Q0013
A commercial or industrial BESS should be sized from interval load data and a defined operating objective—not from annual electricity use or solar capacity alone. Power in MW determines how much load the battery can serve at once; usable energy in MWh determines how long it can do so.
Oversizing raises capital cost and leaves capacity underused. Undersizing may fail to cover the facility’s demand peak, desired backup period or solar surplus, producing disappointing savings and operational performance.
#### Step 1: Define the primary use case
The owner should first rank the objectives:
One system can perform several functions, but one should govern the base design.
#### Step 2: Collect appropriate data
Use at least 12 months of interval data where possible, preferably at 15- or 30-minute resolution, covering:
Annual kWh cannot reveal the size or duration of a peak.
#### Step 3: Determine the power requirement
For peak shaving:
> Required BESS power ≈ uncontrolled peak demand − target grid demand
If a facility peaks at 5 MW and wants to cap grid demand at 4 MW, the battery must provide approximately 1 MW, plus appropriate margin and allowances for inverter limits and auxiliary consumption.
For backup, power must cover the coincident critical load and, where applicable, motor-starting or transient requirements.
#### Step 4: Determine usable energy
For a roughly constant required discharge:
> Usable energy ≈ required power × required duration
A 1 MW reduction lasting two hours requires approximately 2 MWh of usable delivered energy. Installed nameplate energy must be higher after accounting for:
#### Step 5: Model dispatch across the complete year
A valid model should simulate:
Peak shaving fails if the battery discharges too early and empties before the true peak.
#### Step 6: Size solar-related storage from surplus profile
Do not automatically size the battery at one hour of solar capacity unless a regulation requires it. Examine:
For some Malaysian schemes, minimum battery sizing is prescribed. For example, current non-domestic SELCO guidance requires solar installations above 72 kWp to include BESS with at least a one-hour energy rating, subject to the applicable conditions and implementation dates. Energy Commission SELCO guidelines
That is a compliance floor, not proof of economically optimal sizing.
#### Step 7: Size backup from critical loads
Separate essential from non-essential loads. Required energy depends on:
A battery reserved for backup has less capacity available for daily arbitrage.
#### Step 8: Check the electrical connection
Confirm:
#### Step 9: Optimise economically
Compare several power-duration combinations instead of accepting one supplier proposal. Evaluate:
Until site data are analysed, it is not possible to determine:
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