Inverter and Battery Technology
23 May 2013


What is an inverter?
An inverter is an electrical device that is designed to convert Direct Current (DC) to Alternating Current (AC). DC is the power that is produced by solar panels however, AC is the standard power needed to run electrical equipment.
There are two key types of inverters used for solar power system installations: string inverters and micro-inverters.

String inverters:
A string inverter is the most conventional type of inverter used for photovoltaic arrays. It is a single unit (about the size of a meter box) which connects to one or more ‘strings’ of solar modules. All the panels are wired together – combining the output of the solar arrays – which is fed to the inverter, which then converts DC to AC and feeds the power to the grid. Most string inverters have an efficiency of 95% or higher.
Diagram: Solar power system with a string-inverter.

Micro-inverters are smaller inverters mounted on the back of each solar module in an array. Each micro-inverter is rated to handle the output of a single module. They convert DC to AC on the roof and feed power to the grid.
Diagram: Solar power system with micro-inverters



Advantages of string inverters:
Because there is only one inverter, the solar power system cost is lower than having multiple micro-inverters - particularly with large systems.
String inverters come with an integrated monitoring display on the face of the inverter. 
Micro-inverters require a separate communication box that communicates with each inverter and monitors performance. 
String inverters are an established technology with proven reliability, while micro-inverters are a relatively new development and reliability is not yet proven.
Inverters are the most common point of failure in a photovoltaic power system. Many micro-inverters introduce a higher chance of component failure. Replacing a single central string inverter is far easier than replacing a micro-inverter behind a solar module on the roof.
As micro-inverters are mounted on the back of panels on the roof, they are exposed to the elements and heat of the solar modules.  The inverter’s performance and life may be affected by these conditions. 
Advantages of micro-inverters:
Flexible design
A micro-inverter system is more flexible as there is no string configuration required.
Each panel is independent (with its own inverter), and therefore the modules within a single installation can be placed at different orientations.
The performance of the whole array is not affected when there is a loss of output from one panel (for example due to shading). 
Micro-inverter systems are commonly considered safer for installers because there is no high voltage DC power.
If a single micro-inverter fails, the remainder of the solar array continues to produce power. 
Batteries provide energy for use at night or for times that the solar panels are not generating as much electricity required.  They are typically used in off grid systems or in grid connected systems with critical loads or frequent power cuts.  As they do add a lot of cost to the system, most grid connected customers will not require battery backup.
Historically, the only type of battery used for energy storage with solar power systems was lead acid batteries. However Lithium-ion technology is emerging as an alternative for larger scale energy storage.
Lead Acid Batteries:
Lead-acid battery technology has been around for more than 100 years. Lead-acid batteries are still used in most vehicles; therefore, advancements in the efficiency and lifespan of these batteries are still being made.
Lithium-ion Batteries:
Lithium-ion batteries are considered advanced battery technology. The cells in the battery can be fully charged and discharged which improves the efficiency. Lithium-ion batteries are most commonly used in cell phones and laptops.
Advantages of Lead-Acid Batteries:
Lead-acid batteries are significantly cheaper than lithium-ion batteries. The difference is not as apparent with small batteries (such as for phones and computers) and for these items people may not notice a significant cost difference. However, because solar batteries are so much larger, the cost difference is significant. 
Lead-acid batteries can be recycled at the end of their useful life, while lithium-ion batteries are likely to end up in the land-fill as there is no established system for recycling large lithium-ion batteries.
Both types of batteries are capable of going into “thermal runaway” in which the cell rapidly heats and can emit flames, electrolytes and dangerous fumes. However, the likelihood and consequence of an event are higher for lithium-ion as it has a higher amount of energy in a smaller volume.
Further safety concerns for lithium-ion batteries include components made of highly reactive materials.
Advantages of Lithium-ion Batteries:
Lithium-ion batteries are smaller and lighter in weight for the same amount of energy.
Life span
Lithium-ion batteries have an average calendar life span of at least two years longer than lead-acid.
Lithium-ion batteries don’t require regular maintenance. However, new generation lead-acid batteries additionally do not require regular maintenance. 
Deep discharges and cycle life
Unlike lead-acid, lithium-ion batteries are able to be discharged almost completely without loss of function so you can use a smaller capacity battery compared with lead acid.  In addition, lithium-ion batteries generally have a longer cycle life although this varies between manufacturers.




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