Comparing carbon figures across batteries: a matched comparison method

Comparing carbon figures across batteries means judging whether one battery model has a lower life cycle carbon footprint than another, on a like for like basis. A valid comparison is a method, not an impression formed by placing two numbers side by side. The method matches the functional unit, the system boundary, the calculation method, the data quality, the manufacturing plant type, the electricity accounting, and the expected service life basis, and states every difference that remains.

A comparison that matches none of those elements, or that matches some and ignores the rest, is not a comparison. It is a juxtaposition that invites a conclusion the evidence does not support. The method below is written for a reader who has two declarations in front of the reader and needs to decide whether they can be compared at all.

What must match before numbers are compared?

The functional unit must match: kilograms of carbon dioxide equivalent per kilowatt hour of total energy delivered over expected service life, with the same definition of total energy and the same service life basis. The system boundary must match: the same life cycle stages, the same treatment of the use phase and end of life, and the same cut off rules. The method must match: the same JRC rules and delegated act methodology, the same version, and the same allocation rules. The data basis must be comparable: primary data for the same foreground processes, secondary datasets of comparable quality, and the same electricity accounting approach. Where any element does not match, the comparison must either be adjusted in a way the method allows, with the adjustment stated, or must not be made. A comparison that proceeds despite a mismatch, without stating the mismatch, is misleading even when the numbers are quoted accurately.

How are plant and energy differences handled?

Plant differences matter because the declaration is per manufacturing plant. Two models made in different plants, with different energy mixes, different yields and different supplier routes, will have different footprints even where the design is identical. Energy differences matter because electricity is a major contributor and the accounting approach, grid mix versus supplier specific mix, guarantees of origin treatment, and period, can change the result. A comparison should state the plant for each figure, the energy mix and accounting approach for each figure, and whether the difference in figures reflects a design difference, an energy difference, a data difference, or a combination. A comparison that attributes a difference to design, where the difference is in fact an energy mix difference, misleads a buyer who is choosing between designs. The declaration breakdown by life cycle stage helps here, because it shows where the difference arises: materials, manufacturing energy, transport, or another stage.

How is expected service life handled in comparison?

Expected service life determines the denominator of the functional unit: total energy delivered over life. A longer expected life spreads the manufacturing footprint over more delivered energy, producing a lower figure per functional unit, even where the manufacturing footprint per battery is identical. Comparison must therefore check the service life assumption for each figure: the test basis, the cycle life assumption, the depth of discharge, and the end point. A battery with a longer assumed life and a higher manufacturing footprint can show a lower figure per functional unit than a battery with a shorter assumed life and a lower manufacturing footprint. Neither figure is wrong. The comparison is wrong if the service life difference is not stated. A valid comparison states the service life basis for each figure, and, where the basis differs, states whether the comparison is made on the declared basis or on a harmonised basis, with the harmonisation method stated.

What does the comparison checklist show?

How is a comparison reported?

A comparison is reported as a statement with a basis, not as a ranking alone. The report states the two figures, the basis for each, the match result for each element, the differences that remain, and the conclusion that the evidence supports, which may be that no valid comparison can be made on the evidence available. A report that states only that one battery is lower, without the basis, is an incomplete report, even when the ranking is correct on the declared basis. A report that states that no comparison can be made, with the reasons, is a useful report, because it prevents a decision based on a false comparison. The comparison report should be retained with the declarations compared, so that a later reader can repeat the comparison on the same basis.

What limits apply to a comparison?

A comparison of declared footprints compares model level declarations, not single units, not installed performance, and not total impact in use. A lower declared footprint per functional unit does not prove lower total emissions in a specific application, where use patterns, lifetime in service, and end of life treatment differ. Application level assessment requires the declared figures plus use and end of life evidence for the specific case. This guide gives general educational information only and does not give legal advice for a particular battery, comparison or purchasing decision.