Power and energy are two different numbers
The single biggest source of confusion in battery storage is treating “how big is it?” as one question. It is really two.
- Power is measured in megawatts (MW). It is how much a battery can push out at any instant — the size of the tap.
- Energy is measured in megawatt-hours (MWh). It is how much the battery holds in total — the size of the tank.
Duration is simply the tank divided by the tap:
Duration (hours) = Energy (MWh) ÷ Power (MW)
So a 100 MW battery holding 400 MWh is a 4-hour system: run it flat out at 100 MW and it empties in four hours. The same 100 MW paired with only 200 MWh is a 2-hour system. Same power, half the run time, because it has half the cells. This is why a project is almost always written as a pair, like “100 MW / 400 MWh” — you need both numbers to know what it actually does.
C-rate: the same idea, flipped
You will also see C-rate on datasheets. It is just the inverse of duration — how fast the battery charges or discharges relative to its capacity. A 4-hour battery discharges at 0.25C (a quarter of its capacity per hour); a 2-hour battery at 0.5C; a 1-hour battery at 1C. A lower C-rate (longer duration) is gentler on the cells and generally means longer life, which is one reason 4-hour systems have become the comfortable design point for daily cycling.
Why the cost does not simply double
Here is the useful part for budgeting. When you go from a 2-hour to a 4-hour system at the same power, you roughly double the battery cells — but you do not double everything. The inverters, transformers, balance-of-plant and land are sized to the MW rating, and that has not changed. So adding duration adds cell cost but reuses the expensive power-conversion and site infrastructure. Going longer is cheaper per added hour than the first hour was — the same logic that makes very long durations a target for other chemistries like vanadium flow. This also feeds straight into economics; see our BESS price guide for how ₹/kWh moves with system size.
Why India moved from 2-hour to 4-hour
Until recently, Indian tenders leaned on 2-hour systems. SECI’s first standalone battery tender back in 2022 was 500 MW × 2 hours. The newest flagship — a SECI FDRE tender — is 1,200 MW × 4 hours, or 4,800 MWh. Four times the energy in the box. Three things drove the shift:
- The evening peak is a plateau, not a spike. Demand now stays high from roughly 5–11 PM in winter and 7 PM to midnight in summer. Two hours covers only the sharpest moment; four hours covers the real window.
- Firm renewable contracts need it. FDRE and round-the-clock tenders promise dispatchable power across a long window, which only longer-duration storage can honour.
- Cheaper cells made it affordable. Falling LFP prices made 4-hour systems financially viable in a way they were not in 2022.
The market reflects it: 4-hour BESS in India is projected to grow from around 4 GW in 2028 to roughly 67 GW by 2032. Naturally, longer duration costs more per MW of capacity — 2-hour tenders have cleared around ₹1.5–2.5 lakh/MW/month while 4-hour tenders run higher — because you are paying for more stored energy.
Matching duration to the job
| Duration | Typical job | Where you see it |
|---|---|---|
| 1–2 hours | Fast frequency response, quick peak shaving | C&I sites, DISCOM ancillary |
| 4 hours | Evening peak supply, FDRE, RTC | SECI / NTPC national tenders |
| 6–8+ hours | All-day shifting, deep reserves | Flow batteries, pumped hydro |
Note that policies and tender designs change by notification — the duration mandated in a given tender can shift, so verify the current requirement in the specific RFP before you bid or size around it.
What this means for you
Do not buy duration you will not use, and do not under-size it either. The right number falls out of your actual demand shape: measure how many hours you need to cover — the length of your evening peak, the window your Time-of-Day tariff penalises, or the dispatch window a tender specifies — and size energy to that, then size power to the biggest load you must serve at once. A factory trimming a two-hour demand-charge peak needs a very different system from a developer bidding a 4-hour FDRE tender. Our guide on how to size a BESS for your site walks through it, and our savings calculator will estimate the duration and cost for your own load and tariff — or just talk to our team and we will work it out with you.
Reflects tender data and market projections as of July 2026. Tender durations, tariffs and pipeline forecasts change frequently — verify current figures before financial decisions.