Tesla battery health: what the percentage means

State of health in plain words

State of health, or SoH, is a percentage. 100% means the battery can deliver as much usable energy as it could when the car left the factory. 88% means it can deliver 88% of that. The figure compares a pack with its own younger self, so it reads the same way on a small pack and a large one: both hold 88% of what they once held.

It is not the charge level. The percentage on the dashboard says how full the battery is right now. State of health says how big the battery is compared with new. Our vial is built differently on purpose: it is measured in usable kWh against the model’s published figure, and Tesla’s percentage is kept on its own line as degradation.

How Tesla estimates it

Tesla does not publish a step-by-step method, so we will not pretend to describe one. What we can say is where the number comes from: it is Tesla’s own estimate, based on telemetry from the vehicle, returned by the vehicle specification service of Tesla’s Fleet API together with the date of the measurement.

Battery Figures passes that figure on. We do not adjust it, and we do not replace it with a guess from another source. What we add is arithmetic around it: usable kWh, the gap to the model’s published capacity, and how far the car has come since production.

Rounded, stale or missing

• Rounded. Figures are shown to one decimal place. Read 87.7% and 87.8% as the same battery.
• Moving. It is an estimate, not a direct measurement of every cell, so it can move up as well as down between two readings. One reading says where the battery stands now. Several, over time, say more.
• 100% on a young car. A newer car can read 100%. That is the estimate at that moment, and not proof that nothing has been lost.
• Old. Every report shows the date of Tesla’s measurement. The older it is, the less it tells you about the battery today.
• Missing. Tesla does not report an estimate for every VIN. Some cars have none yet and some VINs are unknown to Tesla. When there is no estimate we say so, show whatever else Tesla returned, and charge nothing.

Usable kWh and nominal capacity

Nominal capacity is the total energy the cells hold. Usable capacity is the part the car lets you use; the rest is a buffer the car keeps back to protect the battery. Tesla’s state of health compares usable energy today with usable energy when new, which is why our reports speak in usable kWh.

We do not work out an original size from the health figure: it would rest on two rounded numbers. Where we know a model’s published usable capacity we compare usable kWh with that, and say so. Published sizes can be given either as nominal or as usable, and they differ by market, trim and year, so treat the comparison as a guide.

How we grade a reading

We put each car into one of four plain-language grades. The grade compares usable kWh today with the published usable capacity of the model, as a share of that figure:

• Excellent: 97% and up
• Good: 92% to 97%
• Fair: 85% to 92%
• Weak: under 85%

The scale is ours. It is not a Tesla rating and not a guarantee about the car. It needs a published figure for the model; without one we show no grade. Tesla’s own percentage is never turned into a grade: we show it as degradation, in Tesla’s words.

What it does not tell you

State of health says nothing about how the car was charged, whether the pack has been repaired, the condition of the rest of the car, or what warranty is left. For those, use the checklist in our guide to buying a used Tesla. To understand why the figure changes as a car ages, read battery degradation.