Selecting System Voltage
Windy Dankoff

The independent power system is based on storage batteries and direct 
current (DC) electric power.  Batteries are low voltage modules that 
may be assembled in 6, 12, 24 volt or higher configurations.  Voltage 
is the electrical "pressure" at which the system operates, and part 
of the battery's job is to maintain this pressure at a fairly constant 
level.  Thus, a "12 volt" battery maintains a working voltage within 
the range of about 11 to 14.5 volts-- a STANDARD.  A 12 volt appliance 
will run properly within this range of electrical pressure.

While the voltage remains fairly constant, the CURRENT (measured 
in AMPS) varies according to the power required by the appliance.  
As more lights are turned on in your house, more current is drawn 
from your batteries.  A large bulb draws more current than a small 
one.  Some appliances draw different amounts of current at different 
times; a circular saw draws more current cutting 2" wood than 1/2" 
wood because the motor works harder.

12 volts is the most common standard for alternative energy homes 
only because it is already a conventional standard-- for vehicles!  
As we progress to higher voltages, less current (amps) is required 
to deliver the same amount of power (watts/horsepower).  Wire, switches 
and other in- line components are sized according to the CURRENT 
they carry; the voltage has little bearing on their sizing.  Therefore, 
a 24 volt home electric system is less costly to wire-- it requires 
half the wire size, and less labor to install.  Control systems and 
inverters contain components that the current must pass through, 
so they too can be smaller and less expensive in a higher voltage 
system.

To confirm this for yourself, compare prices of 12 and 24 volt charge 
controllers and inverters.  The 24 volt models handle far more watts 
per dollar!  Efficiencies also tend to increase with higher voltage/lower 
current.  To see an extreme example of relative wire sizes, look 
under the hood of your car and see the BIG wire that goes from the 
battery to the starter.  A typical circular saw requires as much 
power as your starter, but look at the LITTLE wire it uses!  The 
saw uses 120 volts, and requires 1/10 the wire size to carry 1/10 
the current.

The common voltage standards for independent-powered homes are 12 
VOLTS and 24 VOLTS.  Your choice of standard is based on these factors:

(1)	OVERALL SYSTEM SIZE:  Small, cabin-size systems standardize 
on 12 volts, which offers the widest choice of small DC appliances 
and small inverters.  Medium to large homes generally cost less to 
set up on 24 volts, for the reasons below.

(2)	INVERTER SIZE:  Inverter requirements beyond 2,000 watts or 
so indicate 24 volts, for lower cost per watt and higher efficiency.

(3)	DC WELL PUMP OR OTHER LARGE MOTORS:  Motors above 1/4 HP often 
necessitate use of 24 volts, whether they are DC motors or AC run 
by inverter.  Large motors are more efficient at higher voltages.  
High current is required to start most motors so both wire and inverter 
need to be oversized.  So, the potential savings are especially great 
in going to higher voltage for motor circuits.

(4)	WIRING DISTANCES:  Long wire runs from PV or (especially) wind 
or hydro generator, to a DC well pump, or to other buildings can 
be very costly at low voltage/high current.  The longer the distance, 
the larger the wire must be to reduce losses.  So, cutting the current 
in half by using twice the voltage can cut your wire cost by nearly 
75%!

(5)	PLANS FOR FUTURE GROWTH:  If any of the above indicate a
requirement for 24 volts in the FUTURE, set up for it from the start 
so you won't be left with obsolete equipment.  If you see a need 
for higher DC voltage, consult your dealer.

Voltage converters are available for running 12 volt equipment (such 
as electronics) on a 24 volt system.  High quality 24 volt lights 
are nearly as common as 12.  Many large DC motors and pumps are not 
available at all in 12 volts, because the lower voltage motors are 
less efficient and require costly, over-sized wire, breakers and 
switches.

We do not go to 48 volts very often because we cannot get DC lights, 
refrigerators and well pumps at that voltage.  Most PV dealers and 
users agree that DC power still has its place for running the specialized, 
super- efficient DC appliances made specifically for independent 
power.  Direct use of DC in well-engineered appliances reduces both 
energy consumption and inverter requirements.

We are maintaining 12 and 24 volts as our DC home standard because 
it is safer and less costly to use than higher DC voltages.  (1) 
Less battery cells are required (they are 2 volts each) with less 
connections between them.  (2) High DC voltage from batteries (120 
volts) poses a serious shock hazard (twice that of 120 volts AC) 
and (3) high DC voltage poses more fire hazard (it causes much bigger 
sparks) than AC power at the same voltage.  Low voltage virtually 
eliminates these hazards.  120 volt DC is used in industrial power 
systems, but generally not in homes.  Our use of high-efficiency 
appliances and our elimination of electric heating devices keeps 
power consumption low so wire sizes in our DC homes need NOT be 5 
or 10 times oversized for low voltage!

A system dedicated to one specialized purpose need NOT conform to 
the common 12 or 24 volt standard.  When a solar system is designed 
only to power a well pump (with a motor in the range of 1/2 to 1 
HP) we may go to 60 or 120 volts DC if that optimizes economy and 
efficiency.

Remember, the final product of your energy system is not volts-- 
it's light, water, communication, mechanical energy, etc.  The voltage 
selected should be that which produces these ends at the lowest overall 
cost, with a high degree of safety and reliability.

Windy Dankoff is the owner of WINDLIGHT WORKSHOP and FLOWLIGHT SOLAR 
PUMPS, PO Box 548, Santa Cruz, NM 87567