Sizing a Battery Energy Storage System for Peak Shaving

Peak shaving is one of the more straightforward financial cases for a Battery Energy Storage System, and one of the easiest to size incorrectly. The goal is simple: discharge the battery during the site's highest-demand periods so the peak demand recorded by the utility meter is lower, which directly reduces the demand charge component of an industrial electricity bill.

The sizing conversation starts with a load profile, not a battery catalogue. A facility needs at least fifteen-minute interval demand data across a representative billing period — ideally one that includes a seasonal peak — before any storage capacity number means anything. Without that data, a BESS proposal is a guess dressed up as an engineering recommendation.

From the load profile, two numbers matter more than any other: the power rating, in kilowatts, needed to cover the gap between the target peak and the actual peak; and the energy rating, in kilowatt-hours, needed to sustain that discharge for the duration the peak typically lasts. Undersizing power means the battery cannot discharge fast enough to shave the peak. Undersizing energy means it runs out before the peak period ends.

It is tempting to size for the worst peak of the year, but that is usually the wrong target. A system sized for the 95th-percentile peak, with a clear-eyed acceptance that a handful of extreme days each year will see a smaller-than-ideal reduction, is very often the better financial outcome than a system sized for the absolute maximum — because the incremental capacity required to cover that last five percent is disproportionately expensive relative to the demand charge it saves.

Finally, peak shaving rarely justifies a BESS on its own in isolation from other value streams. The stronger financial case almost always stacks peak shaving with solar firming, backup power resilience, and — where available — time-of-use energy arbitrage, evaluated together rather than one at a time.