batteries and should not

be used in PV systems because they are

designed to discharge only about 20% of

their capacity. If drawn much below 20%

capacity more than a few dozen times, the

battery will be damaged and will no

longer be able to take a charge.

Deep-cycle batteries cost from about $65

up to $3,000. The cost depends on the

type, capacity (ampere-hours), the climatic

conditions in which it will operate, how

frequently it will receive maintenance, and

the types of chemicals it uses to store and

release electricity. A PV system may have

to be sized to store a sufficient amount of

power in the batteries to meet power

demand during several days of cloudy

weather. This is known as “days of autonomy.” Consult with your PV dealer before

selecting batteries for your system.

Most types of batteries contain toxic materials that may pose serious health and

safety problems. The National Electric

Code (NEC), battery companies, and PV

system designers recommend that leadacid and wet cell batteries, which give off

explosive hydrogen gas when recharging,

be located in a well-ventilated space isolated from the other electrical components

of the system and away from living

spaces. Allow enough space for easy

access during maintenance, repair, and

replacement. Most important, maintain

the battery according to the manufacturer’s instructions, and recycle the batteries properly when they wear out.

Alternating-Current System 

Equipment

Inverter. AC systems also require an

inverter, which changes the DC electricity

produced by PV modules and stored in

batteries into AC electricity. Different types

of inverters produce a different “quality” of

electricity. For example, lights, televisions,

and power tools can operate on lowerquality electricity, but computers, laser

printers, and other sophisticated electronic

equipment require the highest-quality

electricity. So, you must match the power

quality required by your loads with the

power quality produced by the inverter.

In addition to the 

PV modules, you will

need to purchase 

balance-of-system

(BOS) equipment.

6

Direct-Current (DC) System

Electric load (DC)

Charge controller

PV

modules

Battery

Grounding circuit

Grounding circuit

This figure illustrates the configuration of the PV modules and BOS 

equipment in a basic direct-current system with battery storage. (Circuit 

breakers and safety fuses are not shown.)
Inverters for most stand-alone applications (i.e., those systems not connected to

the utility grid) cost less than $1 per rated

output watt. The cost is affected by several

factors, including the quality of the electricity it needs to produce; whether the

incoming DC voltage is 12, 24, 36, or 48

volts; the number of AC watts your loads

require when they are operating normally;

the amount of extra surge power your AC

loads need for short periods; and whether

the inverter has any additional features

such as meters and indicator lights.

Tell your PV dealer if you plan to add

additional AC loads in the future. If you

are considering building another room

onto your house or adding electrical loads,

consider purchasing an inverter with a

larger input and output rating than you

currently need. This may be less costly

than replacing it with a larger one later.

The National Electric Code

The National Electric Code (NEC) was

established in 1897 to ensure safety in all

systems that generate, store, transport, and

consume electricity. You or the dealer who

installs your PV system should be careful

to follow NEC’s equipment requirements

so that the PV system can be approved by

local electric code officials. Be aware that

many states require all electrical equipment

to be installed by a licensed electrician.

However, many local code officials are not

familiar with PV systems. Though you follow the provisions of NEC, you may have

difficulty convincing a code official that

you have installed a code-approved system. Contact and educate (if necessary)

local code officials before you purchase

and install the system. Throughout the

installation process, invite them to observe

what you or your dealer have done before

you build any enclosure around wiring,

connections, or other components. This

will help ensure that your system receives

approval and will also help future PV

installers to get code approval.

Local insurance providers and lenders

may also need to be educated about the

safety, reliability, and cost-effectiveness of

PV systems. Obtaining insurance will be

easier in states where PV systems are more

common.

What Else Do I Need to Consider?

No PV system is maintenance-free. Schedule regular inspections of your system to

ensure that the wiring and contacts are

free from corrosion, the modules are clear

of debris, and the mounting equipment

has tight fasteners.

You should also monitor the power output

of your PV modules, the state-of-charge

and electrolyte level of your batteries, and

the actual amount of power that your

loads use. Writing this information in a

notebook is a good way to track your system’s performance and help you determine whether your system is operating as

designed. Monitoring will also help you

understand the relationships between

your system’s power production, storage

capability, and load requirements.

