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Microgrid, Off-Grid Knowledge Bank

Types of Off-Grid System

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Overview

An off-grid system typically has five elements:

  • renewable electricity source – solar PV and / or wind;
  • back-up generator (diesel);
  • inverters / converters as required to convert dc electricity to ac (and possibly ac to dc);
  • batteries to store electricity generated by the renewable energy source;
  • master controller and/or battery management system to control the operation of the system.

In addition climate control may be required, depending on the system location and the choice of battery.

The precise selection of equipment depends on whether the loads to be powered are ac, dc, or a mixture of both. Various configurations are discussed below.

DC only system

DC only systems are generally small systems powering lighting and low consumption appliances such as monitoring equipment.  The solar panels or wind generator charge the battery and the battery supplies power to the loads.  All loads are run at the battery voltage, typically 12V or 24V DC. Charge controllers limit the charging when the batteries are full.  Since there is no back-up generator must be sized to meet the load for worst case conditions.

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Mixed DC and AC system

A hybrid DC and AC system is similar to the DC only system, but also includes an AC inverter to enable the use of 230V appliances. The inverter needs to be sized to the peak AC load. Again, there is no back-up and so the system must be sized to worst case conditions.

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AC only system

With an AC only system, all of the loads are run on AC using an inverter to convert power stored in the battery bank to 230V AC.  Grid connected homes all run off 230V ac and thus the wiring for an off-grid home with an AC only system will be the same as the wiring for a grid connected home, using low cost cable and components. Once again, note that without a back-up generator, the system must be sized for worst case conditions.

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AC system with wind and solar

The AC system can have two sources of renewable energy:

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AC system with back-up

To avoid sizing for worst case conditions, a back-up generator is used. A petrol, LPG, or diesel generating set combined with a battery charger can supply power if the batteries become low.

See Sizing an Off-Grid System for more information on system sizing.

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