Usable versus total capacity on a battery label

The big number on the box is usually the total energy the cells can hold. Not all of it is open to you. The part you can actually draw is the usable capacity, and it's smaller.

What the gap is

Many batteries have a limit on how low they can be discharged¹. Rechargeable batteries don't like being run completely flat or filled to the brim. For many battery types, avoiding that is beneficial, or even mandatory for safety reasons².

So a battery management system, which is the control electronics inside the pack, or a charger may keep the battery away from extreme levels of charge². The energy in those top and bottom bands is real. You just can't draw on it.

Other reasons the number shrinks

The held-back band isn't the only cause. The fraction of stored charge a battery can deliver depends on several things, including chemistry, how fast the charge is drawn, temperature and storage time³.

Drawing fast matters too. The higher the discharge rate, the lower the capacity³. A battery rated by a slow, gentle test may give less when you ask for a lot of power at once.

Then there are plain losses. There's always some loss between energy stored and energy output¹. That is a separate idea from usable capacity, and it has its own page: round-trip efficiency.

Which number to plan with

Plan with usable capacity. It's the figure closest to what you can run your devices from.

Two checks keep you honest:

• Look for the words "usable" or "available" on the spec sheet. If the sheet only gives one capacity, read the maker's manual or ask the maker which it is.
• Keep the units straight. A usable figure in kWh is energy. Compare it against the energy your loads use, as explained in kW versus kWh.

A label might list a total and a usable figure close together. The difference between them tells you how much the maker holds back. A small gap isn't automatically better or worse. It reflects the design, and the maker's own documents are the place to confirm what the gap is for.

A picture of the gap

Picture a water tank with a drain slightly above the floor and an overflow slightly below the lid. The tank's full volume is the headline number. The water between the drain and the overflow is the part you can use. The rest is still tank, but it's out of reach.

A battery works the same way in spirit. The cells have a physical size. The control electronics decide which part of that range is open to you. Makers choose the range with lifespan and safety in mind, and the reasons are given in their own documents².

That's why two products with the same headline figure can offer different usable energy. It's also why you shouldn't compare two sheets on the headline number alone. Put the usable figures side by side, and check that both are given under the same kind of test.

How depth of discharge fits in

Usable capacity is often described as a percentage of the total. That percentage is tied to depth of discharge, the share of capacity you're meant to use. The next page, depth of discharge explained, walks through it.