BESS Sizing: Power, Energy and Backup Duration
Estimate BESS power and usable energy from a critical-load profile, then identify the assumptions that require supplier confirmation.
Engineering guidance. Examples are design assumptions, not product guarantees. Reviewed October 29, 2026.
Start with the load, not a cabinet size
Size power and energy separately. Power describes the instantaneous load the system must supply; energy describes how much electricity must be delivered over the required period. Collect a time-stamped load profile and identify which circuits actually need backup. A monthly electricity bill alone cannot establish motor starting requirements or outage autonomy.
Calculate the energy delivered to the load
For each interval, multiply average critical-load power in kW by interval duration in hours, then add the results. An illustrative 40 kW load for three hours requires 120 kWh delivered at the chosen AC measurement boundary. This arithmetic assumes a constant load; use interval data when the load varies. It is not a specification for any product listed on this site.
Translate delivered energy into battery capacity
If a preliminary design assumes 90% one-way battery-to-load efficiency and an 80% usable operating window, the illustrative nominal capacity is 120 / (0.90 × 0.80), or about 167 kWh. Both percentages are design assumptions, not verified product values. Do not substitute round-trip efficiency for one-way discharge efficiency. If the supplier quotes usable AC energy, check what losses and operating limits are already included before applying another allowance.
Check power and operating constraints
Confirm continuous AC output, short-duration overload capability, ambient-temperature derating, battery discharge limits and auxiliary loads against the selected equipment documents. Review starting loads and load sequencing separately from average consumption. Ask for the guaranteed usable energy at the required temperature and end-of-warranty condition. A nominal kWh label does not establish those guarantees.
Specify the operating mode
Backup requires a defined islanding and protection design. A grid-connected storage installation must not be assumed to support an isolated load during an outage. Confirm grid-forming capability, transfer arrangement, neutral and earthing design, restart sequence and the loads allowed during backup. Product-specific capability and local approval requirements remain To be confirmed.
What should you send for a sizing review?
Provide location, voltage and frequency, load interval data, critical-load list, outage duration target, motor starting information, installation conditions and existing solar or generator details. State whether the objective is backup, peak reduction or solar self-consumption. Reserve backup energy explicitly if the same system is also dispatched for another purpose.
FAQ: Is 100 kW / 200 kWh always two hours?
The nominal ratio is two hours. Actual AC runtime depends on usable energy, conversion losses, auxiliary consumption, temperature, power limits and battery condition. Request a runtime calculation at a stated measurement boundary rather than treating the nameplate ratio as a guaranteed duration.
Sources and scope
The terminology for power and stored energy follows EIA's explanation of battery power and energy capacity. Calculations are worked arithmetic examples. The procurement checklist is editorial guidance for defining a project scope, not a compliance standard or a verified product specification.
Continue planning your project
- Battery energy storage systems
- PCS and inverter configuration
- Integrated storage solutions
- BESS Quotation Checklist for EPC and C&I Buyers