HOW DO OFF-GRID POWER SYSTEMS WORK?

Off-Grid power systems, are power systems that are not connected to the electricity grid. They can be powered from solar panel, wind turbines, mini hydro or generator. The most common of these is solar.

1. Solar panels (photovoltaic PV)
2. Charge Controller – controls the changing DC power from the solar panels to charge the battery
3. Appliances – i.e. fridge, lights, washing machine – these are AC
4. Battery charger inverter – this device converts the DC power from the batteries and solar panels to AC power to be used by house hold appliances
5. Battery storage – batteries produce DC power
6. Supplementary generator, can be added to provide power during low sun periods

Wind and mini hydro can either replace or provide supplementary power for the off grid power system.

During the day, electricity is generated by the solar panels.  This electricity fluctuates with the amount of sunlight. The charge controller manages these power changes to optimise battery charging and prevent over charging of the battery. The inverter convertors the DC power batteries and solar panels to AC power for use by appliances in the home. The inverter can also utilise a supplementary from the generator to provide power during periods of low sun and / or low battery state of charge.

There are different ways of configuring an Off Grid system. The configuration type will impact the life expectancy efficiency and reliability of the system

DC COUPLED OFF-GRID POWER SYSTEM

There are two types of system configurations used for Off Grid power systems – AC and DC coupled systems. The above example is of a DC coupled system. This can also be represented as

Advantages

• very high efficiency – up to 99% battery charging efficiency (using MPPT charge controllers)
• low cost setup for smaller scale off-grid systems up to 7kW
• modular – additional panels and controllers can be easily added if required and
• very efficient for powering DC appliances and loads (caravans and marine applications).

Disadvantages

• a little more complex to setup systems above 7kW as often multiple strings are required in parallel
• can become expensive for systems above 7kW as multiple higher voltage solar charge controllers are required and
• slightly lower efficiency if powering large AC loads during the day due to the conversion from solar DC to AC.

An AC coupled power system looks like

AC COUPLED OFF-GRID POWER SYSTEM

Advantages

• higher efficiency when used to power AC appliances during the day such as air-conditioning, and hot water systems
• generally lower installation cost for larger systems above 7kW and
• can use multiple string solar inverters in multiple locations

Disadvantages

• Lower efficiency when charging a battery system
• For systems under 7kW the addition of a solar inverters can be more expensive.
• lower efficiency when powering direct DC loads during the day.