Battery ownership has become a possibility for more households due to the steady decrease in cost of lithiumbased battery chemistries in recent years.
These batteries are compact and light relative to their capacity. They can be efficiently charged and have comparatively long lifespans.
Not all lithium-ion batteries are the same. The two main varieties currently being sold are lithium nickel manganese cobalt oxide (NMC) and lithium iron phosphate (LFP or LiFePO4). NMC batteries have high energy densities but can overheat if discharged too quickly. Lithium iron phosphate batteries are a little larger but are considered safer.
Until recently the most common chemistry used for home batteries was the ‘flooded’ lead-acid battery, however in recent years they have been replaced with safer sealed lead acid (SLA) batteries, where the batteries are effectively sealed and the liquid acid electrolyte is immobilised. These batteries are mostly used in off-grid solar systems although they can be used in gridconnected solar battery systems.
The other main chemistry is ‘flow’ batteries, which use liquids to store energy. Their main advantages are safety, ease of maintenance and the ability to store the charge for long periods. The range of flow battery options are currently very limited for the residential market.
Other chemistries are in various stages of development and commercialisation and could become available in the coming years.
Most battery systems these days combine the batteries and other components in a pre-configured ‘storage in a box’ module for connection to a solar PV system.
These products include an inverter in the same box as the battery. The product is connected to your existing house switchboard, separately from the solar panels and solar inverter. Such a system makes for an easy battery installation or retrofit to an existing solar PV system. The battery can be located far from the solar inverter, for example in a cool location to maximise battery lifespan.
Installation is usually quick as much of the wiring between components has been done, and it often makes for a neater system as many components and their associated wiring are enclosed in a single cabinet. Storage units may be modular so that multiple units can be used to make up the required capacity, and some are designed to have extra battery modules slotted into the case to increase capacity.
Other battery products include the battery and supporting components such as safety switches, cooling devices and battery management system (BMS) only, but don’t include an inverter.
These products must be connected to a compatible inverter or a charge controller, which prevents overcharging. Typically, they are connected to a ‘hybrid’ inverter, which includes a charge controller to allow connection of a DC-coupled battery.
Most currently installed grid-connected solar PV systems will be using a gridinteractive inverter. A grid-interactive inverter converts the energy from solar panels into mains power and feeds it into the house’s electrical wiring—no storage is involved.
When you bring energy storage into the equation, it gets a little more complex, as the inverter needs to deal with both a generation source (like a solar PV system) and batteries, and possibly also the grid. This is when a hybrid inverter is used.
Hybrid inverters can have a vast range of features and this means that they are often capable of being used in place of other types of inverters. For example, you might install a hybrid inverter in a simple grid-connected solar PV system without storage with the view to adding batteries at a later date.
However, the solar industry is evolving very fast, and if you try to purchase a battery in a few years’ time, you may not find one compatible with your ‘old’ inverter. Another approach is to initially install a relatively cheap solar inverter, and then replace it with a hybrid inverter later on when you want to add a battery.
Battery add-ons and smart technology
Talk to your installer about how your proposed system will charge and interact with the grid and whether it needs any add-on systems.
Batteries are not the simple devices they once were. Many storage systems contain considerable computer processing power combined with algorithms that allow them to make the best of the energy available, based on predicting energy consumption patterns from past usage and external data, such as wholesale electricity prices and weather data.
Some systems can use local weather data from online weather services to predict how much solar energy is likely to be available the next day and then charge the battery from the grid overnight (at cheaper rates) if solar energy input is likely to be low.
By combining this sort of data, the battery can decide when it should be charging and discharging to make the most effective use of the available solar energy as well as minimising grid import (and export, if feed-in tariffs are low).
Even for storage systems that don’t have these features built in, add-on energy management systems can provide this sort of functionality.
What warranties are available?
You want your battery to last as long as the warranty and hopefully well beyond. A battery’s lifespan will be influenced by a number of factors including the number of cycles, the charge or discharge rate and the depth of discharge (DoD) employed in its everyday operation.
Systems installed under the Solar Homes Battery Rebate program must have a minimum five-year warranty on the whole system including the workmanship and a minimum sevenyear performance warranty is required under daily cycling operation.
As you would expect, system warranties vary by supplier and component, but you should seek out a ten-year warranty for the battery and any in-built or associated inverter if you can. Some manufacturers provide warranties for specific time periods, while other warranties can refer to a specific number of cycles based on a pre-defined cycling profile, for example 8000 cycles at 80 per cent DoD. Discharging beyond this point may affect the warranty or lifespan.
Battery lifespan may be more than the warranty implies, provided that the system has been correctly sized and the system is used within its recommended cycle depth and frequency ratings.
A workmanship warranty should match both the battery and inverter warranty (ideally 10 years), so that if a battery or inverter require repair or replacement, the labour is included free-of-additional charge.
Solar Victoria’s Solar Buyers Guide has more information about warranties on individual solar components.
Reviewed 03 March 2021