Fronius Economics of PV storage - A brief analysis of technology, costs and payback of battery storage AU Whitepaper [EN]

ECONOMICS OF PV STORAGE

A brief analysis of technology, costs
and payback of battery storage

White Paper
© Fronius Australia Pty Ltd., MF
Version 1.0/2016

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20%

DoD 80%

Usable capacity

INTRODUCTION

Storage solutions for PV systems play a major role in the future of energy supply based on 100% renewable
energy generation for households, a key component of our vision of 24 hours of sun.

As the technology is relatively new and the prices are still high there is currently some uncertainty about how to
select the most suitable technology is and what the real costs of storage are. A common approach is to
compare the upfront costs of different batteries, but, as shown in this paper the real cost of energy storage is
not determined from upfront cost alone.

Key characteristics & criteria for batteries

Not every battery is equal. They can differ in many ways, which leads to a significant impact on battery
characteristics such as lifetime, usable energy and performance capability. This paper summarises the key
characteristics and calculations to consider when choosing a storage solution. Also, an example is given at the
conclusion of this paper showing how to calculate the real cost of storage taking into consideration these key
characteristics.

Depth of discharge (DoD)

This characteristic gives the difference between nominal and usable
storage capacity of the battery. It defines the actual energy which can
be stored in and used from the battery. For example, a battery with
5kWH nominal storage capacity and 80% DoD can actually provide
just 4kWH of usable energy. The given 80% is the maximum DoD of
the battery, which can’t be set to a higher value for example 90%.
The rating of the lifetime or charging cycles is given based on this
value. However, it usually can be set to a lower value, for example
70%, which will increase the lifetime of the battery, as explained
below.

Charging cycles

Charging cycles define the actual lifetime of the battery. One cycle is equal to a full discharging and charging of
the usable storage capacity of the battery based on the maximum DoD value. So, discharging the battery to
50% of the usable capacity and recharging it back to 100% counts as half a cycle, not a full cycle. Reaching the
given cycles number doesn’t mean the battery isn’t usable anymore. There is often a rest capacity left which is
defined as the end of life capacity (EoL) characteristic.

End of life capacity (EoL)

This defines the rest value of the battery after the given cycle life. Batteries don’t have a storage capacity of
100% over their whole lifetime. Their storage capacity decrease in a linear course depending on the number of
charging and discharging cycles. The total amount of usable energy over the lifetime of battery is therefore less
than the usable capacity multiplied by the given charging cycles.

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