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There is no standard battery size for peak-demand reduction. Size the system from the facility’s interval net-load data and utility tariff: kilowatts (kW) determine how much demand the battery can remove at one moment, while usable kilowatt-hours (kWh) determine how long it can hold the grid import below a chosen cap. A defensible design tests both limits in a time-series dispatch model that includes the tariff’s demand windows, state of charge, conversion losses, operating limits and degradation.
Without the site load profile, tariff, operating schedule and target grid-import cap, no responsible engineer can specify a numerical BESS size, savings figure or payback period. The workflow below shows how to produce those figures for a construction project or other facility.
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What “sizing for peak demand” actually means
A battery energy storage system (BESS) used for peak shaving sits behind the utility meter and discharges when the facility’s net load would otherwise exceed a selected grid-import limit. The objective is usually to reduce the demand value used by the utility to calculate a demand charge, not merely to reduce monthly energy consumption.
DOE FEMP describes demand charges as charges based on maximum demand in kilowatts during a billing period, commonly each month. The tariff controls which intervals count, whether seasons or time-of-use periods apply, and whether a separate system-peak component is billed. A battery that misses the tariff’s chargeable interval may provide little demand-charge value even if its nameplate capacity is large.
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| Quantity | What it controls | Question it answers |
|---|---|---|
| Power rating (kW) | Maximum instantaneous AC discharge | How much site load can the battery offset at once? |
| Usable energy (kWh) | Stored energy available within the permitted state-of-charge window | How long can the offset continue before the battery reaches its operating limit? |
| Duration (hours) | Usable energy divided by discharge power | Can the system cover the entire tariff-relevant peak episode? |
| Round-trip efficiency | Energy lost while charging and discharging | How much additional energy must be purchased or generated to deliver the required output? |
Both dimensions are mandatory. A high-power system with too little energy can clip only the first few minutes of a peak. A large-energy system with an undersized inverter cannot remove enough kW when several loads start together.
Start with the tariff, not the battery catalog
Confirm how demand is measured
Obtain the current utility tariff and identify the demand-measurement interval, billing period, ratchet or minimum-demand rules, seasonal periods, time-of-use windows and any coincident system-peak charge. Rates can differ by service voltage and may use the highest monthly kW peak or another defined set of peaks. Do not assume that the facility’s annual maximum load is the only value that drives the bill.
Separate demand and energy charges
Demand charges are based on a kW peak; energy charges are based on kWh consumed. A peak-shaving battery primarily targets the first category, although charging losses and the timing of charging can change the energy bill. If the project also seeks time-of-use arbitrage, resilience or generator-fuel savings, those objectives must be represented as additional dispatch priorities rather than assumed to follow automatically from demand shaving.
Map the chargeable periods
Create a calendar of every interval that can affect the bill: monthly windows, seasonal schedules, weekday and weekend rules, holidays, and any utility-defined system peak. The candidate battery must be tested against those windows. A controller optimized for a daily peak may cycle more frequently than one that discharges once during a monthly peak, affecting degradation and replacement planning.
Data required before calculating kW or kWh
Collect data at the same time resolution and time zone used by the tariff and controls. At minimum, obtain:
- Interval facility load for a period that captures seasonal, production and weather variation.
- The complete current tariff, including demand definitions, rates, billing periods and adjustment clauses.
- Synchronized photovoltaic or other on-site generation output, if present, so the model uses net load rather than gross facility load.
- Operating schedules for major equipment, including temporary construction loads such as cranes, hoists, compressors, pumps, welding equipment, heating, cooling and site offices.
- Planned changes in connected load, construction phases and commissioning dates.
- Any export restriction, interconnection limit, generator constraint or emergency operating requirement.
- The proposed battery’s AC power limit, usable energy, SOC limits, charge limit, temperature range, availability assumptions and control interface.
NREL’s sizing work found that the optimal configuration depends heavily on facility load shape, installed solar capacity and rate structure. Monthly kWh totals cannot reveal whether a short coincident event or a long recurring plateau is responsible for the charge.
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A practical sizing workflow
1. Build the net-load series
For each interval, calculate:
Net load = facility demand − synchronized on-site generation
Use the utility meter’s perspective. If export is prohibited, enforce that constraint in the model. Keep the original measured load, generation and net-load columns so an unexpected result can be traced to the correct source.
2. Identify the peaks that are actually billable
Apply the tariff calendar to the interval series and mark every interval that can set or contribute to billed demand. Record the baseline maximum and the associated date, time, season and operating condition. Also mark repeated near-peak events; a battery sized only for one isolated maximum may be empty when the next chargeable event arrives.
