Three terms turn up on every battery datasheet, every tender specification and every supplier quotation. They are not difficult, but they are frequently read loosely — and reading them loosely is how buyers end up comparing two systems that are not comparable.
C-rate: how fast
C-rate is speed, expressed relative to the battery’s own capacity.
At 1C, the battery delivers its entire rated capacity in one hour. At 0.5C it takes two hours. At 0.25C, four hours.
The useful thing about C-rate is that it scales. A 1 MWh battery at 1C delivers 1 MW. A 100 MWh battery at 1C delivers 100 MW. The ratio is the same statement regardless of size, which is why engineers use it in preference to absolute numbers.
This also means C-rate and duration are the same fact stated two ways. When someone says “a four-hour battery”, they are describing a system operating at 0.25C. Our guide to two-hour versus four-hour storage is, underneath, a discussion about C-rate.
Higher C-rates are harder on a battery. Moving charge faster generates more heat and more internal stress, which is why frequency response applications and energy-shifting applications are specified so differently, and why thermal management matters more on high-rate systems.
There is also a hardware constraint: the power rating is set by the power conversion system, so a high C-rate needs a proportionally larger converter, not just willing cells.
Depth of discharge: how much you use
Depth of discharge (DoD) is the proportion of rated energy you actually draw.
Discharge a 100 kWh battery down by 80 kWh and you have used 80 per cent depth of discharge. The arithmetic is simple:
usable energy = rated energy × depth of discharge
Batteries are not routinely run to zero, because deep discharge and the extremes of the range are hard on cells. So a system’s nameplate capacity and its usable capacity are different numbers — and this is where quotations diverge without anyone lying.
Two suppliers quote 5 MWh. One assumes 90 per cent depth of discharge, giving 4.5 MWh usable. The other assumes 80 per cent, giving 4.0 MWh. That is a 12 per cent difference in what you can actually deliver, hidden inside identical headline numbers.
India’s CEA technical standards set a minimum depth of discharge of 80 per cent for grid-connected systems. That is a capability requirement — the system must be able to go that deep — not an instruction about how to operate it day to day. See CEA technical standards.
Depth of discharge also interacts with warranty. Degradation guarantees are written against an assumed envelope of depth, cycles and temperature; operating outside it can affect the cover even if the system permits it technically. Our guide to warranties and degradation guarantees covers what to check.
State of charge: how full it is now
State of charge (SoC) is the battery’s fuel gauge — how full it is at this moment, as a percentage.
The subtlety is that state of charge cannot be measured directly. There is no dipstick for stored charge. The battery management system estimates it from voltage, current history and a model of the cells, which means accuracy varies between suppliers and drifts over a cell’s life.
That matters commercially. Your energy management system makes dispatch decisions using the state-of-charge figure it is given. If the estimate is off by five per cent, the EMS makes confidently wrong decisions — bidding energy it does not have, or leaving energy unsold. This is one of the practical reasons the EMS and BMS division of labour is worth understanding.
How the three fit together
| Term | Answers | Set by | Where it shows up |
|---|---|---|---|
| C-rate | How fast? | PCS rating and cell capability | Your MW rating and duration |
| Depth of discharge | How much? | Cell design and warranty terms | Your usable MWh |
| State of charge | How full now? | BMS estimation | Every dispatch decision |
A worked example. A 5 MWh system at 80 per cent depth of discharge has 4 MWh usable. Run it at 0.5C and it delivers 2.5 MW for two hours. If its state of charge is currently 60 per cent, it has roughly 2.4 MWh available before hitting its lower limit — enough for about an hour at that rate.
None of that is on the front page of the datasheet. All of it determines what the asset can do this evening.
What this means for you
- If you are a C&I buyer: ask for usable energy and the depth of discharge assumed, and compare on that basis. It is the single most common way otherwise honest quotations become non-comparable. The savings calculator works on usable energy for the same reason.
- If you are a developer: state C-rate rather than duration in specifications where precision matters, and confirm the PCS supports it. Duration is a marketing word; C-rate is an engineering one.
- If you are writing a tender: specify depth of discharge, C-rate and the test conditions for capacity verification together. Capacity measured at 0.5C and 25°C is not the same number as capacity at 1C on a hot afternoon — which is exactly what acceptance testing is designed to pin down.
- If you are still sizing: our product range states usable energy and power rating separately, and our team can size a system against a real load profile — get in touch.
Standards and minimum requirements change by notification, and supplier definitions of usable capacity vary. Treat this as an August 2026 snapshot and confirm the definitions used in your own quotations and contracts.