How To Solar Power

The Basics of Solar Power for Pro­duc­ing Elec­tric­ity
Learn the essen­tial basics of using solar power so you can under­stand your project.
Plan­ning your project begins with under­stand­ing the basics found in this sec­tion.

Solar power works well for most items except large elec­tric appli­ances that use an elec­tric heat ele­ment such as a water heater, clothes dryer and elec­tric stove — for exam­ple — or total elec­tric home heat­ing sys­tems. It is not cost effec­tive to use solar power for these items. Con­ver­sion to nat­ural gas, propane or other alter­na­tives is usu­ally rec­om­mended. Solar power can be used to oper­ate a gas clothes dryer (May­tag, etc) because the elec­tri­cal require­ment is lim­ited to the drum-motor and/or ignito-lighter, but not a HEAT ele­ment for dry­ing the clothes, for exam­ple.
We rec­om­mend you also review our page Help­ful Solar Power Glos­sary which will pro­vide use­ful terms and Gen­eral Con­struc­tion Tech­niques for insight into energy effi­cient new con­struc­tion or retro­fitting energy effi­cient prin­ci­ples into your build­ing project.

The basics of solar power:
Using solar power to pro­duce elec­tric­ity is not the same as using solar to pro­duce heat. Solar ther­mal prin­ci­ples are applied to pro­duce hot flu­ids or air. Pho­to­voltaic prin­ci­ples are used to pro­duce elec­tric­ity. A solar panel (PV panel) is made of the nat­ural ele­ment, sil­i­con, which becomes charged elec­tri­cally when sub­jected to sun light.

Solar pan­els are directed at solar south in the north­ern hemi­sphere and solar north in the south­ern hemi­sphere (these are slightly dif­fer­ent than mag­netic com­pass north-south direc­tions) at an angle dic­tated by the geo­graphic loca­tion and lat­i­tude of where they are to be installed. Typ­i­cally, the angle of the solar array is set within a range of between site-latitude-plus 15 degrees and site-latitude-minus 15 degrees, depend­ing on whether a slight win­ter or sum­mer bias is desir­able in the sys­tem. Many solar arrays are placed at an angle equal to the site lat­i­tude with no bias for sea­sonal periods.

This elec­tri­cal charge is con­sol­i­dated in the PV panel and directed to the out­put ter­mi­nals to pro­duce low volt­age (Direct Cur­rent) — usu­ally 6 to 24 volts. The most com­mon out­put is intended for nom­i­nal 12 volts, with an effec­tive out­put usu­ally up to 17 volts. A 12 volt nom­i­nal out­put is the ref­er­ence volt­age, but the oper­at­ing volt­age can be 17 volts or higher much like your car alter­na­tor charges your 12 volt bat­tery at well over 12 volts. So theres a dif­fer­ence between the ref­er­ence volt­age and the actual oper­at­ing volt­age.
The inten­sity of the Suns radi­a­tion changes with the hour of the day, time of the year and weather con­di­tions. To be able to make cal­cu­la­tions in plan­ning a sys­tem, the total amount of solar radi­a­tion energy is expressed in hours of full sun­light per m², or Peak Sun Hours. This term, Peak Sun Hours, rep­re­sents the aver­age amount of sun avail­able per day through­out the year.

It is pre­sumed that at “peak sun”, 1000 W/m² of power reaches the sur­face of the earth. One hour of full sun pro­vides 1000 Wh per m² = 1 kWh/m² — rep­re­sent­ing the solar energy received in one hour on a cloud­less sum­mer day on a one-square meter sur­face directed towards the sun. To put this in some other per­spec­tive, the United States Depart­ment of Energy indi­cates the amount of solar energy that hits the sur­face of the earth every +/- hour is greater than the total amount of energy that the entire human pop­u­la­tion requires in a year. Another per­spec­tive is that roughly 100 square miles of solar pan­els placed in the south­west­ern U.S. could power the country.

