Ask for a battery quote and the first question is usually “how big?”. The answer needs two numbers, not one. A battery energy storage system (BESS) is described by its power, in kilowatts, and its energy, in kilowatt-hours. Get the first wrong and the battery can't carry your load. Get the second wrong and it runs out before your peak or outage ends.
The tap and the tank
Think of a battery as a water tank with a tap. The tank's size is energy, in kilowatt-hours (kWh): how much the battery stores. The tap's size is power, in kilowatts (kW): how fast energy flows in or out. A 250 kW / 500 kWh battery can deliver 250 kW for about two hours.
The tap is set mostly by the power conversion system (PCS), the two-way inverter between the battery's DC and your site's AC. The tank is set by how many cells, modules and racks sit behind it. The two are chosen separately, so the same 500 kWh of cells can sit behind a small PCS or a large one.
Your electricity bill already uses both ideas. Energy charges are per kWh: the tank. On a Tamil Nadu high-tension (HT) bill, the demand charge is per kVA of maximum demand: the tap.
Duration and C-rate
Duration is energy divided by power: 500 kWh ÷ 250 kW = 2 hours. C-rate says the same thing the other way round: a battery that empties in two hours runs at about 0.5C, and in four hours about 0.25C. On a commercial site, duration follows how long your peak or outage lasts.
Most sizing questions lean one way or the other:
| The job on your site | Mostly a question of | Why |
|---|---|---|
| Capping a 15-minute demand spike | Power (kW) | The spike is steep but short |
| Covering a two-hour evening peak | Energy (kWh), with power to match | The battery must keep delivering for the whole window |
| Bridging the gap until a genset takes over | Power (kW) | The gap lasts seconds to minutes, at full site load |
| Backing up critical loads through a long outage | Energy (kWh) | It has to last as long as the outage |
A higher C-rate needs more PCS for each kWh and works the cells and cooling harder. A lower C-rate is gentler on the cells but costs more for each kW. The right balance comes from your load data, not from a catalogue.
Not every kWh is usable
The number on the nameplate is not the number you can use every day. Three things sit between them.
- Depth of discharge (DoD) is the share of the nameplate you cycle. The Central Electricity Authority's (CEA) 2026 construction standards set a minimum of 80%.
- Round-trip efficiency is what you get back for what you put in, measured AC to AC including cooling and auxiliaries. The CEA's floor is 70%, and industry planning references such as NREL's use about 85%.
- Degradation: capacity fades with cycles and time, so plan for it, or for augmentation (adding modules later) if the energy must be held.
The CEA's 2026 construction standards, in force from 1 April 2027, set depth of discharge of at least 80% and AC-to-AC round-trip efficiency, including auxiliaries, of at least 70%, with performance guarantees of at least 90%, 80% and 70% at 5, 10 and 15 years.
Industry planning references such as NREL's use about 85% round-trip efficiency and about a 15-year life, with augmentation, for lithium-ion storage.
Ask where each figure is measured. Efficiency quoted at the cell or on the DC side is well above what reaches your meter, so compare usable AC energy at the point of connection.
A worked example
Real sizing starts from your data, not a round number. A 15-minute load profile often shows a peak that is narrower, wider or more jagged than the bill suggests. It also shows whether the same battery can take on other jobs, such as capping demand or covering outages, which changes the size again.
In Tamil Nadu, the tariff windows shape the answer too. See what a battery can and can't do under the peak-hour tariff.
What we need to size yours
To size a battery for your site, we start with these. If you don't have them all, start with the bills; we can help collect the rest.
- Last 12 months of electricity bills
- 15-minute interval data from your meter, if available
- Contract demand and sanctioned load
- Single-line diagram of your electrical network
- Gensets: rating, age, running hours and fuel records
- Solar: capacity, inverter make, metering or open-access details
- Outage history, even rough notes
Send what you have and talk to an engineer. We'll come back with a size, the assumptions behind it and the jobs it can do.