3. Select a candidate grid-import cap
Choose a provisional cap below the relevant baseline peaks. A lower cap generally requires more instantaneous battery power and may require more stored energy, but its value depends on which tariff peaks are reduced. There is no universally optimal cap. Evaluate several caps against the actual demand rate, the number of affected intervals and the cost of the additional equipment.
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For each interval, the ideal AC discharge needed to hold the grid at the cap is:
Required battery discharge = max(0, net load − grid-import cap)
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The battery cannot exceed its inverter discharge limit, and the controller must reduce output when SOC, temperature, availability or interconnection rules impose a constraint. The design power should reflect repeated peaks and coincident events, not just the single largest subtraction.
5. Calculate energy across each peak episode
For each consecutive group of intervals above the cap, sum the required discharge energy:
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Then convert that requirement to installed usable capacity using the permitted SOC window and the modeled charge/discharge losses. A simple peak-power-times-duration calculation is only an initial screen. Irregular peaks, pauses, and a second event before recharge require time-series dispatch.
6. Simulate dispatch and the tariff bill
Run candidate kW/kWh combinations through a model that includes the tariff, charge rules, SOC limits, efficiency, standby consumption, availability, degradation and any PV or generator interaction. NREL’s SAM battery dispatch documentation describes a behind-the-meter controller that forecasts the peak period and attempts to meet a grid-power target subject to battery capacity and SOC. Compare each candidate’s billed demand and energy charges with the identical baseline load without the battery.
7. Test life-cycle operation
Repeat the dispatch over representative years or a suitably conservative operating profile. Check whether the battery can recharge between events, how often it cycles, when usable capacity falls below the target, and whether augmentation or replacement is needed. Confirm the warranty’s definition of usable capacity and power availability; nameplate ratings are not automatically deliverable at every temperature or SOC.
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Equations and constraints for a transparent model
A simple interval model can expose the sizing logic before detailed software is used. Let Lt be net load, C the grid-import cap, Pmax the battery’s AC discharge limit, and Δt the interval length.
- Unconstrained requested power: preq,t = max(0, Lt − C).
- Actual discharge power: pt = min(preq,t, Pmax, available power at SOC and temperature).
- Delivered energy in an interval: et = pt × Δt.
- SOC update: subtract discharge energy adjusted for the model’s discharge efficiency and add charging energy adjusted for charging efficiency; enforce minimum and maximum SOC at every interval.
- Tariff result: recalculate the demand value using the utility’s exact billing rule, then add energy charges and any applicable export or standby charges.
Use AC-side quantities when comparing the battery with the utility meter. A vendor’s DC battery capacity, inverter rating and round-trip efficiency may use different measurement boundaries, so ask for the precise definition of each figure.
Illustrative calculation (not a project estimate)
Suppose a hypothetical facility has a 700 kW net-load interval and the design team is evaluating a 500 kW grid-import cap. The requested discharge for that interval is 200 kW. If the same 200 kW requirement continues for two hours, the AC energy delivered is 400 kWh before accounting for SOC limits and losses. If a model assumes an 80% usable SOC window, the installed DC energy would need to be larger than 400 kWh; the exact value depends on the battery’s AC/DC boundary and efficiency assumptions.
This arithmetic does not establish a recommended size. The model must also test whether another 200 kW event occurs before recharge, whether the inverter can sustain the output at the expected temperature, and whether the utility actually bills that two-hour period as demand.
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Use a defined efficiency boundary
Round-trip efficiency is a modeling input, not a universal product rating. NREL’s 2024 Annual Technology Baseline uses 85% as a representative round-trip-efficiency assumption for its utility-scale battery model. The 2022 ATB used 86%. Those values are dated national modeling assumptions, not specifications for an arbitrary current system. Use the selected system’s current efficiency at the stated power, temperature and measurement boundary.
Do not generalize study-specific inverter figures
A 2013 NREL demand-charge sizing study modeled a lithium-ion system with a 93% efficient inverter. That figure describes the study’s assumptions and should not be treated as a current market norm. A proposal should state whether efficiency is measured DC-to-DC, AC-to-AC, or at another boundary and whether auxiliary loads are included.
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Model usable capacity, not nameplate capacity
Reserve SOC for battery protection and operating requirements. Apply temperature derating, inverter limits, planned outages and controls availability. Then model capacity fade and the point at which augmentation or replacement is required. SAM’s battery models include degradation and replacement considerations; omitting them can make a system appear adequate only in its first year.