The daily aver­age of Peak Sun Hours, based on either full year sta­tis­tics, or aver­age worst month of the year sta­tis­tics, for exam­ple, is used for cal­cu­la­tion pur­poses in the design of the sys­tem. To see the aver­age Peak Sun Hours for your area in the United States, you can click the fol­low­ing link which will open a new win­dow — just close it [X] when youre done to return here; U.S.-Solar Inso­la­tion Choose the area clos­est to your loca­tion for a good indi­ca­tion of your aver­age Peak Sun Hours.
So it can be con­cluded that the power of a sys­tem varies, depend­ing on the intended geo­graph­i­cal loca­tion. Folks in the north­east­ern U.S. will need more solar pan­els in their sys­tem to pro­duce the same over­all power as those liv­ing in Ari­zona. We can advise you on this if you have any doubts about your area.

Com­po­nents used to pro­vide solar power:

The four pri­mary com­po­nents for pro­duc­ing elec­tric­ity using solar power, which pro­vides com­mon 120 volt AC power for daily use are:

Solar pan­els, charge con­troller, bat­tery and inverter.

Solar pan­els charge the bat­tery, and the charge reg­u­la­tor insures proper charg­ing of the bat­tery. The bat­tery pro­vides DC volt­age to the inverter, and the inverter con­verts the DC volt­age to nor­mal AC volt­age. If 240 volts AC is needed, then either a trans­former is added or two iden­ti­cal invert­ers are series-stacked to pro­duce the 240 volts.

Solar Pan­els:
The out­put of a solar panel is usu­ally stated in watts, and the wattage is deter­mined by mul­ti­ply­ing the rated volt­age by the rated amper­age. The for­mula for wattage is VOLTS times AMPS equals WATTS. So for exam­ple, a 12 volt 60 watt solar panel mea­sur­ing about 20 X 44 inches has a rated volt­age of 17.1 and a rated 3.5 amperage.

V x A = W

17.1 volts times 3.5 amps equals 60 watts

If an aver­age of 6 hours of peak sun per day is avail­able in an area, then the above solar panel can pro­duce an aver­age 360 watt hours of power per day; 60w times 6 hrs. = 360 watt-hours. Since the inten­sity of sun­light con­tact­ing the solar panel varies through­out the day, we use the term “peak sun hours” as a method to smooth out the vari­a­tions into a daily aver­age. Early morn­ing and late-in-the-day sun­light pro­duces less power than the mid-day sun. Nat­u­rally, cloudy days will pro­duce less power than bright sunny days as well. When plan­ning a sys­tem your geo­graph­i­cal area is rated in aver­age peak sun hours per day based on yearly sun data. Aver­age peak sun hours for var­i­ous geo­graph­i­cal areas is listed in the above section.

Solar pan­els can be wired in series or in par­al­lel to increase volt­age or amper­age respec­tively, and they can be wired both in series and in par­al­lel to increase both volts and amps. Series wiring refers to con­nect­ing the pos­i­tive ter­mi­nal of one panel to the neg­a­tive ter­mi­nal of another. The result­ing outer pos­i­tive and neg­a­tive ter­mi­nals will pro­duce volt­age the sum of the two pan­els, but the amper­age stays the same as one panel. So two 12 volt/3.5 amp pan­els wired in series pro­duces 24 volts at 3.5 amps. Four of these wired in series would pro­duce 48 volts at 3.5 amps. Par­al­lel wiring refers to con­nect­ing pos­i­tive ter­mi­nals to pos­i­tive ter­mi­nals and neg­a­tive to neg­a­tive. The result is that volt­age stays the same, but amper­age becomes the sum of the num­ber of pan­els. So two 12 volt/3.5 amp pan­els wired in par­al­lel would pro­duce 12 volts at 7 amps. Four pan­els would pro­duce 12 volts at 14 amps.

Series/parallel wiring refers to doing both of the above — increas­ing volts and amps to achieve the desired volt­age as in 24 or 48 volt sys­tems. The fol­low­ing dia­gram reflects this. In addi­tion, the four pan­els below can then be wired in par­al­lel to another four and so on to make a larger array.

Charge Con­troller:

A charge con­troller mon­i­tors the bat­terys state-of-charge to insure that when the bat­tery needs charge-current it gets it, and also insures the bat­tery isnt over-charged. Con­nect­ing a solar panel to a bat­tery with­out a reg­u­la­tor seri­ously risks dam­ag­ing the bat­tery and poten­tially caus­ing a safety concern.

