Energy Storage Sizing Calculator
Commercial and grid-scale energy storage systems are sized for specific applications: peak demand shaving, renewable energy time-shifting, or backup power. Each application requires both an energy capacity (kWh) and a power capacity (kW). This calculator covers peak shaving and backup power sizing using NREL methodology. Enter your demand profile and backup requirements to get recommended storage specifications.
Energy storage sizing formulas (NREL methodology)
Peak Shaving Energy (kWh) = Peak kW x Peak Duration (hrs) / (DoD/100) / (RTE/100)
Backup Energy (kWh) = Backup kW x Backup Duration (hrs) / (DoD/100) / (RTE/100)
Total Nameplate (kWh) = max(Peak Shaving kWh, Backup kWh)
Required Power (kW) = max(Peak kW, Backup kW)
System Cost = Total kWh x Cost per kWh
This methodology is consistent with NREL's commercial storage sizing guidance published in NREL/TP-6A20-73505. The system must satisfy both the energy requirement (kWh) and the power requirement (kW) simultaneously. Use whichever is the binding constraint for final sizing.
NREL Annual Technology Baseline: storage cost trends
- 2023 NREL ATB commercial Li-ion storage: approximately $350-400/kWh installed (4-hour duration system).
- 2030 projected NREL ATB cost: approximately $200-250/kWh installed, driven by manufacturing scale and cathode material improvements.
- Utility-scale (100 MW+) costs are lower than commercial (below 10 MW) due to economies of scale in procurement and installation.
- Duration matters: cost per kWh decreases for longer-duration systems (8h, 12h) as the inverter and balance-of-system cost is spread over more kWh.
Energy storage sizing: frequently asked questions
How is energy storage sized for peak shaving?
For peak demand shaving, storage capacity (kWh) = Peak Shaving Target (kW) x Duration of Peak (hours). The storage system must discharge at the peak shaving kW rate for the full duration of the peak demand period to avoid demand charges. This is the method used in FERC Order 841 and NREL commercial storage studies.
What is C-rate and how does it affect storage sizing?
C-rate is the rate of charge or discharge relative to the battery's capacity. A 1C rate means the battery charges or discharges fully in 1 hour. A 0.5C rate (also written C/2) takes 2 hours. For peak shaving applications requiring high power for short periods, a higher C-rate (2C or 4C) battery may be needed. LFP batteries can typically sustain 1C-2C rates continuously.
What is the difference between energy capacity (kWh) and power capacity (kW)?
Energy capacity (kWh) is how much total energy the system can store. Power capacity (kW) is the maximum rate at which it can charge or discharge. Both must be sized correctly. A system with large energy capacity but low power rating cannot respond quickly to demand spikes. NREL recommends specifying both kWh and kW independently.
What are typical round-trip efficiency values for grid storage?
NREL's 2023 Annual Technology Baseline (ATB) uses 85% round-trip efficiency for lithium-ion battery storage in grid models. Pumped hydro storage achieves 70-80% RTE. Compressed air energy storage (CAES) achieves 40-70% RTE. Vanadium flow batteries achieve 65-75% RTE. For commercial LFP systems, 90-95% RTE is typical.
What is the FERC Order 841 significance for storage?
FERC Order 841 (2018) requires regional transmission organizations and independent system operators to allow electric storage resources to participate in energy, capacity, and ancillary services markets. This order enables energy storage to compete with traditional generation assets and has accelerated commercial storage deployment in the US.
Official sources
- NREL Annual Technology Baseline: NREL ATB (storage cost and performance data).
- NREL: NREL/TP-6A20-73505: Behind-the-Meter Battery Energy Storage.
- FERC Order 841: Electric Storage Participation in Markets (FERC).
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.