Direct answer: Battery peak shaving sizing is the process of choosing storage power and duration to reduce the highest demand intervals that drive a site’s demand charges. Start from time-stamped interval data, set the demand target, and size the battery to cover the gap for the duration of the peaks, while allowing for efficiency losses and forecast error. The result is a kilowatt rating and a kilowatt-hour duration, both derived from the load shape rather than guessed.
Peak shaving pays when a site has demand charges that reward lowering the highest short-term load. The battery discharges during those peaks and recharges when the load is low. The economics depend on how high and how long the peaks are, which is why interval data is the foundation of the calculation.
This guide explains how to turn interval data and a demand target into a storage size. It is a planning reference; the final size and dispatch must be confirmed with the utility tariff and an engineering review.
Start with time-stamped interval data
Interval data records the load in short steps, often 15 minutes or one hour, over many months. The interval length matters because a demand charge is typically based on the highest average load over a fixed window. A battery sized against daily totals will miss the spikes that actually set the charge.
Collect at least a year of data where possible so the sizing covers seasonal variation. Shorter histories can still be useful, but they should be treated as a planning assumption and checked against the expected annual profile.
Check the data for gaps and outliers before trusting it. A missing day or a metering error can hide the highest interval, and the battery would then be sized against an incomplete peak. Clean the data and note any assumptions made.
Set the demand target before the battery size
The demand target is the load level the battery will try not to exceed. Lowering the target saves more on the demand charge but requires a larger battery, so the target and the battery size are decided together. Start with the utility tariff to understand how the charge is calculated and which window sets the bill.
For each candidate target, count the intervals that exceed it and the size of the excess. That excess, not the total energy use, is what the battery must cover.
Power, duration and the actual load shape
The battery power rating must cover the largest excess in any single interval, and the energy capacity must cover the excess across the longest run of consecutive peak intervals. A site with one short sharp spike needs high power and little energy, while a site with a broad afternoon plateau needs more duration for the same power.
Reading the load shape answers both questions. The same kilowatt rating can require very different kilowatt-hour capacities depending on whether the peaks are isolated or clustered.
When several peaks have similar height but different durations, size for the worst combination the project intends to cover. Choosing a shorter duration saves cost but leaves the longer peaks partially unshaven, which may be acceptable if the tariff only charges on a shorter window.
Dispatch logic and forecast error
No dispatch is perfect. The battery may start a peak at the wrong state of charge, the load may arrive earlier than modeled, or a forecast error may leave the battery empty just before the real peak. The sizing should include a reserve for forecast and dispatch error so the system can still meet the target on an imperfect day.
| Input | What it determines | Typical source |
|---|---|---|
| Interval data | Peak magnitude and duration | Utility meter |
| Demand target | Allowed residual peak | Project economics |
| Efficiency losses | Extra capacity needed | Manufacturer or test |
| Forecast reserve | Margin for imperfect dispatch | Project assumption |
As an illustrative example, if the highest demand interval exceeds the target by 100 kW for two consecutive hours, the battery needs roughly 100 kW of power and 200 kWh of delivered energy before efficiency and reserve. The exact figure depends on the tariff and the dispatch rules.
Recharge window matters too
The battery must recharge between peaks without creating a new demand spike. If the recharge is scheduled during a high-load period, the charger itself adds load and can set a new peak. Model the recharge schedule with the same interval data, and cap the charge rate so the combined load stays below the target.
Efficiency losses and usable energy
The energy the battery must store is larger than the energy it delivers. Round-trip efficiency, auxiliary loads and the allowed depth of discharge all increase the installed kilowatt-hour rating. A 90 percent round-trip efficiency means the battery must charge about 11 percent more energy than it delivers, before any depth-of-discharge limit.
Auxiliary loads such as cooling fans and controls also consume energy while the system operates. They should be counted as part of the round-trip loss so the net delivered energy is the number the dispatch can rely on.
Carry these factors explicitly. Treating the delivered kilowatt-hours as the installed rating will undersize the system and leave the demand target unmet on real days.
Utility rules change the optimum
The tariff defines what counts as the peak, how the demand is measured and whether there are time-of-use limits. Some tariffs use a ratchet based on a past peak, some average over a specific window, and some treat exports differently. The optimum battery size and the recharge schedule both depend on these rules, so the sizing should be rechecked when the tariff changes.
A dispatch that is technically possible may not be economic under a different tariff. Confirm the measurement window and the rate structure before locking the battery size.
Frequently asked questions
How do I size a battery for peak shaving?
Start with time-stamped interval data, choose a demand target, then size the power to the largest excess interval and the energy to the longest run of excess intervals. Add efficiency losses and a forecast reserve.
Why is interval data more important than monthly totals?
Because demand charges are based on the highest short-term load, not total energy. Monthly totals hide the spikes that set the charge, so interval data is needed to see the peak magnitude and duration.
What is a forecast reserve?
It is a margin for imperfect dispatch and load forecast error. Without it, a mistimed discharge can leave the battery empty before the real peak and miss the demand target.
Does the tariff change the battery size?
Yes. The measurement window, ratchet rules and time-of-use structure change which intervals matter and how much value the battery creates, so the size should be rechecked against the actual tariff.
What to send for a technical review
Share the interval data, the utility tariff, the demand target and the operating schedule with the AllGrand engineering team. The team can review the proposed power, duration and dispatch against the product range.
Review the 522 kWh liquid-cooled energy storage cabinet or read more energy storage selection guides. AllGrand Group has manufactured batteries since 1987 and supplies energy storage systems, lead-acid batteries and cells to customers in more than 100 countries. Final sizing must be confirmed for the actual tariff and site.
Sources and further reading
These sources were used to verify general peak-shaving and demand-charge concepts. The sizing steps and worked example are original AllGrand editorial work.