Our methodology
Every number we publish has a source, a verification date, and an honest uncertainty range. Here is exactly how the calculator and state reports work.
What the calculator outputs
The Worth It? Calculator produces three outputs: an annual savings range (low and high), an estimated payback period, and a backup-hours estimate. These are illustrative ranges, not quotes or guarantees. Your actual numbers depend on your specific utility tariff, installer quote, and usage pattern.
Electricity rate data
We use state-level blended average residential rates from the U.S. Energy Information Administration (EIA) and state energy commission data (e.g., the California Energy Commission for CA). Rates are verified and dated. We only publish data for states we have sourced rates for: CA, TX, and FL in v1. We do not publish a national-average fallback as a state-specific result.
Savings range model
Annual savings are modeled as: (estimated battery throughput kWh per year) multiplied by (the applicable rate spread). For time-of-use states, we use the peak-to-off-peak spread. For flat-rate states, we use the arbitrage potential (charging off-peak, using on-peak). The range reflects uncertainty in actual dispatch frequency, battery degradation, and tariff structure. We present a low estimate and a high estimate, never a single point.
Payback calculation
Payback = (system net cost after incentives) divided by (annual savings midpoint). System cost uses a representative installed-cost range for a mainstream 13.5 kWh system ($14,000 to $18,000). Net cost subtracts applicable incentives from the data table (SGIP in CA; no federal purchase credit for 2026 outright buyers). Payback is displayed as a range, and we note when it exceeds 15 years.
Backup hours
Backup hours are estimated from a standard 13.5 kWh usable capacity, derated for real-world efficiency (90 percent round-trip), divided by the total wattage of loads the user selects. Each load (fridge, lights, medical device, etc.) has a sourced typical wattage. The result is a rough hours estimate, not a guarantee: actual backup time depends on your specific appliances and the battery's state of charge at outage start.
Incentives
Incentives are sourced directly from program administrators (CPUC for SGIP, IRS for federal credits). Every incentive in our data table has a source and a verification date. We do not include the Section 25D homeowner purchase credit because it terminated December 31, 2025. A 2026 outright buyer receives no federal purchase credit. We include Section 48E in the context of lease and PPA routes, where a third-party owner claims it.
Independence stance
We take no money from installers or manufacturers to change a verdict. Affiliate links and installer-match fees are disclosed on every page that earns them. Our ratings and rankings are set before any affiliate relationship, and we publish systems we recommend against. The full incentive and rate data files are versioned in our codebase.
Data freshness
Each data point in our tables carries a verification date (e.g., "Verified 2026-06"). We review rates, incentive amounts, and program availability at least quarterly. If a program ends or a rate changes significantly, we update the data and the verification date. The calculator result card shows the verification date so you know how current the data is.
Questions about the model or data sources? The underlying rate and incentive data files are versioned in our public codebase.
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