Peak-Shaving Arithmetic for Commercial Rooftops
Commercial tariff structures reward load reduction at specific hours. Sizing storage to the billing peak is a different calculation from sizing to energy.
Key takeaways
- Commercial and industrial customers pay for two things: energy consumed and, in many markets, the maximum demand recorded in a billing period.
- Size the block to clip the top of the load curve rather than to serve the whole peak period.
- After commissioning, compare billed demand against the pre-installation baseline for three to six billing cycles.
Start from the bill, not the roof
Commercial and industrial customers pay for two things: energy consumed and, in many markets, the maximum demand recorded in a billing period. Demand charges can dominate the bill for a site with a spiky profile. Storage that shaves the spike can therefore earn more from capacity than from energy, and the sizing target is set by the billing structure.
Hotel and office profiles
Hotels, retail units and offices have evening loads that persist after the sun sets. A rooftop array without storage exports its surplus at midday and buys power at night. Adding under-panel storage converts the midday surplus into evening self-consumption, which reduces both the imported energy and the peak period between the two tariffs.
The rooftop constraint
Most commercial decks were designed for the roof itself, not for a battery cabinet's point loads and fire separation needs. That is where distributed storage changes the arithmetic: energy spread under the panels needs no structural reinforcement, no plant-room allocation and no separate fire-engineering scope, which removes cost lines that often make a rooftop project uneconomic.
A worked approach
Take a year of half-hourly load data, identify the ten highest billing peaks, and size the storage block to clip the top of that curve. Then check the available noon surplus against the required evening discharge. If the surplus covers the peak, the project is primarily a peak-shaving investment; if it does not, the case rests on self-consumption and yield gain.
Choosing the storage block size
Size the block to clip the top of the load curve rather than to serve the whole peak period. Clipping the highest demand intervals captures most of the billed saving with a fraction of the capacity, and it keeps the asset cycling rather than idling. Where the tariff includes a separate peak period, combine clipping with a modest energy shift into that window.
Planning around load growth
Commercial sites change: equipment is added, occupancy shifts and charging loads appear. Because capacity arrives in small increments, the storage block can grow with the building rather than being sized for a decade-old load profile. Review the recorded demand curve annually and add units where the shape of the curve has changed most.
Battery sizing for clipping duty
Clipping duty is demanding because the storage block charges and discharges at high rates for short periods. Size for the energy needed across the clipped interval rather than for the peak power alone, and check that the discharge duration covers the number of billing intervals being targeted. Under-sizing the energy while matching the power is a frequent mistake that leaves the peak partly uncovered.
What to review monthly
After commissioning, compare billed demand against the pre-installation baseline for three to six billing cycles. Demand charges can change without warning when a site adds equipment, so ongoing review is what preserves the savings that justified the project in the first place.
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