Engineering Methodology

1. Battery Sizing Algorithm

Our sizing calculator does not simply multiply "Load x Days." That approach leads to system failure during winter. We use a derating-based model that accounts for real-world inefficiencies.

Core Formula

The required Nameplate Capacity (C_total) is calculated as:

Variable Definitions

• Daily Load: Measured in kWh. We recommend using your highest winter monthly bill divided by 30 to ensure worst-case coverage.
• Days of Autonomy: The number of days the system must run without solar input (critical for off-grid or storm resilience).
• DoD (Depth of Discharge): We assume a safe DoD of 80% for most calculations to prolong battery life, even if manufacturers claim 100%.
• Efficiency: We apply a standard 90% round-trip efficiency factor (AC-to-DC-to-AC losses).
• Reserve Multiplier: We default to a 15% reserve buffer (1.15 multiplier) for safety margin.
• Winter Multiplier: If "Winter Mode" is enabled, we apply an additional 20% buffer (1.2 multiplier) to account for higher heating demands and lower array efficiency in cold temperatures.

2. Data Sourcing Standards

Garbage in, garbage out. Our models are only as good as the data feeding them. We adhere to a strict hierarchy of data sources.

Tier 1: Primary Engineering Data (Preferred)

• Manufacturer Datasheets: Source of truth for C-rates, cycle life, and continuous power output.

Excluded Data

We explicitly exclude press releases and unverified marketing claims from our database. If a company claims a "breakthrough" density without a whitepaper, we do not model it.

3. Cost & ROI Modelling

Calculating the Return on Investment (ROI) for batteries is complex because it depends on utility rate structures. Our cost guides use the following framework:

The "Hybrid ROI" Model

We assume three revenue streams for a home battery:

1. Access to Time-of-Use (TOU) Rates: Arbitrage savings (buy low, sell high). We model a conservative 15¢/kWh spread.
2. Resilience Value: The "Insurance Value" of avoiding spoiled food and hotel stays during outages. Modeled at $200/day of avoided outage.
3. Incentives: Direct cash rebates (e.g., California SGIP, NY-Sun) or state tax credits where available. Note: the US federal Section 25D residential ITC expired for new installations after December 2025.

Levelized Cost of Storage (LCOS)

For comparisons, we calculate LCOS to normalize value across chemistries:

This exposes why cheap Lead Acid batteries are actually expensive long-term (low cycles) compared to LFP (high cycles).

4. Incentive Verification

Incentive programs are legally binding and complex. Our methodology for verifying rebates includes:

• Legislation Review: We read the actual bill text and subsequent amendments (e.g., the "One Big Beautiful Bill" H.R. 1, July 2025, which terminated Section 25D for residential installations placed in service after December 2025).
• Program Status Checks: We verify if a program is "Open," "Waitlisted," or "Closed" (e.g., SGIP Step status).
• Cap Analysis: Many rebates have funding caps. We note when a program is nearing exhaustion.

5. Safety & Compliance

• NEC 2023 (National Electrical Code): Specifically Article 690 (Solar) and 706 (Energy Storage).
• NFPA 855: Standard for the Installation of Stationary Energy Storage Systems.
• UL 9540: Fire safety certification requirements.

We prioritize safety over DIY savings. If a procedure is dangerous (e.g., high voltage wiring), we explicitly advise hiring a certified electrician.

6. Update Cadence

Update Cadence: Pricing benchmarks, incentive availability, and programme status are reviewed when legislation changes or on a best-efforts basis. We cannot guarantee a fixed quarterly cadence. Always verify current programme status with the administering authority before making a purchase decision.

Methodology Version 2.2. Last Updated: August 2026. Questions about our math? Email engineering@batteryblueprint.com.