The gross capacity stated on the data sheet is not what you can actually use. The depth of discharge (DoD) indicates what proportion of the battery is extractable – the rest remains as a reserve to protect the cells. What remains is the usable capacity, and that is exactly what counts. That is why one always compares battery storage systems per usable kilowatt hour. (As of: July 2026)
Key points in brief
- Gross capacity is the total battery capacity – usable capacity is the part that you can actually extract.
- The depth of discharge (DoD) describes this usable proportion; a small reserve deliberately remains unused in order to protect the cells.
- Modern batteries – especially LFP – can be discharged comparatively deeply, the usable capacity lies close to the gross capacity.
- For the comparison, only the usable capacity counts – and the price per usable kilowatt hour.
- Whoever confuses gross and usable values is comparing apples with oranges.
Why can I not completely empty a battery storage?
Because a battery lives longer if you do not squeeze it out to the last drop. Complete discharging to zero strains the cells and shortens their lifespan. Therefore, the systems deliberately hold back a small reserve: A part of the battery remains as a buffer that does not go into everyday life. This is no defect and no trick of the manufacturers, but a protective measure that ensures a long life for the battery storage.
From this protective reserve follows the distinction that so often causes confusion when buying a battery storage: between what is in the battery, and what you can use of it. Both are on the data sheet, and confusing both leads to false comparisons.
Gross capacity, usable capacity, depth of discharge
Three terms, one connection – and once you have understood it, every data sheet reads more clearly.
Gross capacity is the entire capacity built into the battery. It is the larger, more promotional number – but not the one that you calculate with in everyday life.
Usable capacity is the part that you can actually extract after the protective reserve is deducted. This number decides how much electricity the battery storage really delivers in the evening.
Depth of discharge (DoD) describes the ratio: what proportion of the gross capacity is usable. The higher the depth of discharge, the closer the usable lies to the gross capacity.
| Term | What it means |
|---|---|
| Gross capacity | entire built-in capacity (the large number) |
| usable capacity | the actually extractable part (the relevant number) |
| Depth of discharge (DoD) | usable proportion of the gross capacity |
| Protective reserve | deliberately unused rest, protects the cells |
Simplified overview, As of: July 2026. The information of the respective manufacturer is authoritative.
How much do modern battery storage systems yield?
The good news for today's buyers: The difference between gross and usable has become smaller. Modern batteries – especially the LFP chemistry widespread in the home battery – tolerate a comparatively deep discharge without ageing rapidly. Leading manufacturers now state the usable capacity of their current LFP systems at a depth of discharge of 100 per cent (As of: July 2026) – the protective logic is then in the battery management and in the cell design itself. In practice, many inverters additionally limit the minimum state of charge to a few per cent, so that the really extractable amount can lie just below the data sheet specification. The withheld reserve is thus significantly narrower than with earlier battery generations – lead batteries, for instance, could only be discharged by about half in a cell-protecting manner.
But that does not mean that the distinction does not matter. Firstly, the usable proportion varies from system to system, and secondly, it is exactly this number that you need for a fair comparison. A battery storage with a larger gross capacity, but a lower usable capacity can deliver less in everyday life than a smaller one on paper – whoever only reads the gross number falls prey to this effect.
Why one compares battery storage systems per usable kilowatt hour
This brings us full circle to the purchasing decision. Because only the usable capacity counts in everyday life, it is also the only fair basis for price comparison. The reliable benchmark is therefore: price per usable kilowatt hour, not per gross kilowatt hour – exactly as the guide on battery storage costs sets as a basis for comparison.
The practical advice from this is simple: if you place two quotes side by side, make sure that both mean the same size. If one states the gross capacity and the other the usable capacity, you are comparing apples with oranges – and the seemingly cheaper one is possibly the smaller one. A serious quote shows the usable capacity; where only a large round gross number is prominently displayed, it is worth asking. And for the right battery storage size, the following applies anyway: what matters is how much you can use in the evening – i.e. the usable number, not the gross number.
The confusion of gross and usable is one of the most common stumbling blocks when customers compare quotes themselves. There are two offers on the table, one says «10 kWh», the other «9.5 kWh usable» – and at first glance, the first one seems larger. On closer inspection, the first means the gross capacity, the second the usable capacity, and suddenly they are closer together than they seemed, perhaps even the other way around. In the consultation, we therefore always calculate down to the usable kilowatt hour and clearly show it in our quotes. With modern battery storage systems, the gap between the two numbers has admittedly become smaller – but as long as it is there, it determines the fair comparison. Those who compare the right number make the right choice; those who compare the large number sometimes make the wrong one.
Frequently asked questions
What is the difference between gross and usable capacity?
The gross capacity is the total capacity built into the battery; the usable capacity is the part that you can actually draw from after a protective reserve has been deducted. In everyday life, the usable capacity counts – it determines how much electricity the battery storage provides in the evening.
What does depth of discharge (DoD) mean?
The depth of discharge – Depth of Discharge – indicates what proportion of the gross capacity can be drawn. A high depth of discharge means that the usable capacity is close to the gross capacity. The retained reserve protects the cells and extends their service life.
Why can I not empty the battery completely?
Because a complete discharge stresses the cells and shortens their service life. Therefore, the systems deliberately retain a small reserve – a protection, not a defect. Modern batteries such as LFP tolerate a deep discharge well, which is why the usable capacity today is close to the gross capacity.
Which number is in the quote – gross or usable?
That is exactly the point you should look out for: a serious quote shows the usable capacity. Where there is only a large gross number, it is worth asking. Only compare two offers fairly if both mean the same size – ideally the usable kilowatt hour.
How does this affect the battery storage size?
The deciding factor for sizing is the usable capacity, because only this covers your evening and night consumption. Those who base the size on the gross capacity tend to plan too tightly. Therefore, calculate with what the battery storage actually yields.
Free initial consultation
Clear numbers instead of large numbers.
We show the usable capacity in every quote and calculate the price per usable kilowatt hour – so you can compare what really counts.
Want to calculate yourself first? Solar calculator with official roof data →
Prefer to talk? +41 78 830 83 35
Have a battery storage system installed →
Swissolar member and Solarprofi · ESTI installation authorisation (Art. 14 NIV) · in Zurich since 2017 · over 150 systems completed · a personal answer from the specialist company, no call centre
Sources: Manufacturer data sheets of current LFP home batteries (capacity data at 100% depth of discharge, including footnotes in the data sheets of leading providers); HTW Berlin (study on capacity data in data sheets); EnergieSchweiz; empirical values from the planning practice of ecoEn GmbH, Zurich region. (As of: July 2026)
Last updated: 9 July 2026 · Author: ecoEn editorial team
