System Types Explained
A grid-tied PV system is a semi-autonomous electrical generator that produces and diverts power to feed the loads within the property, with any excess being supplied back to the utility grid. A grid tied PV system works in conjunction with the grid to supply the loads within the property, when insufficient energy is available from generation the grid can supply the shortfall. A pure grid-tied system with no storage or load management for a user with fixed rate power charges is a viable option, but the system needs to be sized in accordance with local regulations and load requirements inside the property. In a post FiT environment (where it is more economical to use generated power instead of exporting it), system sizing should reflect the requirements of the homeowner and should not be oversized. Essentially the PV system should be sized to generate only sufficient power for the base load during the day, i.e. the fridge, freezer, pool pump and other permanently on devices. The low investment cost of a small PV system with a high self-consumption rate should make them quite attractive especially for households with family at home during the day.
A system designed to increase self-consumption incorporates an energy storage system (ESS) to soak up any excess energy generated throughout the day. PV systems will generate the most power around the middle of the day when the sun tends to be the highest in the sky. This contrasts heavily with the typical usage profile of most residential properties, where the occupants are likely to be out of the property at their places of work or school. This means during the peak operational hours of the day a lot of the excess energy is essentially wasted as it must be exported back to the grid with not enough loads active within the property to absorb it. A storage system will alleviate this issue by storing excess energy generated throughout the day to be used later in the evening when the occupants return and usage spikes. This type of system is referred to as Enhanced Self-Consumption as it allows the customer to maximise their existing generation by storing and distributing when needed.
Grid-backup systems can operate with no grid supply for periods of time. Adding a charge controller or a hybrid inverter along with a battery makes it possible to discharge energy from the stored battery to power at least the essential loads in the property. The capacity and discharge rating of the battery required would be dependent on the rating of the essential loads that would need to remain energised during periods of no power. In order to guarantee that stored power is available during a power cut it is recommended to set a reserve on the battery. This will ensure that during normal operation the battery does not discharge to a point where there is no leftover energy stored in the event of loss of power.
Where a PV/storage system is used to limit the amount of higher cost electricity consumed by storing energy during the hours of sunlight and releasing it during the high cost periods. This may require additional timing controls to limit the time of usage of stored energy to these higher charge periods. Any system with this feature will have a TOU (Time Of Use) mode which will allow the user to define the hours of charging and discharging.