Charge con­trollers (or often called charge reg­u­la­tor) are rated based on the amount of amper­age they can process from a solar array. If a con­troller is rated at 20 amps it means that you can con­nect up to 20 amps of solar panel out­put cur­rent to this one con­troller. The most advanced charge con­trollers uti­lize a charg­ing prin­ci­pal referred to as Pulse-Width-Modulation (PWM) — which insures the most effi­cient bat­tery charg­ing and extends the life of the bat­tery. Even more advanced con­trollers also include Max­i­mum Power Point Track­ing (MPPT) which max­i­mizes the amount of cur­rent going into the bat­tery from the solar array by low­er­ing the pan­els out­put volt­age, which increases the charg­ing amps to the bat­tery — because if a panel can pro­duce 60 watts with 17.2 volts and 3.5 amps, then if the volt­age is low­ered to say 14 volts then the amper­age increases to 4.28 (14v X 4.28 amps = 60 watts) result­ing in a 19% increase in charg­ing amps for this example.

Many charge con­trollers also offer Low Volt­age Dis­con­nect (LVD) and Bat­tery Tem­per­a­ture Com­pen­sa­tion (BTC) as an optional fea­ture. The LVD fea­ture per­mits con­nect­ing loads to the LVD ter­mi­nals which are then volt­age sen­si­tive. If the bat­tery volt­age drops too far the loads are dis­con­nected — pre­vent­ing poten­tial dam­age to both the bat­tery and the loads. BTC adjusts the charge rate based on the tem­per­a­ture of the bat­tery since bat­ter­ies are sen­si­tive to tem­per­a­ture vari­a­tions above and below about 75 F degrees.

Bat­tery:

The Deep Cycle bat­ter­ies used are designed to be dis­charged and then re-charged hun­dreds or thou­sands of times. These bat­ter­ies are rated in Amp Hours (ah) — usu­ally at 20 hours and 100 hours. Sim­ply stated, amp hours refers to the amount of cur­rent — in amps — which can be sup­plied by the bat­tery over the period of hours. For exam­ple, a 350ah bat­tery could sup­ply 17.5 con­tin­u­ous amps over 20 hours or 35 con­tin­u­ous amps for 10 hours. To quickly express the total watts poten­tially avail­able in a 6 volt 360ah bat­tery; 360ah times the nom­i­nal 6 volts equals 2160 watts or 2.16kWh (kilowatt-hours). Like solar pan­els, bat­ter­ies are wired in series and/or par­al­lel to increase volt­age to the desired level and increase amp hours.

The bat­tery should have suf­fi­cient amp hour capac­ity to sup­ply needed power dur­ing the longest expected period “no sun” or extremely cloudy con­di­tions. A lead-acid bat­tery should be sized at least 20% larger than this amount. If there is a source of back-up power, such as a standby gen­er­a­tor along with a bat­tery charger, the bat­tery bank does not have to be sized for worst case weather conditions.

The size of the bat­tery bank required will depend on the stor­age capac­ity required, the max­i­mum dis­charge rate, the max­i­mum charge rate, and the min­i­mum tem­per­a­ture at which the bat­ter­ies will be used. Dur­ing plan­ning, all of these fac­tors are looked at, and the one requir­ing the largest capac­ity will dic­tate the bat­tery size.

One of the biggest mis­takes made by those just start­ing out is not under­stand­ing the rela­tion­ship between amps and amp-hour require­ments of 120 volt AC items ver­sus the effects on their DC low volt­age bat­ter­ies. For exam­ple, say you have a 24 volt nom­i­nal sys­tem and an inverter pow­er­ing a load of 3 amps, 120VAC, which has a duty cycle of 4 hours per day. You would have a 12 amp hour load (3A X 4 hrs=12 ah). How­ever, in order to deter­mine the true drain on your bat­ter­ies you have to divide your nom­i­nal bat­tery volt­age (24v) into the volt­age of the load (120v), which is 5, and then mul­ti­ply this times your 120vac amp hours (5 x 12 ah). So in this case the cal­cu­la­tion would be 60 amp hours drained from your bat­ter­ies — not the 12 ah. Another sim­ple way is to take the total watt-hours of your 120VAC device and divide by nom­i­nal sys­tem volt­age. Using the above exam­ple; 3 amps x 120 volts x 4 hours = 1440 watt-hours divided by 24 DC volts = 60 amp hours.

