DC system voltage
The system voltage parameter determines the nominal DC voltage for the system. The voltage chosen for the system will impact the the sizing and selection of all other system components. Off-grid PV systems are typically built at several standard voltages - 12V, 24V, or 48V - 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.
Contents
12V 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 700W. Not recommended for use with inverters larger than 800W due to current/wire sizing issues.
Advantages
- 12V deep-cycle lead acid batteries are widely available.
- Wide selection of available components (inverters, charge controllers).
- Wide selection of 12V DC lighting and DC appliances.
Disadvantages
- Use with a PWM charge controller requires 12V (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 12V can require significant amounts of current. This can create issues for wire sizing and overcurrent protection device sizing.
- Operating a system at 12V can create voltage drop, which can require large wire sizes to ensure the system performs properly.
24V system
Commonly used with systems that have a PV source with a power rating above 150W and below 1500W. Not recommended for use with inverters larger than 1200W 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 sizing and overcurrent protection device sizing.
- Less voltage drop issues compared to a 12V system.
Disadvantages
- There are significantly fewer lighting and DC appliance options. that operate at 24V. A DC-DC converter can be incorporated into the system to power 12V loads if necessary.
48V system
Commonly used for any systems that has a PV source with a power rating above 1200W or an inverter larger than 1200W.
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 12V or 24V systems.
Disadvantages=
- Very limited lighting and DC appliance options that operate at 48V. A DC-DC converter can be incorporated into the system to power 12V loads if necessary.
- Requires a minimum battery bank size of 4 x 12V batteries.
- Smaller selection of available components (inverters, charge controllers).
