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Battery Backup



A battery back-up system adds two levels of functionality to a grid-tie solar system:
A battery back-up system from Current Generation will charge during the day and then release the stored energy overnight, with no disruption of power supply to the home. Energy will only be ‘imported’ from the national grid if it is necessary, and less energy imported from the grid means a lower power bill. This also protects the consumer from rising electricity prices.
· In the event of a power cut, the solar array will continue to work without any disruption of power supply to the home. As long as the batteries are being sufficiently topped up each day with solar energy, the power cut will not affect the home.
Battery back-up systems are ideal for adding value to a grid-tie power system (solar, wind or hydro power), however they can also be used in a normal home or business without a renewable energy system for emergency back-up, and/or to maximise benefits from off-peak electricity rates.
The best size depends on the budget and the energy consumption of the home, and what the consumer’s expectations are. A well designed system will minimise the ‘depth-of-discharge’ of the batteries, which in turn increases their service life. A decent battery bank is a significant investment, so it is important to get it right the first time. A medium sized lead-acid battery bank for a normal home fits in a battery box which is about half the size of a standard chest freezer. Lithium-ion batteries are smaller and lighter than lead-acid batteries, however their use in renewable energy systems is still an emerging technology.
Extensive monitoring systems are available, which put the consumer in charge of their power system. Real-time information can be accessed from anywhere in the world via the internet, and apps are available for smart phones and computers.

A battery back-up system is the only sensible way to provide back-up and to allow your PV system to remain active during a power cut. There is another incentive to installing a battery back-up system – the ‘self-consumption’ of the energy your PV system generates increases dramatically. This adds value in a way that a generator can’t (because running a generator will never save you money).
To explain what I’m talking about, here are some screenshots of the monitoring from a GTBB (Grid Tie Battery Back-up) we recently installed:
Battery SOC (State of Charge)-217
The batteries were fully charged about lunchtime, and were discharged by about 20% overnight.
Battery Voltage and Current-326
The battery voltage (blue) climbed during the morning (the dips are when the sun is behind a cloud, or when a large load is drawn). During the afternoon it stayed steady (float charge). At sunset, the bank is no longer being charged and the battery bank's voltage drops, and goes down slowly overnight. The ‘blips’ overnight will be the fridge compressor turning on and off.
During the day, the house is running mainly from solar with minimal draw from the batteries or grid. Only ¼ of a kWh was taken from the grid for the whole 24 hour period ($0.07 worth).
Solar Yield-824
This shows how energy is sent to the batteries until they’re full and then exported to the grid after that. During the winter months, there would be far less energy exported. Ideally, the grid is only really used for back-up and most of the energy used is produced on-site.
Live Feed-344
This is the live feed from the same system the following morning. The energy being yielded is not being exported to the grid – it gets sent straight to the batteries and the home until the batteries are full. When this snapshot was taken, 223W was being sent to the batteries.
So, the system works very well and adds multiple layers of functionality to a grid-tie system. As very few units are being imported (bought from the retailer), the money saved offsets the cost of the batteries over time. In the event of a power cut, the system remains active. It might be necessary (depending on the power use at your property) to have certain circuits switch off during a power cut to prevent over-discharge of the batteries, but this is fairly easy to automate. It can also be done manually of course. A generator can also be added if you need to be able to run large loads during extended power cuts. The battery bank needs to be sized based on one's consumption and PV yield. If a battery bank that is too small is used, the voltage will drop too much when large loads turn on, causing importing of energy from the grid. This can also shorten the battery life. Furthermore, if a large load is switched off while in ‘off-grid mode’, a sudden in-rush of current from the PV to the batteries can happen, which is not ideal.  A large battery bank is better for the batteries, but a balance needs to be struck between the service life of the batteries, their performance, and the capital investment.
In my opinion, a battery back-up system is the best way to add value to a grid-tie solar system. It will cost more up-front, but this will even out over time, all the while giving you added functionality.

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