An EV battery has two capacity figures. Gross (total) capacity is all the energy the pack physically holds; usable capacity is the smaller amount the car actually lets you access for driving — typically about 95-99% of the total. The difference is a deliberate buffer the battery management system reserves to protect the cells from being fully charged or fully drained, both of which accelerate degradation. For example, a VW ID.4 has an 82 kWh gross pack but ~77 kWh usable. Range estimates and efficiency are always based on usable capacity, so it's the number that matters for real-world driving.
Usable vs gross capacity at a glance
| Term | What it is | Why it matters |
|---|---|---|
| Gross (total) capacity | All energy the pack holds | Marketing/spec headline |
| Usable capacity | What you can actually use | Drives real range (~95-99% of gross) |
| Buffer | Reserved by the BMS | Protects cells, extends battery life |
| Range estimate | Based on usable | The number to plan around |
Two capacity numbers, one battery
Look closely at EV specs and you'll often see two battery figures, or a single number that doesn't quite match the range. That's because a pack has a gross (total) capacity — every kilowatt-hour it can physically store — and a usable capacity, the smaller portion the car actually permits you to use. The gap between them is an intentional buffer, and understanding it explains why your '82 kWh' car drives like a 77 kWh one.
What the buffer is and why it exists
The buffer is the slice of total capacity the battery management system (BMS) walls off and never makes available for normal driving. It exists for one main reason: longevity. Lithium-ion cells degrade faster when held at very high voltage (fully charged) or drained too low (fully empty). By keeping the cells away from those damaging extremes, the buffer protects them, improves durability and keeps the pack operating in a safer, more efficient window.
In practice this means when your dashboard reads 100%, the cells themselves aren't truly full, and when it reads 0%, they aren't truly empty. The car is showing you the top and bottom of the usable range, not the absolute limits of the cells — a hidden safety margin working in your favour.
How big is the buffer?
It varies by manufacturer and chemistry, but usable capacity is usually around 95-99% of the total. Some examples illustrate the range:
| Example model | Gross capacity | Usable capacity | Buffer |
|---|---|---|---|
| BMW i7 | 105.7 kWh | 101.7 kWh | ~4 kWh |
| Volkswagen ID.4 | 82 kWh | ~77 kWh | ~5 kWh |
| Typical EV | Stated total | ~95-99% of total | ~1-5% |
Some makers quote only the usable figure, some only the gross, and some both — which is a frequent source of confusion when comparing cars. LFP-based EVs often run smaller buffers because the chemistry tolerates full charging better, while NMC packs may reserve a little more.
Why this matters to you
- Range is based on usable capacity. The car's range estimate and efficiency figures use the usable number, so that's what determines how far you actually go — not the headline gross figure.
- Comparing cars fairly. A 'bigger' battery on paper (gross) isn't necessarily bigger in practice; check usable capacity for a true comparison.
- The buffer is protecting you. That reserved energy is why charging to '100%' is gentler on an EV than it sounds, and it contributes to the slow ~2.3%/year degradation EVs typically show.
- LFP nuance. Because LFP tolerates full charge, makers often recommend charging it to 100% regularly — the buffer plus the chemistry means that's still safe.
When comparing EVs or estimating range, use usable capacity, not gross. It's the energy you can actually drive on. If a spec sheet only gives the gross figure, assume usable is roughly 95-99% of it — and remember the range estimate already reflects usable capacity.
Does the buffer ever change?
The reserved buffer is set by the manufacturer and generally fixed, though software updates can occasionally adjust how the BMS manages the pack. Separately, as a battery ages and its total capacity fades through normal degradation, the usable capacity falls too — which is the real reason an older EV shows less maximum range even when charged to an indicated 100%. The buffer protects against premature aging; it doesn't stop the slow, normal decline measured as state of health.
The takeaway
The mismatch between gross and usable capacity isn't a trick — it's a feature. The buffer is one of the quiet reasons modern EV batteries last so well, keeping the cells off the damaging extremes that would otherwise shorten their life. As an owner, the practical lesson is simple: judge an EV by its usable capacity and its range estimate, both of which already account for the buffer, and trust that the hidden reserve is working to protect your battery for the long haul.
Frequently asked questions
What's the difference between usable and gross battery capacity?
Why don't EVs let you use the whole battery?
Which capacity figure should I use to estimate range?
How big is the battery buffer?
Does the buffer mean charging to 100% is safe?
Sources & further reading
- InsideEVs — What is 'usable' kWh on a battery pack?
- Recurrent — Usable vs total battery capacity for an EV
Figures, prices and policy details were current at the last-updated date above. Automotive pricing, incentives and regulations change frequently — verify time-sensitive details with the linked primary sources. Read our editorial policy and fact-checking standards.
