230 The Fourth Year

By Windy Dankoff

he independent home power sys-tem is based on storage batteries anddirect current (DC electric power.Batteries are low voltage modules thatmay be assembled in 6, 12, 24-volt orhigher configurations. Voltage is theelectrical “pressure” at which the sys-tem operates, and part of the battery’sjob is to maintain this pressure at afairly constant level. Thus, a “12-volt”battery maintains a working voltagewithin the range of about 11 to 14.5volts—a standard. A 12-volt appli-ance will run properly within thisrange of electrical pressure.

While the voltage remains fairlyconstant, the current (measured inamps) varies according to the powerrequired by the appliance. As morelights are turned on in your house,more current is drawn from your bat-teries. A large bulb draws more cur-rent than a small one. Some appli-ances draw different amounts of cur-rent at different times; a circular sawdraws more current cutting 2” woodthan ½” wood, because the motorworks harder.

Twelve volts is the most commonstandard for alternative energy homesonly because it is already a conven-tional standard—for vehicles! As weprogress to higher voltages, less cur-rent (amps) is required to deliver thesame amount of power (watts/horse-power). Wire, switches, and otherinline components are sized accordingto the current they carry; the voltagehas little bearing on their sizing.Therefore, a 24-volt home electricsystem is less costly to wire—itrequires half the wire size, and lesslabor to install. Control systems andinverters contain components that thecurrent must pass through, so they toocan be smaller and less expensive in ahigher voltage system.

To confirm this for yourself, com-pare prices of 12 and 24-volt chargecontrollers and inverters. The 24 voltmodels handle far more watts per

dollar! Efficiencies also tend toincrease with higher voltage/lowercurrent. To see an extreme example ofrelative wire sizes, look under thehood of your car and see the big wirethat goes from the battery to thestarter. A typical circular saw requiresas much power as your starter, butlook at the little wire it uses! The sawuses 120 volts, and requires 1/10 thewire size to carry the current.

The common voltage standards forindependent-powered homes are 12volts and 24 volts. Your choice ofstandard is based on these factors:

1. Overall system size: Small, cabin-size systems standardize on 12 volts,which offers the widest choice ofsmall DC appl iances and smallinverters. Medium to large homesgenerally cost less to set up on24 volts, for the reasons below.

2. Inverter size: Inverter require-ments beyond 2,000 watts or so indi-cate 24 volts, for lower cost per wattand higher efficiency.

3. DC well pump or other largemotors: Motors above ¼ HP oftennecessitate the use of 24 volts,whether they are DC motors or ACrun by inverter. Large motors aremore efficient at higher voltages. Highcurrent is required to start mostmotors, so both wire and inverter needto be oversized. So the potentialsavings are especially great in goingto higher voltage for motor circuits.

4. Wiring distances: Long wireruns from PV (photovoltaics, or solarcells) or (especially) wind or hydrogenerators, to a DC well pump, or toother buildings, can be very costly atlow voltage/high current. The longerthe distance, the larger the wire mustbe to reduce losses. So cutting the cur-rent in half by using twice the voltagecan cut your wire cost by nearly 75%.

5. Plans for future growth: If anyof the above indicate a requirementfor 24 volts in the future, set up for itfrom the start so you won’t be leftwith obsolete equipment (such as elec-tronics) on a 24-volt system. Highquality 24-volt lights are nearly ascommon as 12. Many large DC

motors and pumps are not available atall in 12 volts, because the lower volt-age motors are less efficient and requirecostly, over-sized wire, breakersand switches.

We do not go to 48 volts very often,because we cannot get DC lights,refrigerators, and well pumps at thatvoltage. Most PV dealers and usersagree that DC power still has its placefor running the specialized, super-effi-cient DC appliances made specificallyfor independent power. Direct use ofDC in well-engineered appliancesreduces both energy consumption andinverter requirements.

We are maintaining 12 and 24 voltsas our DC home standard because it issafer and less costly to use than higherDC voltages. (1) Less battery cells arerequired (they are 2 volts each) withless connections between them. (2)High DC voltage from batteries (120volts) poses a serious shock hazard(twice that of 120 volt AC). (3) HighDC voltage poses more fire hazard (itcauses much bigger sparks) than ACpower at the same voltage. Low volt-age virtually eliminates these hazards.120 volt DC is used in industrialpower systems, but generally not inhomes. Our use of high-efficiencyappliances and our elimination ofelectric heating devices keeps powerconsumption low, so wire sizes in ourDC homes need not be 5 or 10 timesoversized for low voltage!

A system dedicated to one special-ized purpose need not conform to thecommon 12 or 24-volt standard. Whena solar system is designed only topower a well pump (with a motorrange of ½ to 1 HP) we may go to 60or 120 volts DC if that optimizeseconomy and efficiency.

Remember, the final product of yourenergy system is not volts—it’s light,water, communication, mechanicalenergy, etc. The voltage selectedshould be that which produces theseends at the lowest overall cost, with ahigh degree of safety and reliability. ∆

Selecting the right PV voltage

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ALTERNATIVE ENERGY