Start from the duty cycle, which is how often the battery is charged and discharged, how deeply, and for how many hours at a time. A battery cycled once a day for years has different requirements from one that sits ready and discharges rarely. The chemistry follows from that pattern rather than from a headline specification.
Start with the pattern of use
A battery is not one product. The same installed capacity can be asked to do jobs that have almost nothing in common.
Shifting solar output by a few hours into the evening means one deep cycle every day, every day, for years. Smoothing the short fluctuations of a windy afternoon means many shallow cycles and high power for short bursts. Holding reserve against a grid failure means sitting fully charged for months and discharging rarely, but discharging completely when it happens.
Those three jobs wear a battery in different ways, and they are the reason the first question is never which chemistry. It is what will this thing actually be doing, how often, and for how long.
The four properties that matter
Cycle life. How many charge and discharge cycles before capacity falls to a stated fraction of the original. This dominates when the battery cycles daily, because a chemistry with twice the cycle life can cost more per unit of capacity and still be cheaper over the life of the site.
Depth of discharge. How much of the stored energy can be used routinely. Two batteries with the same nominal capacity deliver different usable energy if one tolerates deep discharge and the other does not, and discharging deeper than a chemistry likes shortens its life sharply.
Rate. How fast energy can go in and come out. A battery that is asked to deliver its whole capacity in under an hour is doing something quite different from one discharging gently over four, and the internal resistance and the cooling design follow from that.
Temperature tolerance. How the chemistry behaves in real ambient heat, and what cooling it needs to stay in its comfortable range. Cooling is a parasitic load: energy spent keeping the battery happy is energy the site does not sell.
Cost per unit delivered, not cost per unit installed
The figure that decides a project is the cost of every unit of energy the battery delivers across its whole life, once the replacements, the losses and the cooling are counted.
That figure can rank chemistries in the opposite order to the purchase price. A cheaper battery cycled hard may need replacing within the term of the offtake contract. A dearer one that lasts the term does not.
It also means the contract shapes the choice. A power purchase agreement that runs for a long term sets how long the storage has to keep working, and therefore what it is worth paying for durability.
What the battery is really for
On a renewable site, storage is not primarily backup. It is what turns variable generation into something a buyer will sign a contract for.
Solar arrives in the middle of the day. Wind often arrives when solar is weakest. Demand arrives when it arrives. Storage moves energy by hours so the combination can meet a firm commitment, which is what makes the output contractable rather than merely available.
It also reduces exposure to curtailment, because surplus has somewhere to go other than nowhere.
The questions to settle before anyone quotes
Six, and they should be answered before a specification is issued rather than after.
How many hours of storage are needed, and to cover what: the evening peak, a full night, or a cloudy day.
How many cycles a year does that imply, and at what depth.
What is the maximum power in and out, not just the total energy.
What ambient temperature range will it actually sit in, on that site, in that month.
How long does the offtake contract run, and does the storage have to last that long or be replaced within it.
What happens at end of life, who takes it back, and what that costs.
A quotation that arrives without these settled is a price for a product, not a price for the job.
Where this fits
Storage is the component that makes an integrated site work as one system rather than as separate generators sharing a field. Sizing it follows the load the buyer needs covered, which keeps the cost proportionate to what it earns.
See also battery storage, capacity factor and curtailment in the glossary.
Related questions
- What does duty cycle mean?
- The pattern of use: how many full charge and discharge cycles a battery goes through in a year, how deep each discharge goes, how fast the power is drawn, and what temperature it operates at. Two batteries of the same size doing different duty cycles will have very different lifetimes and very different costs per unit delivered.
- Why is energy density often the wrong thing to optimise?
- Energy density matters when weight and volume are constrained, which is why it dominates in vehicles and phones. A battery sitting in a container on a site has space. What matters there is cost per unit delivered across the whole life, cycle life, safety and how the chemistry behaves in local heat.
- Does Indian heat change the answer?
- Yes. High ambient temperature accelerates degradation in most chemistries and it increases the cooling load, which consumes some of the energy the battery stores. A chemistry that tolerates heat well can be worth more on a site in an Indian summer than one with better numbers measured in a laboratory at mild temperature.
- How is battery life actually measured?
- In cycles to a stated remaining capacity, at a stated depth of discharge and temperature. All three conditions have to be quoted together. A cycle count given without the depth of discharge and the temperature it was measured at cannot be compared with another.