How construction projects change the sizing problem
Temporary and changing loads
Construction demand can move sharply as phases change. A system sized for early earthworks may be too small during fit-out, while a system sized for the final building can be uneconomic during a short project. Build separate load cases for each phase and identify the tariff months in which each phase occurs.
Motor starts and sensitive equipment
Cranes, pumps, compressors and HVAC can create short high-power events. Verify whether the tariff records those events in its demand interval and whether the inverter can provide the required ramp rate. If the objective includes power quality or ride-through, specify those requirements separately from monthly peak shaving.
Limited recharge opportunities
On a constrained temporary service, the battery may not be able to recharge immediately after a morning peak. Include generator or PV availability, charging limits, site access restrictions and the next scheduled peak in the dispatch model.
Relocation and interconnection
For a mobile or leased system, document the service voltage, protection settings, export controls, grounding, communications and commissioning requirements at every site. A kW/kWh calculation that ignores interconnection limits is not an installable design.
Compare candidate systems on the measures that affect the bill
| Comparison axis | What to request | Why it matters |
|---|---|---|
| Power | Continuous and short-duration AC discharge and charge limits | Determines whether the system can clip the required interval peak. |
| Usable energy | Guaranteed kWh at the stated SOC window, temperature and end-of-life point | Determines how long peak shaving can continue. |
| Efficiency | AC round-trip efficiency, test conditions and auxiliary-load treatment | Sets charging losses and the energy-charge impact. |
| Operating limits | SOC range, temperature derating, ramp rate and availability | Shows what the controller can actually deliver on site. |
| Degradation plan | Capacity warranty, cycling limits, augmentation and replacement assumptions | Determines whether the tariff target survives the project life. |
| Tariff fit | Modeled reduction in the utility’s billed demand periods | Connects equipment capability to actual savings. |
| Economics | Installed cost, operating cost and current local bill-savings model | Allows a life-cycle decision using present site conditions. |
Common sizing mistakes and how to correct them
- Using monthly kWh alone: Replace it with interval load and synchronized generation data.
- Choosing a battery from its kWh label: Check continuous AC kW, SOC limits and duration at the required power.
- Targeting the annual maximum automatically: Identify the tariff-defined peaks and any coincident system-peak rule first.
- Ignoring repeated events: Test recharge time and the next peak, not just one isolated spike.
- Applying a generic efficiency: Use a vendor value with a stated boundary; treat ATB assumptions as reference scenarios only.
- Ignoring aging: Size and dispatch against warranted end-of-life capacity or include a funded augmentation plan.
- Assuming demand savings equal total bill savings: Recalculate energy charges, charging losses, standby charges and any export effects.
- Mixing AC and DC ratings: Label every input and output at its measurement point.
- Using a dated cost threshold: The historical 2013 NREL study’s cost threshold depended on its modeled facilities, rates and assumptions; obtain a current local proposal instead.
Procurement and commissioning checklist
Before selecting a system, require the supplier and engineer to provide:
- The tariff version, demand interval and all modeled billing rules.
- At least one full interval-load data set covering the project’s seasonal and construction-phase operation.
- A documented net-load calculation including PV or other generation.
- A table of candidate caps with required AC kW, usable kWh, duration and residual billed demand.
- Dispatch traces showing SOC, charge and discharge power, recharge opportunities and missed targets.
- Efficiency, auxiliary-load, temperature and availability assumptions with their test conditions.
- End-of-life usable-capacity results and the augmentation or replacement schedule.
- Interconnection, protection, communications, fire-safety and emergency-shutdown requirements.
- A bill comparison using the same baseline interval data with and without the BESS.
When a battery is the wrong size—or the wrong solution
If peaks are rare, very short or outside the battery’s recharge capability, a large energy system may be uneconomic. Operational changes, staggered equipment starts, permanent load controls or a tariff change may address the same peak with less storage. Conversely, a facility seeking backup power must reserve SOC for outages; that resilience reserve can require more kWh than demand shaving alone. State the priority order—demand charge, energy arbitrage, resilience, emissions or power quality—before optimizing the dispatch.
The decision rule
Select the smallest candidate that meets the tariff-defined grid-import target across the modeled load cases, remains within power and SOC limits, and retains adequate warranted capacity over the project life. Validate the result with current local rates and a vendor proposal. If interval data, tariff rules or operating objectives are missing, the correct next step is data collection—not a guessed battery size.
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