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Anexampleloadcalculation
BasicsofSolarPVSystemDesignPostedonFebruary21,2013
Asuccessfuldesigninvolvesaccurateknowledgeofdailyelectricalloadcalculationandaccountsforallworstcasescenarioswhichmightpossiblyoccurduringoperation.Agooddesignerwillbepragmaticandkeepthecostsdownbycuttingonunnecessaryoversizingthesystem.Intheexampleofatypicalmiddleclasshome(shownattheleft)allthelightshavebeenreplacedwithLEDlightswhichconsumefarlesselectricitycomparedwiththeusualCFLlightsortubelights.ThislowersdailypowerconsumptionwhichenablesyoutomanagewithsmallersolarPVsystem.Itwouldhavebeenstillbetterifthefanswerealsoreplacedwithenergyefficientfans.InIndia,CromptonGreaveswillsoonbeginoffering1200mmceilingfans(MAXIMUS)thatwillconsumejust35watts!!TheproductionisexpectedtostartinMarchApril2013.USHAhasa45Wattceilingfan(Technix).Ifsuchfansare
installedthePVsystemsizewillbefurtherreduced(whichmeansstilllowerinvestment!).
Forenergyefficientfans,pleasecheckoutthispageonSuperFans:SuperEfficientFans.itwaspointedbyareadercommentingbelow.
Step1:CalculateTotalPowerConsumption
ThefirststepindesigningasolarPVsystemistofindoutthetotalpowerconsumptioninwatthours.Itinvolvesmultiplyingthewattageofeachappliancewiththenumberofhoursitrunsdailyandaddingthemall.Thesideimageshowshowitcanbedone.Weshalluseitasanexamplehere.So,thetotaldailypowerconsumptionis4300wh.
Step2:SizingthePVModules(Wp)
Besidessolarpanelswhichgeneratephotocurrent,asolarPVsystemismadeupofmanycomponentschargecontroller,inverter,batteriesallconnectedbywires.Noneoftheseare100percentefficient.Forinstance,every100amphourdrawnfromthebatteryrequiresputtinginabout115120amphoursofchargeintoit.Invertersaretypicallyonly85percentefficientandlikewiseforchargecontroller.CablesofferelectricalresistancetotheflowofcurrentwhichissubstantialbecauseofratherlowDCvoltagesinvolved.PeopletrykeepingitlowbyworkingwithhigherDCvoltageswherepossibleandyetkeepwiringcostandresistancelosseslow(saylessthan3%).Therefore,inordertoaccountforallsuchlossesthemodulesmustputinsomeextraenergyintothesystemnotmerelythedailywatthourload.
Wecanguessthisextraamountifweassumesay80%efficiencyfortheinverter/chargecontroller,85%forthebatterybankand3%wirelosses.Thus,wemustoversizetherequirementfromthemodulesby1/(0.80x0.85x0.97)orby1.51orsimply1.5.
SolarPowerforOrdinaryPeopleSolarEnergyisidealforacountrylikeIndiawhichhasplentyofsunshinewithwellover300averageannualsunnydays.
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So,energyneededfromthemodulesforourexamplehouseisabout4300x1.5=6450wh.Duringthedaylighthoursmodulesmustproduceatleastthismuchenergy.Thisbringsintheconceptofpeaksunhoursintothepicture.Sincesolarmodulesarecharacterizedfor1000W/m
(1kW/m
)ofsolarradiationintensity,itisusefultorepresentthedailysolarradiationintermsofnumberofhoursof1000W/m
.Thus,a5kW/m
ofdailyaveragesolarinsolationisequivalentto5hoursof1000W/m
solarintensity.Monthlyaverageddailysolarinsolationdata(mapsanddatainMicrosoftexcelworksheet)areavailablefromtheMNREwebsiteforIndianlocations.Thesedailysunhourshelpyoufigureoutthepeakwattrequirementfromthepanels.
Forexample,inNewDelhithenumberofaveragedailysunhoursis5.5formodulesmountedhorizontallyand6.2whenthemodulesaremountedatthelatitudeangleofDelhiwhichisabout28degrees.AlthoughmodulesshouldbeideallyinclinedatthelatitudeanglefacingsouthinIndia(sothatsunraysfallnormallyonthesolarmodules),but
thetableattheleftshowsthedifferenceisnothighlysignificant.AgoodwaytoharnessthemaximumsolarenergyfromthemodulesistochangetheangleofinclinationtwiceduringanyearinMarchforsummerangle(generally15degreeslessthanthelatitude)andinSeptemberforwinterangle(typically15degreesmorethanthelatitude).Thiscanbeeasilydoneforsmallsystems.