PV Can Power Your Future

PV systems can be cost-effective options

for providing electricity to your home or

remote site. However, they are not appropriate for all situations. Deciding whether

a PV system is right for you depends on

many factors. Therefore, conduct careful

research and consult with PV equipment

dealers and others who have installed these

systems. If you then decide that a PV system is right for you, the power of the sun

will take on a new meaning in your life.

7

The National 

Electric Code 

contains provisions

and requirements 

for PV systems.

Alternating-Current (AC) System

Electric load (AC)

Inverter

Charge controller

PV

modules

Battery

Grounding circuit

Grounding circuit

Most household appliances operate on alternating current (AC). This illustrates a

basic configuration of the PV modules and BOS equipment in an AC system.

(Circuit breakers and safety fuses are not shown.)
8

Source List

The following are just a few of the many organizations

that can help you with locating PV equipment dealers in

your area and designing and installing PV systems.

The Energy Efficiency and Renewable Energy 

Clearinghouse (EREC)

P.O. Box 3048

Merrifield, VA 22116

(800) 363-3732

Fax (703) 893-0400

E-mail: doe.erec@nciinc.com

EREC provides free general and technical information to the

public on the many topics and technologies pertaining to

energy efficiency and renewable energy, including PV systems,

solar energy, and solar radiation data.

Equipment, Dealers, and Installers

Renewable Energy & Efficiency Training 

Institute (RETI)

1800 M Street, NW

Suite 300

Washington, DC 20036

(202) 496-1417

Fax: (202) 496-1494

RETI offers customized PV design, installation, and maintenance programs to meet the needs of a wide range of customer

groups.

Solar Energy Industries Association (SEIA)

122 C Street, NW

4th Floor

Washington, DC 20001

(202) 383-2600

Fax: (202) 383-2670

SEIA is the national trade organization of PV and solar thermal

manufacturers and component suppliers.

Training Programs

Florida Solar Energy Center (FSEC)

Photovoltaic System Design Assistance and 

Training Center

1679 Clearlake Road

Cocoa, FL 32922-5703

(407) 638-1000

Fax: (407) 638-1010

FSEC offers workshops on a variety of topics related to 

PV system design and use. 

Siemens Solar Industries (formerly Arco Solar)

Photovoltaic Technology and System Design 

Training Course

4650 Adohr Lane

Camarillo, CA 93012

(805) 388-6561

Fax: (805) 388-6395

Siemens offers a one-week training program on PV technology

and system design.

Solar Energy International (SEI)

P.O. Box 715

Carbondale, CO 81623

(970) 963-8855

Fax: (970) 963-8866

SEI offers training programs on PV system design and installation, as well as on wind energy, mini-hydro systems, and solar

home design. SEI also sells books on a variety of renewable

energy topics.

On-Line Renewable Energy Information

Energy Efficiency and Renewable Energy Network

(EREN)

http://www.eren.doe.gov

EREN is the Department of Energy’s premier resource for

information about renewable energy and energy efficiency

technologies, including solar radiation and photovoltaic data.

National Renewable Energy Laboratory (NREL)

http://www.nrel.gov

NREL, one of the Department of Energy’s national laboratories, leads the nation toward a sustainable energy future by

developing renewable energy technologies. Its Web site

includes information on many renewable energy topics. See

NREL’s Renewable Resource Data Center, at

http://www.rredc.nrel.gov, for solar radiation information.

Reading List

Periodicals, Books, Pamphlets, and Reports

Consumer Guide to Solar Energy, S. Sklar, Bonus Books,

Chicago, 1991.

Home Power Magazine: The Hands-On Journal of HomeMade Power, Home Power, Inc., P.O. Box 520, Ashland,

OR 97520; (916) 475-3179; www. homepower.com.

Photovoltaic Fundamentals, National Renewable Energy

Laboratory, Document No. DE-91015001, available from

National Technical Information Service, U.S. Department

of Commerce, 5285 Port Royal Road, Springfield, VA

22161, 1991.

The Solar Electric House, S. Strong, Sustainability Press,

Still River, MA, 1993.

Solar Electricity: A Practical Guide to Designing and

Installing Small Photovoltaic Systems, S. Roberts, Prentice

Hall, NJ, 1991.

Stand-Alone Photovoltaic Systems: Handbook of Recommended Design Practices, Sandia National Laboratory,

Document No. SAND87-7023, available from National

Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161,

1991 (revised).