By Jeffrey R. Yago, P.E., CEM

Most of my past articlesdescribed solar systemsthat included a batteryor batteries to store the

collected solar energy. There aremany types of solar power systemswith battery backup that can be con-nected to the utility grid. However,technically speaking, a “grid-tie” sys-tem refers to a solar power systemthat transfers all of the electricalpower generated directly back intothe local utility grid and does nothave a battery bank.

Not needing a battery bank cangreatly simplify system complexityand reduce installation costs, but bycode the system is not allowed to pro-duce any power when the utility gridis down, which means a grid-tie solarsystem is not suitable for use as anemergency backup power system. It

will, however, significantly reduceyour monthly utility costs, and sincethere are no batteries to maintain andperiodically replace, there is almostno maintenance involved with a grid-tie solar system.

SSyysstteemm ccoommppoonneennttssA grid-tie solar photovoltaic system

consists of individual solar moduleswired together to create a solar array,one or more inverters to convert thesolar DC power to AC grid qualitypower, and associated switchgear andwiring.

Although almost any size solarmodule can be used, most grid-tiesolar inverters operate at much highervoltages than a typical 12 to 48-voltbattery based solar power system,which allows more solar modules tobe wired into each series “string.”Solar modules for grid-tie systemsare usually chosen in the larger 100-

to 300-watt permodule sizerange, and usu-ally include pre-wired weather-proof male andfemale plugconnectors asshown in photo#2.

This meanshigh voltagesolar arraywiring consistsof just “plug-ging” the posi-tive connector ofone module intothe negativeconnector of thenext moduleuntil they are all

connected together, with a final posi-tive and negative connection made atthe start and end of the array string. Aword of caution is in order. Sincemost grid-tie inverters will allow 16or more individual modules to bewired in each series string, you couldeasily be dealing with voltages over340 volts DC, and this will generate avery large arc and very hazardousconditions if you do not use the prop-er components and wiring.

This grid-tie project requiredmounting thirty-two 165 watt solarmodules on a raised seam metal barnroof. Fortunately, the solar industrynow provides precut mounting sys-tems for almost any solar modulemounting arrangement, and we founda design that included clampingblocks that anchored the mountingrails directly to the raised seams ofthe metal barn roof.

Photo #3 shows how theseadjustable high strength aluminum

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Photo 2: Back view of 165-wattsolar module showing preinstalled

weatherproof male and female connectors

Photo 1: One of two Fronius 3-kW grid-tie inverters. Note the DC-rated safety disconnect and all high

voltage wiring installed in metal conduit.

mounting rails and related stainlesssteel hardware were attached to thisbarn roof.

Photo #4 shows the array installa-tion beginning with the attachment ofthe solar modules to the mountingrails. Since each module just “plugs”into the next, the array wiring is com-pleted as each module is physicallyattached to the mounting rack.

UUttiilliittyy iinntteerrccoonnnneeccttiioonnSince you are using the local utility

grid as your “battery,” not all electricutilities will look kindly to sendingelectric power back into their utilitylines and turning electric meter back-wards in the process. You will need tocheck with your local utility companybefore installing a grid-tie solarpower system to make sure they

allow this type of intercon-nection.

Most states that dorequire their electric utili-ties to offer net-meteringhave a simple, no costapplication process. Someutilities will require chang-ing out your existing elec-tric meter to allow them toseparately record your elec-tric flows in both direc-tions, while a few electricutilities are still down righthostile to anyone wantingto back-feed power to thegrid. Older style mechani-cal dial electric meters willrun backwards for anyreverse electrical flow andsubtract from the totalmonthly reading, whilemost new electronic metersare programmed to ignoreany reverse flow. There arecurrently 35 states thatallow utility interconnec-tion or net-metering.Although the utilities thatdo allow interconnectionhave different rate struc-tures and billing arrange-ments, most will allow a

monthly carry-forward credit balance,which means those summer monthswhen you generate more power thanyou use will offset purchased electri-cal power you used later during thewinter months.

Ideally you would like to have anysolar electric power sold at the samerate that you purchase electricity.This is called net-metering since youare just paying the difference betweenwhat you use and what you produce.However, some utilities only pay avery low rate for the solar power theypurchase even though it is being sup-plied during their afternoon peak loadperiod when their high demand gen-eration costs are much higher.

Utilities operating in states that doallow net-metering usually require

your system to include a manual dis-connect switch located next to theelectric meter so they can “lock-out”the solar power grid connection whenthey are doing any maintenance onthe nearby power lines. Since utilitylinemen will always earth ground anddisable any power lines before theystart working, and will most likelynot even know if you are located neartheir repair location, it is doubtfulthat this switch will ever be actuallyused. Utilities will also require thatyou provide documentation that yourinverter meets UL 1741, IEEE 929,and NEC Art. 690 code requirements,which ensure the inverter will auto-matically disconnect from the utilityline anytime the grid is not energized.

Photo #6 shows our completed5,280 watt installation on the south-facing roof of the barn. This systemwill generate almost all of the powerneeds of this hay barn, whichincludes a repair shop, carpentryshop, and small office.

Although the simplicity of a grid-tiesystem makes it easy for a do-it-your-

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Photo 3: Solar array mounting rails and raisedseam metal roof clamps

Photo 5: Utility required emergencysolar disconnect located next

to utility meter

Photo 4: Start of installation of solar modules onraised mounting rails

selfer to install, if you are not anexperienced electrician, I stronglyrecommend hiring a professional dueto the very high voltages involvedwith these systems. A list of profes-sionally trained and certified solarphotovoltaic installation professionalslocated in each state can be found onthe Internet under the NorthAmerican Board of Certified EnergyPractitioners, (NABCEP).

On a final note, although solarpower system costs for the same sizesystem will be competitive from stateto state, different utility rates, localtax credits, and state rebate or buy-down programs offered in some statesand not others can make this either areally good investment or a very poorone. Be sure to find out what utility

rates and incentive programs areavailable in your area before makingyour final purchasing decision.

(Jeff Yago’s latest text titled, AchievingEnergy Independence - One Step At A Time,provides a very good introduction to off-gridliving and battery based power systems. It isavailable from Backwoods Home Magazine’sBookstore (page 95) or by calling 804-457-9566.) ∆∆

July/August 2005 Backwoods Home Magazine

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Useful referencesNABCEP Certified Solar

Installers:www.nabcep.org

List of states allowing inter-connection:

www.dsireusa.org

Fronius grid-tie inverters:www.fronius.com

Solar mounting rail systems:www.directpower.com

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Photo 6: Completed solar array on south facing barn roof

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