Lead-acid bat­ter­ies are the most com­mon in PV sys­tems because their ini­tial cost is lower and because they are read­ily avail­able nearly every­where in the world. There are many dif­fer­ent sizes and designs of lead-acid bat­ter­ies, but the most impor­tant des­ig­na­tion is that they are deep cycle bat­ter­ies. Lead-acid bat­ter­ies are avail­able in both wet-cell (requires main­te­nance) and sealed no-maintenance ver­sions. AGM and Gel-cell deep-cycle bat­ter­ies are also pop­u­lar because they are main­te­nance free and they last a lot longer.
DIY — Home Made Energy

Using an Inverter:

An inverter is a device which changes DC power stored in a bat­tery to stan­dard 120/240 VAC elec­tric­ity (also referred to as 110/220). Most solar power sys­tems gen­er­ate DC cur­rent which is stored in bat­ter­ies. Nearly all light­ing, appli­ances, motors, etc., are designed to use ac power, so it takes an inverter to make the switch from battery-stored DC to stan­dard power (120 VAC, 60 Hz).

In an inverter, direct cur­rent (DC) is switched back and forth to pro­duce alter­nat­ing cur­rent (AC). Then it is trans­formed, fil­tered, stepped, etc. to get it to an accept­able out­put wave­form. The more pro­cess­ing, the cleaner and qui­eter the out­put, but the lower the effi­ciency of the con­ver­sion. The goal becomes to pro­duce a wave­form that is accept­able to all loads with­out sac­ri­fic­ing too much power into the con­ver­sion process.
Invert­ers come in two basic out­put designs — sine wave and mod­i­fied sine wave. Most 120VAC devices can use the mod­i­fied sine wave, but there are some notable excep­tions. Devices such as laser print­ers which use tri­acs and/or sil­i­con con­trolled rec­ti­fiers are dam­aged when pro­vided mod-sine wave power. Motors and power sup­plies usu­ally run warmer and less effi­ciently on mod-sine wave power. Some things, like fans, ampli­fiers, and cheap flu­o­res­cent lights, give off an audi­ble buzz on mod­i­fied sine wave power. How­ever, mod­i­fied sine wave invert­ers make the con­ver­sion from DC to AC very effi­ciently. They are rel­a­tively inex­pen­sive, and many of the elec­tri­cal devices we use every day work fine on them.

Sine wave invert­ers can vir­tu­ally oper­ate any­thing. Your util­ity com­pany pro­vides sine wave power, so a sine wave inverter is equal to or even bet­ter than util­ity sup­plied power. A sine wave inverter can “clean up” util­ity or gen­er­a­tor sup­plied power because of its inter­nal processing.

Invert­ers are made with var­i­ous inter­nal fea­tures and many per­mit exter­nal equip­ment inter­face. Com­mon inter­nal fea­tures are inter­nal bat­tery charg­ers which can rapidly charge bat­ter­ies when an AC source such as a gen­er­a­tor or util­ity power is con­nected to the invert­ers INPUT ter­mi­nals. Auto-transfer switch­ing is also a com­mon inter­nal fea­ture which enables switch­ing from either one AC source to another and/or from util­ity power to inverter power for des­ig­nated loads. Bat­tery tem­per­a­ture com­pen­sa­tion, inter­nal relays to con­trol loads, auto­matic remote gen­er­a­tor starting/stopping and many other pro­gram­ma­ble fea­tures are available.

Most invert­ers pro­duce 120VAC, but can be equipped with a step-up trans­former to pro­duce 120/240VAC. Some invert­ers can be series or par­al­lel “stacked-interfaced” to pro­duce 120/240VAC or to increase the avail­able amperage.

Effi­ciency Losses:
In all sys­tems there are losses due to such things as volt­age losses as the elec­tric­ity is car­ried across the wires, bat­ter­ies and invert­ers not being 100 per­cent effi­cient, and other fac­tors. These effi­ciency losses vary from com­po­nent to com­po­nent, and from sys­tem to sys­tem and can be as high as 25 per­cent. Thats why its a good idea to speak to some­one who has exten­sive design expe­ri­ence — like us! — to prop­erly con­fig­ure the right equip­ment for you.

CONGRATULATIONS!
IF YOU HAVE READ AND BASICALLY UNDERSTAND ALL THE SECTIONS ABOVE, THEN YOU CAN BEGIN PLANNING OF A POWER SYSTEM USING SOLAR POWER.