Evidence before estimates
Methodology, data sources, and model boundaries
Every result should be traceable to an input, a formula, or a named source. This page documents what the software currently uses, what remains an estimate, and what it cannot determine.
Last methodology review: August 11, 2026
How the model works
The core calculator is deterministic: changing an input changes the same documented equations. API data calibrates location, utility rate, and production when available.
- Annual usage
- Monthly kWh x 12, or monthly bill divided by electricity rate and multiplied by 12.
- System capacity
- Target annual energy divided by peak sun hours x 365 x (1 - system loss).
- Panel count
- Required kW x 1,000 divided by panel wattage, rounded up, then capped by entered usable roof area including a 10% layout margin.
- Production
- Installed kW x peak sun hours x 365 x (1 - system loss).
- Annual savings
- Self-consumed generation at the retail rate plus exported generation at the export rate.
- Simple payback
- Gross installed cost divided by estimated first-year savings; financing is not included.
- 25-year benefit
- Annual savings with configurable utility escalation, panel degradation, maintenance, inverter replacement, and initial cost.
- Quote score
- A documented rules engine covering price, production, sizing, capacity consistency, and financing risk.
Active data sources
API keys stay server-side. State pages cache their baseline for 30 days; an address lookup requests fresh provider data for the submitted location. DSIRE is used only to discover policy candidates. State incentive amounts are maintained in a structured configuration and verified against primary government or utility sources; DSIRE receives no user address data.
| Source | Used for | Refresh |
|---|---|---|
| NLR PVWatts v8 | Annual and monthly AC production, solar radiation, and capacity factor | Monthly state snapshot; on-demand address lookup |
| EIA Open Data API v2 | Latest available state residential average electricity price | Monthly state snapshot; on-demand address lookup |
| Google Geocoding API | Address normalization, coordinates, and state identification | On demand; monthly state baseline |
| IRS and state agencies | Tax, net-metering, and consumer-policy descriptions | Manual legal-source review |
| DSIRE public Programs database | Internal policy discovery and cross-checking; raw program records are not published on state pages | Low-frequency research snapshot followed by manual primary-source verification |
Fallback and confidence rules
The interface labels provider fallbacks instead of presenting them as live measurements.
External result
The page names the provider, effective date where available, baseline location, and relevant model inputs.
Fallback estimate
If an API is disabled or unavailable, the calculator uses a configured planning default and explicitly marks it as an estimate.
What is not modeled
Hourly household load, interval-level time-of-use billing, demand charges, fixed bill charges, battery dispatch, and utility-specific tariff eligibility are not yet simulated.
Roof-plane geometry, structural condition, shading, setbacks, fire code, permitting, service-panel upgrades, insurance, and interconnection studies require local professional review.
Tax eligibility depends on the taxpayer and installation date. The software does not calculate tax liability or guarantee an incentive.
These are configurable illustrative benchmarks, not live market prices. Installer availability, equipment mix, labor, and financing can move actual prices.
Planned, not active
These sources are listed for transparency. No result currently claims to use them.
| Source | Potential use | Status |
|---|---|---|
| OpenEI Utility Rate Database | Utility-specific tariff structures and time-of-use detail | Not currently used |
| Google Solar API | Roof planes, shading, usable area, and imagery-derived solar potential | Not currently used |
| Green Button | Customer-authorized interval usage import | Not currently used |
Default rate registry
| Region | Fallback rate | Source class | Confidence |
|---|---|---|---|
| US | $0.17/kWh | Configured fallback | low |
| CA | $0.31/kWh | Configured fallback | low |
| TX | $0.15/kWh | Configured fallback | low |
| FL | $0.16/kWh | Configured fallback | low |
| AZ | $0.16/kWh | Configured fallback | low |
| NV | $0.15/kWh | Configured fallback | low |
| CO | $0.16/kWh | Configured fallback | low |
| NC | $0.14/kWh | Configured fallback | low |
| NY | $0.24/kWh | Configured fallback | low |
| MA | $0.28/kWh | Configured fallback | low |