Data Sources and Provenance
Datasets behind HelioCalc: EIA August 2026 state rates, EnergySage and LBNL cost benchmarks, NREL solar bands, IRS 25D guidance and verified state incentives.
Electricity rates — verbatim
All 50 states and DC use the residential column of U.S. EIA Electric Power Monthly, Table 5.6.A, August 2026 single month, released October 23, 2026. The U.S. total is 18.16 ¢/kWh. EIA flags 2026 values as preliminary estimates. Table 5.6.B is a year-to-date cumulative series (U.S. total 18.19 ¢/kWh) and is deliberately not mixed into this dataset. Retrieved November 7, 2026.
Installed cost — measured for 12 states, benchmarked for the rest
EnergySage September 2026 marketplace state averages are used for Arizona (2.27), California (2.52), Florida (2.12), Hawaii (3.43), Illinois (2.96), Massachusetts (2.91), Minnesota (3.23), New Jersey (2.58), New York (2.76), North Carolina (2.44), Pennsylvania (2.59) and Texas (2.25) $/W. For every other state the model applies a 2.80 $/W national modeling benchmark, bracketed by the 2026 EnergySage national marketplace average of about $2.60/W and the LBNL Tracking the Sun 2025 install-year cash-purchase median of about $3.00/W (host-owned median $3.60/W). LBNL's 2026 data update covers installations through 2025 — there is no 2026 install-year full-market median. Every page labels which value it uses.
Federal tax rule
IRS FS-2025-05, "FAQs for modification of sections 25C, 25D, 25E, 30C, 30D, 45L, 45W and 179D under Public Law 119-21" (August 21, 2025), Q1 and Q7. Section 25D is not allowed for expenditures made after December 31, 2025; §25D(e)(8)(A) treats the expenditure as made when the original installation is completed. The commercial section 48E credit survived but has its own construction and placed-in-service deadlines. This page is not tax advice.
Solar resource
Peak sun hours are six planning bands derived from NREL NSRDB state-level annual climatology: Southwest 6.3, Mountain 5.6, Southeast 5.2, Midwest 4.6, Northeast 4.3, Northwest 4.0 hours/day. These are rounded planning values, not station measurements. For an address-level production estimate use NREL PVWatts.
State incentives, export regimes and local programs
Statewide credits and grants were verified in a November 2026 review against DSIRE listings and program documentation for New York, Massachusetts, Arizona, South Carolina, New Mexico, Utah, Montana and Iowa. Export-credit regimes were verified for California (NEM 3.0 avoided-cost credits), Arizona, Nevada, North Carolina, Georgia, Utah, Texas, Florida, Illinois, Massachusetts, New Jersey and New York. Programs with registration-dependent payments (New Jersey SuSI / ADI, Massachusetts SMART, Illinois Shines, Connecticut RSIP, California SGIP, NY-Sun, North Carolina PowerPair) are described but not modeled. Where no statewide rule could be verified, the calculator defaults to $0 incentives and a conservative 5 ¢/kWh export credit and says so on the page.
Other constants
55 sq ft of usable roof per kW; 450 W per module for panel-count display; 0.84 system derate; 0.855 lb CO₂/kWh; 48 lb CO₂/tree/year; 0.89 lb CO₂/car-mile. Each is a labeled planning default that can be overridden in the calculator.
Frequently asked questions
Why are some states missing a $/W value?
Because we only had verified state-level marketplace prices for twelve states. A labeled national benchmark is used instead of an invented state figure.
Are EIA rates my actual tariff?
No. They are statewide residential averages, flagged preliminary for 2026. Fixed charges and time-of-use periods are not included.
How do I report a data error?
Use the contact page. Substantive corrections are logged publicly on the corrections page with the date and the affected pages.