IfthehouseislocatedinDelhi,wecanassume5.5hoursofdailysunshine.Sothepeakwattratingofmodulesforoursystemwillbe6450/5.5=1173Wproundingitup,1.2kWpmoduleswillberequired.Ifyouwanttobemoreliberaluse5sunhoursitgives6450/5=1290Wpor1.3kWp.Thus,thereisnorigidformulaforsystemdesigningtheonlythumbruleistobesomewhatliberaldependinguponyourpocketsize.
FormostlocationsinnorthIndia,onecanassume55.5dailysunshinehours.
Step3.InverterSizing
AninverterisusedinthePVsystemwhen220VACpoweroutputisneeded.ItsratingshouldneverbelowerthanthetotalwattofappliancesandinputDCvoltage(12V/24V/48V)mustmatchthevoltageofthebatterybank.Theinvertersizeshouldbegenerally2530%biggerthantotalwatts
ofappliances.Iftheloadincludesanappliancesuchasmotororcompressortheninvertersizeshouldbeminimum3timesthecapacityoftheseappliancessothatitcanhandlethestartingsurgecurrentforfewseconds.
Forexample,iftotalwattageallappliancesisabout500W(asisthecaseinourexamplehome),theinvertersizeshouldbearound650W.
Step4:batterySizing
Thebatterybankshouldbelargeenoughtoprovidethedesiredbackup.Batterycapacitycalculationinvolvesaccountingforbatterylosses(typically15percent),maximumallowabledepthofdischarge(DoD)anddaysofautonomy.Althoughthedeepcyclebatteriescanbeoccasionallydischargedupto80percent,butforlongerlifeitisbettertokeeptheroutinedischargelimittoonly50percent.GiventheunpredictablenatureofPVsystems,onceinawhilebatteriespayvisittothe80percentlimitofDoD.
Notalltheenergygeneratedbythepanelswillbestoredinthebatteries.Somepartofthedaytimeloadwill
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receivepowerdirectlyfromthepanelsthroughtheinverter(powerconditioningunit).Whatisnotconsumedwillbedivertedtothebatteries.
Letsassumethatforaverage5.5hoursdailytheworkingappliancesreceivepowerdirectlyfromthepanelthroughtheinverter.Insummermonths,itcanbeassumedthatfor5.5hoursallthedaytimeloadoffans,TVandthelaptopreceivespowerfromthepanelthisis400Wfor5.5hoursor2200wh.Thus,energyonlyfortheremaining2100wh(ofthetotaldaily4300wh)needstobestoredinthebatteries.
Nowagainwemustaccountforefficienciesofbatteriesdischargeandinverter/chargecontroller:1/(0.85x0.80)=1.47(orsimply1.5).Thus,thestoragerequiredis2100x1.5=3150wh.Thismuchstoredenergywillbedrawndailyfromthebatteries.
InDelhithetemperaturegoesdownto3Cinthewinter.Insidehomewherebatterieswillbelocatedisexpectedtobearound10C.Batterycapacitiesaregenerallyratedfor25C.Soadropof15Cwillreducethebatterycapacitybyabout10%.Applyingthiscorrection:3150/0.90=3500wh.
IfthesystemisdesignedforDC24V,thenthedailyamphourcapacityneededis3500wh/24V=146Ah.For50%allowabledischargeweget292Ah.Therefore,a24V300Ahbatterybankshouldprovidesufficientbackup.
Summarizing:
DesigningasolarPVsysteminvolvesfirstcalculatingtheaveragedailyenergyconsumptioninwatthours.Asasimplerule,overratethedailyWhloadby1.5forpanelcapacity.LikewiseoverrateWhloadtobestoredinthebatteriesalsoby1.5forbatterysizing.Forbetteraccuracyofdesigngettheactualefficienciesofthechargecontroller,inverter,batteriesandcablestobeused.
Featuresofthesystemdesignhereare
Maximumdailyenergyconsumptionis4300Wh.Maximumallowableloadatanypointoftimeis500W.Whenfullycharged,thebatterybankcansupportthedailyusageof4300Wh.Chargedepletionofthebatterieswillbereplenishedonaclearsunnyday.
Similarpages
SolarSystemSizingworksheet
SizingbatteriesandinvertersforSolarPVSystem
ANEXERCISE:TrycalculatingspecificationsofasolarPVsystemforafactoryfloorthatuses50CFLlightsof20Weach.Theselightsaretoberunfor12hourseverydayandinthe12hoursofnight17oftheselightswillremainON.Whathappensifyoureplacealltheselightswith12WLEDlights.TheseareagainACloadliketheexampleusedabove.
Youcanshareyourcalculationsthroughthecommentboxanddiscussdifficulties.
