SOC over the day · Peak shaving · TOU savings
This simulator models a battery energy storage system (BESS) dispatching against a commercial load profile over a 24-hour period, calculating time-of-use (TOU) savings and peak demand reduction. Engineers use it to size BESS capacity and power rating for peak shaving and TOU arbitrage applications.
The dispatch model steps through each hour of the day, applying a TOU rate schedule to charge the battery during off-peak hours (when solar surplus is available) and discharge during on-peak hours to reduce grid imports. Round-trip efficiency (RTE) is applied as √RTE on both the charge and discharge sides, so a 90% RTE system applies a 94.9% factor at each step.
Peak shaving compares the maximum grid import with and without the battery to determine demand reduction. Savings are computed as the difference in energy cost: (baseline grid import × rate) minus (battery-modified grid import × rate) summed over all 24 hours.
NEC Article 706 governs energy storage systems, requiring appropriate disconnecting means, overcurrent protection, and labeling. Battery chemistry selection (LFP, NMC) affects both the achievable RTE and the safe state-of-charge (SOC) operating window, typically 10–90% to protect cycle life.
NEC Article 706 (Energy Storage Systems) is the primary electrical code governing BESS installations, covering disconnecting means, overcurrent protection, and system voltage ratings. UL 9540 provides the system-level safety standard for energy storage systems, while UL 1973 covers the battery units themselves.
For utility interconnection, IEEE 1547-2018 governs the interconnection and interoperability of distributed energy resources including BESS. NFPA 855 sets installation requirements for stationary energy storage systems, including separation distances, ventilation, and suppression.
For commercial TOU rate structures, FERC Order 841 and state utility tariffs define peak demand windows and demand charge structures that determine the economics of peak shaving.
The ratio of battery energy (kWh) to power (kW) — the C-rate — determines how long the battery can sustain discharge. A 100 kWh battery with 50 kW discharge runs for 2 hours at rated power; specifying too low a power rating limits peak shaving effectiveness during short, sharp demand spikes.
SOC limits must be respected: operating LFP below 10% or above 95% degrades cycle life and can trigger battery management system (BMS) protection trips. The simulator enforces a 0–100% SOC range; real systems should be configured with 10–90% usable window.
Demand charges on commercial utility bills often account for 30–50% of total electricity cost; even a modest 10–20 kW of peak shaving can yield significant monthly savings. Always confirm the utility demand measurement interval (typically 15 or 30 minutes) since coincident peaks within the interval determine the actual demand charge.
Enter the battery energy capacity (kWh) and maximum power (kW), then set the round-trip efficiency for your battery chemistry (LFP: 90–96%, lead-acid: 75–85%). Set the solar array size and the site peak load to define the net load profile.
Adjust the on-peak and off-peak TOU rates to match your utility tariff. The orange shaded region on the chart marks the on-peak window (16:00–21:00 default). The chart shows battery SOC % (yellow) and grid import (orange) over the 24-hour period.
Review the Dispatch Results card for daily savings and peak demand reduction. If minimum SOC drops below 10%, increase battery capacity or reduce the peak load served. Use the results to right-size BESS for a target payback period alongside the Solar Payback & IRR calculator.
Modern LFP (lithium iron phosphate) BESS systems achieve 92–96% round-trip efficiency at the DC terminals. At the AC level (including inverter losses), expect 88–94%. Use 90% as a conservative planning value for utility-scale systems; premium residential/commercial systems may exceed 94%.
Commercial and industrial utility rates include a demand charge based on the highest 15- or 30-minute average power draw in the billing month. By discharging the battery during demand peaks, the measured peak is reduced, cutting the demand charge. Savings = (peak reduction in kW) × (demand charge in $/kW/month).
TOU arbitrage involves charging the battery at off-peak energy rates (typically $0.08–0.15/kWh) and discharging during on-peak periods ($0.30–0.60/kWh). The spread must exceed round-trip losses to be profitable. With a 90% RTE, you need on-peak rates at least 10% higher than off-peak rates just to break even on energy — demand charge savings typically drive the economics.
NEC Article 706 (Energy Storage Systems) is the primary code. It requires a disconnecting means within sight of the battery, overcurrent protection, appropriate labeling, and compliance with UL 9540. NEC 706.20 addresses interactive systems, and NEC 706.30 covers stand-alone systems. The AHJ may also require NFPA 855 compliance for separation distances and fire suppression.
For a 2-hour on-peak window and 100 kW target, you need at least 200 kWh usable energy and 100 kW power rating. Add margin for SOC limits (use only 80% of rated capacity) and round-trip losses: 200 kWh ÷ 0.80 ÷ 0.90 ≈ 278 kWh nameplate. Always simulate the actual load profile as the required energy depends on when in the on-peak window the demand occurs.
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