Choosing System Voltage

12 Volt vs. 24 Volt vs. 48 Volt Systems
Batteries
PV Array
Inverters
DC Motors
Upgrading

12 VOLT vs 24 VOLT vs 48 VOLT SYSTEMS:

The trend has been that 12 volt systems are the most cost-effective for small to medium sized systems. And 24 volt systems were best for larger systems where larger inverters, larger water pumps and larger DC motor-powered appliances were required. At 24 volts, smaller wire can be used to deliver power efficiently. Compared to 12 volts it takes only 1/4 the size of wire to conduct the same amount of electricity at 24 volts. A 48 volts it takes only 1/8 the size.

BATTERIES:

A 12 volt battery requires six healthy (lead-acid) cells, a 24 volt system requires 12 and a 48 volt battery requires 24., it is ideal to have a parallel set of batteries, or two sets at the same voltage. Two parallel sets double the capacity, and in the event of an accident or cell failure, your system need not shut down while you find a cell or set replacement. It is preferable to have large cells in your battery bank versus a lot of small cells to provide an appropriate amount of storage. Large cells are more massive and less prone to structural failure. Also the fewer cell to cell connections the better. Large Cells, Less Cells, Less Trouble. (For information on Batteries see Battery section on p. 20)

PV ARRAY:

A single PV module is designed to charge a 12 volt battery. It takes two modules, wired in a series, to charge a 24 volt battery and it takes four in series at 48 volts. So buying an even number of like modules is required for both 24 and 48 volt systems (*Different size modules can be in the same array as long as the modules in each pair or foursome are identical). Because of the line loss advantage at 24 volts, the PV array can be located much further from the batteries if necessary to maximize solar exposure. An electronic device called a Linear Current Booster (LCB, see p.31) can also be used to work like an ÎDC step-down transformerâ to allow wiring the array in series for higher voltage (higher than system voltage) so that transmission will be very efficient on smaller wire for greater distances and you can still efficiently charge a 12 or 24 volt battery.

INVERTERS:

On the low power end of the scale 50 watts to 2500 watts, there are more 12 volt inverters to choose from than 24 volt. For power inverters ranging over 2500 watts, 24, 36 or 48 volt batteries are required. Some inverters like Trace and Heart are Îstackableâ--meaning that two units can work in tandem to double the output. The Trace DR and SW Series inverters (p.42-44) also supply you with 240VAC when they are stacked. Most 12 and 24 volt inverters in the mid-range (700 to 2400 watts) are comparable in efficiency and price per watt. 24 volt inverters tend to have better surge ratings (motor starting capability) than 12 volt inverters of the same wattage. Most of the Trace line of inverters include standard built-in battery chargers. This gives the option of using a gas generator or utility power as a back-up power source.

DC MOTORS :

DC motors inherently use a lot less energy than equally sized AC motors. Thatâs why in a solar electric system long term loads that use motors should use DC motors. We offer DC fans and water pumps. Thatâs the way to go if possible. Inverters are practical for running short term AC motor loads (circular saws, drills vacuum cleaners, washing machines etc.). Long duration AC motor loads like the array of different AC fans (ceiling, attic, window, table, etc.) are significant energy hogs. If operating various stationary shop tools is a high priority in your system, then we would recommend going with a 24 volt system and running your bigger tools with 24 volt DC permanent magnet motors. PM motors are the most efficient of all the different types (series, compound, etc.). For converting tools like table saws, drill presses, planers, etc, with DC motors, you will find a greater selection at 24 volts than at 12--especially for 3/4 HP and up. Tools with pulley and belt drive are the easiest to convert since the RPM differences in the motors can be adjusted by changing the pulley size. Check out different mail order surplus catalogs for bargains on DC motors. (Surplus Center 800-488-3407) A general rule on motor conversions: A DC motor can generally replace an AC motor with 1.5 to 2 times the horsepower rating!

UPGRADING:

Upgrading from 12 to 24 volts requires: either adding 6 more cells to your battery bank or reconfiguring two 12 volt sets in a series; or starting over with a new 24 volt set. Two identical 12 volts sets in parallel will be equal in age and capacity and can be connected in series to make one 24 volt set. Mixing old and new batteries is not recommended, especially connecting them in series. One old, dying cell can be a constant drain on your whole bank. If you have an odd number of PV modules then another one will be necessary to make a pair for the series-parallel connection. *If two modules of different current ratings are wired in series, then, the output of that series will be equal to the output of the module with the lower current. Some charge controllers will work on 12 or 24 volt systems, otherwise to upgrade to 24 volt another controller will be necessary. Changing from 12 to 24 volts makes your 12-volt appliances obsolete, unless you employ an electronic converter (like the Vanner Battery Equalizer) that will allow you to discharge equally from both sides of a 24 volt battery. *Careful consideration of your present and future electrical needs will aid in choosing the most appropriate system voltage.

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