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AboutGoodpalIamafirmbelieverinhealthypeople(mindandbodyboth),healthysocietiesandhealthyenvironment.Ialsoundertakecontentwritinganddocumentationprojects.Pleasefeelfreetocomment,shareandbroadcastyourviews.Ifyouwishtowriteforthisblog,pleasecontactmeatvj.agra@yahoo.comThanksforstoppingby.HaveaGoodDay!ViewallpostsbyGoodpal
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poloolopsays:May29,2013at10:32pm
GoodarticleonPVsystemscapacityandotherfactorstothinkabout.AnyideaifCGMaximusfansareavailablenowordelayed.Iamnotabletofindanythingaboutthesamefromtheirwebsite.
Reply
Goodpalsays:June4,2013at1:15am
TheMaximusmodelwassupposedtohavebeeninthemarketinApril2013,butprobablyithasbeendelayed.
Reply
Sukumarsays:June25,2013at7:11am
Thereisanotheronecalledsuperfan,whichconsumes35wattsattopspeed(22Wonspeed4,14Wonspeed3).Anditworks
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atfullspeedevenonlowvoltagesandcomeswitharemote.ItcostsaroundRs3000(X1=2900A1=3200V1=3100).TheysayyoucansavearoundRs1000/yrwithreplacinga1620hrsrunningnormalfan.Ofcourse,replacingbecomesobviouswhenpurchasingsolarpanel,otherwisetheextracostwouldgointhepanel,byreplacingyougetanewsuperfanatthesamesolarinvesment.
Reply
Sukumarsays:June25,2013at7:12am
Forgottomentionthesite:http://superfan.in
Reply
vishalsays:October8,2014at12:56am
thankssukumar.canuplssuggestusthelowerpowerconsumptionfansinapricerangeof1300to1800
Reply
sukumarsays:December1,2014at8:53am
Iamnotanyfandealerorlikethat,butatcostsabout1300,youwillonlygetinefficientandunbrandedfans(70to85+watts).Eventhebrandedones(likeusha,orient,etc)costaround2200+thesedays(highspeedandqualitymodels).So,3200isnotabigdealforanefficientfan(itwillpayforitselfinabout3years)
Goodpalsays:June25,2013at10:12am
ThanksSukumar,foraddingvaluetothearticle.
Iinvitereaderstosharesuchusefulinformationwhichareaboutinnovationorenergysaving.
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Venkatsays:July8,2013at9:12pm
Thereisonemorefromhttp://www.parasgreen.comSeetheproductpageathttp://www.parasgreen.com/productprofile.htmlandthebrochurePDFhttp://www.parasgreen.com/PDFDCCeilingFans.pdf
PS:theirinfo@andsales@parasgreen.comemailaddressesarenotworkingbuttheyrepliedviatheirgmailaddress.ItalsocostsRs3000(withouttax)buttheyoffernotaxifpurchasedwithsolarpanel.AndIguessitdirectlyrunsonsolar/mainsinwithoutbatterycondition.
But,Ihavepurchasedasuperfan(X1model,230vAC)anditisreallyasuperfan.Thoughsuperfandoesntrunondirectsolar,theyhavea24vDCversion(Rs500extra).
Reply
Sukumarsays:July9,2013at8:47am
SuperFanseemtohave2newmodelsX7andV7withlowerprices(aboutRs400less)thanmodelsX1andV1,probablythereasonbeingthattheydontcomewithremote(X7isRs2521andV7isRs2715).
AndIguessspeedcontrolisdoneviathemainswitch(turningon/offtheswitchmultipletimesquickly),Ireadthis
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SolarPowerforOrdinaryPeople
somewhereontheirsiteawhileback,probablyoncommentspage.Ofcourseitremembersthelastspeed,sothereisnoextraworkifyouusethesamespeedmostly.
Simplysuperb.
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ChanderManiSharmasays:November19,2013at5:11am
NicelyDoneMr.Goodpal
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ChandanPrakashsays:May1,2014at12:35pm
Hi,Thanksforthearticle.Butwhataboutthechargecontroller?Ireckonchargecontrollerhastobetherebetweenpanelandbatteries.Inverterwillbeusedwhenweuseelectrictyfromthebattery.Sohowdowedesignchargecontroller,putdifferently,whatshouldbethecapacityofthechargecontroller??Ihavereadsomethingaboutmpptchargecontrollerbutdonotknowhowtopickacorrectsize.
Reply
shubhamsays:June16,2014at2:05am
ExcellentArticle
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DrTShankerRaosays:November30,2014at11:40pm
Beautifularticle.Anybodycanunderstand.afterreadingonecancalculateandinstallPVSYSTEMATHOME
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