Utility rate data
This section documents the source and interpretation of the state-level residential rates used throughout the site.
Source: U.S. Energy Information Administration (EIA) Electric Power Monthly, residential sector average rates by state, latest available year-to-date snapshot through February 2026.
We use a single average rate per state rather than utility-specific or time-of-use rates. Actual rates can vary meaningfully within a state, so you should always verify against your own utility bill before acting on the output.
Solar production inputs
Source: National Renewable Energy Laboratory PVWatts v8 using state-capital assumptions and fixed-tilt residential defaults.
Formula: annual_kwh = system_kw × peak_sun_hours × 365 × 0.80
The 0.80 efficiency factor captures common real-world losses such as inverter inefficiency, wiring loss, soiling, and temperature effects.
Battery sizing formula
Formula: required_kwh = (daily_kwh × backup_days) / dod_decimal
For appliance-list sizing, daily_kwh is estimated from typical daily runtime assumptions for each selected load rather than a blanket 24-hour run time. That keeps the workflow conservative without forcing every appliance into an all-day usage model.
Backup runtime formula
Formula: runtime_hours = usable_battery_kwh / load_kw
Motor-driven loads can draw more than their running wattage for a short startup window. PowerSizing estimates startup load as steady running load plus the largest additional motor-start event so runtime and power warnings stay internally consistent across the battery tools.
Solar payback formulas
simple_payback = net_cost / (annual_kwh × utility_rate)
npv = Σ(savings_n / 1.05ⁿ) − net_cost, for n = 1 to 25
Discount rate: 5%, approximating a homeowner’s risk-adjusted opportunity cost of capital.
Rate escalation: user-adjustable, default 3% per year.
Not included: utility-specific net metering differences, most state incentives beyond the federal credit, panel degradation, or home value effects.
Battery product data and estimates
Battery specifications come from manufacturer datasheets as of Q1 2026. Installed cost estimates are regional survey averages and should be treated as planning ranges rather than quote-ready pricing.