[Off Grid] Is DC better than AC in Off-grid Power Systems?

There are two different ways of configuring an off-grid power system, AC-Coupling and DC-Coupling, and each have unique advantages and work in different ways.

The configuration type that is used will have an impact on the life expectancy, efficiency and flexibility of your off-grid power system.

AC Coupled configuration

What is AC power?

Alternating current (AC) power is the standard electricity that comes out of power outlets and is defined as a flow of charge that exhibits a periodic change in direction.

The most recent innovation in stand-alone power is the AC (Alternating Current) coupled system. Energy generators (E.g. Solar array) are coupled into the system via regular 230V AC wiring.

Advantages

• Higher Efficiency as it provides direct power to household during daylight, bypassing the batteries
• Reduced battery cycling
• The excess power charges the batteries via the inverter
• Lower solar array wiring efficiency loss
• Increased off-grid power system performance
• Flexible placement of solar arrays and other equipment
• Battery bank life expectancy increased
• The fuel powered generator is also fully AC coupled. It provides a significant system advantage at times of high system demand or low renewable energy input.
• Less and thinner AC cabling
• PV array can be far away from the installation

Disadvantages

• The equipment cost is typically a little higher, but easily recoverable in medium to large stand-alone power systems.
• Inverter manufacturers that use the AC coupled configuration (Eg. SMA, Victron Energy) use the frequency shift method of solar output control. In some cases sensitive devices such as some inverter type air conditioners, low quality lights may not operate at certain times of the day. Digital clocks (Eg. oven and microwave clocks) also won’t keep accurate time due to the power frequency variation.
• Programming needed
• System will not recover from low battery if there is no backup generator
• Potential clock and timer issues
• Not suitable for boats or vehicles

DC coupled configuration

What is DC power?

Direct current is a bit easier to understand than alternating current. Rather than oscillating back and forth, DC provides a constant voltage or current.

Until recently the traditional method has been used a DC (Direct Current) connection method in which energy generators are directly coupled into the off-grid power system battery bank via a regulator or charge controller.

Advantages

• Simple and has lower component costs for smaller systems requiring lower power.
• Considered to have some safety advantages.
• Better for charging batteries
• More efficient if most energy needs to be stored
• Fail-safe in case batteries are too far discharged

Disadvantages

• Copper cables are required to carry power and can be very costly for larger systems.
• Cost cutting techniques can result in inferior off-grid power system performance.
• All power is delivered to the home via the main off-grid battery inverter. These inverters have limited capacity so may limit the system performance at times as it can't transfer all available solar array power as its availability.

Our consultants are off-grid power system experts and have an in-depth understanding of the different currents. We are happy to recommend the most suitable solution for each situation.