DC system voltage

The DC system voltage chosen for the system will impact the sizing and selection of all other system components. Off-grid PV systems are typically built at several standard voltages - 12 V, 24 V, or 48 V - as all system components must be rated to designed to operate at the same voltage. Each voltage has different advantages and disadvantages that make it appropriate for different system sizes and types. One limitation that is common to all lead acid battery bank designs is that it is not recommended to incorporate more than 3 parallel strings of batteries into a battery bank as it can create charging issues - see Battery wiring. This constraint often determines the system voltage.

12 V system

Ideal for smaller systems and direct current (DC) only systems. Commonly uesd with systems that have a PV source with a power rating below 700 W. Not recommended for use with inverters larger than 800 W due to current/wire sizing issues.

Advantages

• 12 V deep-cycle lead acid batteries are widely available.
• Wide selection of available components (inverters, charge controllers).
• Wide selection of 12 V DC lighting and DC appliances.

Disadvantages

• Use with a PWM charge controller requires 12 V (36-cell) nomimal modules. 36-cell modules typically cost more per-watt than more common modules like 60-cell and 72-cell modules.
• Operating loads with high power ratings at 12 V can require significant amounts of current. This can create issues for wire sizing and overcurrent protection device sizing.
• Operating a system at 12 V can create voltage drop, which can require large wire sizes to ensure the system performs properly.

24 V system

Commonly used with systems that have a PV source with a power rating above 150 W and below 1500 W. Not recommended for use with inverters larger than 1200 W due to current/wire sizing issues.

Advantages

• A PWM charge controller permits the use of cheaper 72-cell modules.
• Wide selection of available components (inverters, charge controllers).
• Larger battery bank with fewer parallel strings.
• Less current required to power loads with high power ratings. This creates less issues for Wire and overcurrent protection sizing and selection
• Less voltage drop issues compared to a 12 V system.

Disadvantages

• There are significantly fewer lighting and DC appliance options. that operate at 24 V. A DC-DC converter can be incorporated into the system to power 12 V loads if necessary.

48 V system

Commonly used for any systems that has a PV source with a power rating above 1200 W or an inverter larger than 1200 W.

Advantages

• Allows the highest capacity battery bank with minimal parallel strings.
• Minimal current required to power loads with high power ratings. This minimizes issues for wire sizing and overcurrent protection device sizing.
• Less voltage drop issues compared to 12 V or 24 V systems.

Disadvantages

• Very limited lighting and DC appliance options that operate at 48 V. A DC-DC converter can be incorporated into the system to power 12 V loads if necessary.
• Smaller selection of available components (inverters, charge controllers).