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    How to size a BESS

    6 min readLast reviewed: August 2026
    Short answer

    Size a BESS in two steps. First set power in kW from the largest peak you must shave or the load you must carry. Then set energy in kWh from how long you must sustain it, divided by round trip efficiency and usable depth of discharge, with an allowance for capacity fade to 80 percent at end of life. Everything else follows from these two numbers.

    A BESS quoted only in megawatt hours tells you nothing. The interval data behind the number is where the project succeeds or fails.

    Start from interval data

    Ask the utility or the meter for at least twelve months of 15 minute interval data. From it you can see the true peak, how often it occurs, how long it lasts, and whether it is seasonal. Sizing from a monthly bill overstates energy and understates power almost every time.

    A worked example

    A factory peaks at 900 kW against a 700 kW target, for up to 90 minutes, several times a month. Power required is 200 kW. Energy at the terminals is 200 kW multiplied by 1.5 hours, which is 300 kWh. Divide by 0.90 round trip efficiency and 0.90 usable depth of discharge, giving about 370 kWh. Add 20 percent for end of life capacity retention and the installed size is roughly 450 kWh behind a 250 kW power conversion system.

    What people forget

    Auxiliary consumption for cooling, which can reach several percent in summer. Cycle count against warranty, since a system cycled twice daily uses its warranty far faster than one cycled weekly. Grid connection limits, which may cap charging power. And the physical envelope: foundation, access, fire separation distances, and cable routes are often the real constraint on a tight industrial site.

    Frequently asked

    How much data do I need before sizing?
    Twelve months of 15 minute interval data is ideal, because it captures seasonal peaks. Three months can support a feasibility study but should not be the basis of a final purchase.
    What round trip efficiency should I assume?
    85 to 92 percent measured at the AC terminals for a modern lithium system, including conversion losses and auxiliaries. Anything above that in a proposal deserves a written explanation.
    Should the BESS also provide backup?
    It can, but it must be specified for it. Backup needs a grid forming converter and a changeover arrangement, and reserving energy for outages reduces what is available for peak shaving.

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