Fast chargers are built to deliver a lot of energy in a short stop. The catch sits behind the charger: every kilowatt it can draw has to come through your connection, your transformer and your bill. A battery energy storage system (BESS) placed between the grid and the charger changes that arithmetic, where the site and the tariff suit it.
Why fast chargers are hard on a connection
A 60 kW charger draws 60 kW only while a vehicle is charging, but your connection, transformer and demand charges must be sized for that peak. Between sessions it may draw almost nothing. So a site can end up paying, every month, for capacity it uses for a few busy hours.
Three things usually bite at once:
- Sanctioned load. The distribution company (DISCOM) supplies up to the load it has agreed. A fast charger can push a site past it, which means an application for more load and, often, new cabling or protection.
- Transformer and upstream works. A bigger connection can need a larger or new distribution transformer, cabling and switchgear. These take time and money before the first session.
- Demand-related charges. Where fixed or demand charges are billed on connected or recorded demand, one short, sharp charging peak can set the bill for the month.
The Ministry of Power's 2024 guidelines allow low-tension (LT) connections for charging up to 150 kW. Above that, a site generally needs a high-tension (HT) connection, with the equipment and approvals that brings.
How a buffer works
A buffer battery sits between the connection and the charger. It separates what the grid supplies from what the vehicle receives: the grid only has to supply the average, and the battery supplies the peak.
- Fill steadily. The grid, and solar if you have it, charges the battery within the connection you already have.
- Boost on demand. When a vehicle plugs in, the charger draws from the grid and the battery together.
- Refill between sessions. The battery recharges between sessions, ideally in cheaper hours or from solar.
An energy management system (EMS) runs the three steps. It watches the site's total load, sets how hard the battery charges and discharges, and keeps enough energy in reserve for the next car. The power conversion system (PCS), the two-way inverter between the battery's DC and the site's AC, is sized for the boost. Some designs connect the battery on the site's AC side; others feed the charger's DC stage directly. Which architecture suits a site depends on the chargers, the space and what else the battery has to do.
Designed well, a buffer can help you:
- Avoid or defer transformer and connection upgrades
- Limit demand peaks
- Keep charging through short outages, where the system is designed and approved to run without the grid
- Use more of your own solar
Sizing the buffer
Two numbers describe the battery: power in kilowatts (kW), how fast it can deliver, and energy in kilowatt-hours (kWh), how much it holds. kW vs kWh explains the difference. The power side is simple: the battery's discharge power plus what the grid can spare must cover the charger's rating. The energy side takes more thought.
The inputs come from how the site will be used:
- Which vehicles will charge, and how much energy each session delivers
- When the busy hours fall, and how many sessions arrive close together
- What the connection can spare after the site's other loads, hour by hour
- Whether solar is available to refill the battery during the day
- How much reserve to hold back for outages or late arrivals
Get the busy-hour pattern wrong and the battery either runs flat mid-session or sits oversized. That is why we model the charging pattern before choosing any hardware, and check it again once real session data comes in. Allow for capacity fade too: a buffer often cycles hard several times a day, so the usable energy it holds in later years matters more than its nameplate on day one.
When a bigger connection is better
A buffer is not always the answer. A bigger connection is often simpler and cheaper when:
- The chargers will be busy with steady use all day, leaving little quiet time to refill a battery
- Upstream costs are low: the transformer has spare capacity, or the line is close by
- Demand or fixed charges for EV connections are low or waived, so a small connection saves little
Every battery also loses some energy in each round trip, needs space and fire-safety design, and needs maintenance of its own. Those costs sit against the upgrade it avoids. Fixed and demand charges for EV connections can change with each tariff order, so check the current position before deciding. We model the current tariff and a bigger-connection option side by side, and show the assumptions.
In Tamil Nadu, EV charging energy is cheapest from 9 am to 4 pm and dearest from 6–9 am and 6–10 pm. Smart time-of-day meters are mandatory.
EV charging has its own time-of-day (ToD) tariff categories in Tamil Nadu, LT-VII and HT-V, which include battery swapping and charging stations. So a battery can store daytime energy, from the grid or from solar, for evening charging, as long as the windows stay where they are.
Policy support
National guidelines already make room for storage at charging sites. The Ministry of Power's 2024 guidelines do so in two clauses, cl. 18(4) and cl. 19:
Public charge point operators may complement stations with storage, and solar carports with battery storage are an alternative to grid-dependent charging.
NFPA 855 (2026 edition) covers the installation of stationary energy storage, including EV charging with integrated storage.
Neither makes a buffer compulsory, and NFPA 855 is a US standard, not an Indian rule. It is still a useful reference for spacing, detection and suppression when a battery stands beside a public charger. We design a buffer to the same layered approach as any battery system: qualified cells, cell-level monitoring, detection, suppression and separation distances.
If your connection is the constraint, send us the sanctioned load, the charger plan and a year of bills, and we'll model both routes. Talk to an engineer, or see how buffers fit into our EV charging work.