BESS Sizing Calculator

Size a battery energy storage system for backup, peak shaving, solar self-consumption or time-of-use — usable and nameplate kWh, PCS kW, racks, footprint and cycle life, with the formulas shown.

Application



Solar PV
Battery parameters (editable)

Worked example

A plant wants 4 hours of backup for 100 kW of critical load. With LFP cells (90 % DoD), 92 % round-trip efficiency, 97 % inverter efficiency, 80 % end-of-life capacity and a 5 % margin:

  • Usable energy = 100 kW × 4 h = 400 kWh
  • Nameplate = 400 ÷ (0.90 × 0.92 × 0.97 × 0.80) × 1.05 ≈ 654 kWh — about 7 racks of 100 kWh
  • PCS = 100 kW, C-rate 0.15 C, footprint about 23 m²

Switch to peak shaving — 800 kW peak, 600 kW ceiling, 2-hour peak — and the tool gives 400 kWh usable, about 654 kWh nameplate and a 200 kW PCS, cycling once a day for an indicative 15-year life.

How it is calculated

1. Usable energy by application

  • Backup: E_usable = P_load × t_backup
  • Peak shaving: E_usable = (P_peak − P_threshold) × t_peak; PCS = P_peak − P_threshold
  • Solar self-consumption: surplus = (0.75 × kWp − P_day) × sun hours; E_usable = surplus
  • Time-of-use: E_usable = daily kWh shifted; PCS = E_usable ÷ discharge hours

2. Nameplate capacity

E_nameplate = E_usable ÷ (DoD × η_round-trip × η_inverter × EoL) × (1 + margin)

3. Power and C-rate

C-rate = PCS kW ÷ E_nameplate kWh. Keep ≤ 0.5 C for daily cycling, ≤ 1 C for backup.

4. Life and throughput

Throughput per year = E_usable × cycles per day × 365. Indicative life = 6,000 cycles ÷ cycles per year, capped at 15 calendar years.

5. Solar payback (optional)

Annual saving = surplus kWh/day × (import tariff − export tariff) × 365. Payback = E_nameplate × ₹22,000 ÷ annual saving. Demand-charge and DG-diesel savings are not included and often dominate.

Assumptions and limits

  • LFP chemistry: 90 % DoD, 92 % round-trip, 6,000 cycles; 100 kWh per rack; 0.035 m² per kWh including clearances.
  • Installed cost ₹22,000 per kWh for the payback hint (2026 indicative, varies with size and PCS rating).
  • PV output at 75 % of kWp for the stated full-sun hours; Indian sites typically see 4.5–5.5 effective hours.
  • Backup sizing assumes no PV contribution during the outage (conservative).
  • A measured 15-minute load profile over at least one month is required for a firm design.
  • Standards referenced: IS 16270, IEC 62619, IEC 62933, UL 9540A, NFPA 855, CEA regulations.

Frequently asked questions

What is the difference between usable and nameplate capacity?

Usable energy is what the load actually receives. Nameplate (rated) capacity is larger because of depth of discharge, round-trip and inverter losses, and the capacity the battery loses by end of life. Nameplate = usable ÷ (DoD × η_rt × η_inv × EoL). With typical LFP values that ratio is about 1.55.

How many hours of backup should we size for?

Size for the longest outage you must ride through fully, not the longest outage you have ever had. Many industrial sites choose 30–60 minutes of full load (enough to shut down safely or start a DG set) and longer durations only for critical loads such as servers, cold rooms or life-safety systems.

What C-rate is acceptable?

C-rate is power divided by energy (kW/kWh). LFP systems are comfortable at 0.5C for daily cycling and up to 1C for short backup duty. Above 1C, cell heating and degradation increase; the calculator flags it.

Is peak shaving worth it on an Indian C&I tariff?

Usually when the contract-demand charge is ₹300 per kVA per month or more and the peak lasts under 2–3 hours. A 200 kW / 400 kWh system that removes 200 kVA of demand saves roughly ₹7–12 lakh a year before any time-of-day benefit.

DC-coupled or AC-coupled with solar?

DC-coupled shares one inverter with the PV array, is cheaper and loses less energy, and is the natural choice when PV and storage are installed together. AC-coupled adds storage to an existing PV plant without touching its inverters and can also work with a DG set.

Which standards apply to a BESS in India?

IS 16270 and IEC 62619 for cells and batteries, UL 9540A for propagation testing, NFPA 855 for installation, IEC 62933 for system performance and safety, and CEA technical standards for connectivity. Fire detection, gas detection and a BMS with remote monitoring are part of every Syncsine design.

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