Methodology
Every number on this site comes from public data and a model we describe fully here. If you think an assumption is wrong, you can change it in the calculator, and we'd like to hear about it.
How the calculator works
- Location. We geocode your address (U.S. Census Geocoder) or use your ZIP code's center, and find the utilities that serve it.
- Production. For each roof face, we run PVWatts v8 for your coordinates, direction, tilt and shading, which returns expected output for every hour of a typical weather year. Standard system losses are 14% (soiling, wiring, mismatch, light-induced degradation, availability), plus your shading. Panel-level electronics (microinverters and optimizers) reduce the shading penalty by 30%.
- Usage. We convert your bill, yearly kWh or monthly kWh into an hourly profile using typical residential load shapes by season and day type, spread across months with a climate pattern for your state. If you upload hourly utility data, we use it directly.
- Billing. We bill every hour of the year with and without solar under your rate plan: flat or tiered prices, time-of-use periods, fixed charges and minimum bills. Exported power is credited under your state's or utility's rule: full retail net metering with monthly rollover and annual true-up, monthly netting at a reduced rate, or net billing with a fixed or hourly export value.
- Batteries. When included, the battery charges from excess solar and discharges either whenever you use power or only during peak-price hours, with 90% round-trip efficiency, a 20% backup reserve and capacity fade of 2.5% a year.
- Years ahead. We repeat the billing for every year of the analysis with panel degradation and electricity price increases, then build cash flows for each way to pay: cash, solar loan (with dealer fee), home equity loan, lease, PPA and community solar.
- Sizing. In "best value" mode, we test system sizes from 30% to 120% of your usage (limited by roof space) and choose the one with the highest value today when paying cash.
- Verdict. We compare the present value of your cash flows through the year you expect to move (including a loan payoff and a modest home-value effect if you sell) with your net cost.
The verdict rule
For options you own, we divide the value-at-move-out (in today's dollars, at your discount rate) by the net cost. Worth it: at least +15%. Close call: between −10% and +15%. Not worth it: below −10%. For $0-down options (leases, PPAs), we compare the discounted savings with your discounted bills over the same years: at least 12% savings is worth it, 3–12% is a close call. Thin margins are labeled close calls on purpose, because estimates carry uncertainty.
Default assumptions
| Assumption | Default | Why |
|---|---|---|
| Analysis length | 25 years | Typical panel warranty term. |
| Electricity price increase | Your state's 2015–2025 average, limited to 1.5–4%/yr (U.S.: about 3%) | EIA history. U.S. nominal prices rose about 3.1%/yr from 2005–2025. |
| Export credit increases | Same as retail under full net metering; flat otherwise | Net billing export rates are typically locked or falling. |
| Discount rate | 5% | A reasonable return on safe alternatives; adjustable. |
| Panel degradation | 0.5%/yr | NREL's median for modern panels. |
| Installed price | State cash benchmark (U.S. about $3.00/W at 7 kW), plus $3,500 of fixed soft costs | LBNL 2025 cash median; EnergySage state data scaled to that level. |
| Federal tax credit | 0% for systems you own | Section 25D ended for systems installed after December 31, 2025. |
| Maintenance and repairs | $25 per kW per year, rising with inflation | Covers monitoring, minor repairs and occasional component failures. |
| Inverter replacement | None for microinverters; year 13 for string or optimizer systems (about $2,500–3,000) | Typical warranty terms and lifetimes. |
| Battery | $1100/kWh installed; replacement at year 15 when the analysis runs longer | EnergySage and LBNL 2026 data. |
| Roof remove and reinstall | $1,500 + $0.35/W | Our assumption; prices vary, so get a quote. |
| Home value if you sell | $2.00/W for a new owned system, falling to zero over its life | Conservative compared with published studies; none for leases. |
| Solar loan | 7% APR, 20 years, 25% dealer fee | CFPB found fees typically 10–30%. |
| Lease | Priced at about 85% of first-year savings, 2.9% escalator, 25 years | Typical market terms; enter your quote. |
State, city and utility estimates
Programmatic pages use the same engine for a "typical home": the average residential usage and price of the state or utility (EIA-861), PVWatts production for a south-facing roof at a 20° tilt in each city (population-weighted for states), the state cash price benchmark, the export rule for the main utility, and statewide incentives that apply to most homes. They pay cash, use microinverters, and pick the best-value size. They're a starting point, not a quote.
Data sources
| What | Source |
|---|---|
| Solar production | PVWatts v8 (National Laboratory of the Rockies, formerly NREL), hourly typical-year weather from the National Solar Radiation Database. |
| Utility prices and customers | EIA Form 861, 2025 final data (released October 2026), adjusted to 2026 using each state's change in EIA-861M monthly prices. |
| State price history | EIA annual state residential prices, 2010–2025. |
| Time-of-use rates | Current tariff sheets from 45 of the largest utilities (verified October 2026), and California utilities' 2026 hourly export value tables. |
| Utility for each ZIP code | NLR U.S. Electric Utility Companies and Rates by ZIP code (2024 edition). |
| Net metering and incentives | Our state-by-state research of public utility commission orders, utility tariffs, DSIRE and the NC Clean Energy Technology Center's 50 States of Solar reports, current as of October 2026. |
| Installed prices | Lawrence Berkeley National Laboratory Tracking the Sun (2026 edition) and its August 2026 data update; EnergySage marketplace reports (H1 2026); California DGStats. |
| Installer track records | LBNL Tracking the Sun (27 states through 2025) and California DGStats interconnection records (through August 2026). |
| Equipment specifications | California Energy Commission Solar Equipment Lists (October 2026). |
| Homes and demographics | U.S. Census Bureau American Community Survey 2020–2024 5-year estimates; 2025 population estimates; 2026 Gazetteer files. |
| Address lookup | U.S. Census Bureau Geocoder; building outlines from OpenStreetMap contributors. |
Known limitations
- We don't model tiered baseline allowances in detail for every utility, demand charges, or every utility-specific rider. Entering your actual rate from your bill fixes most of this.
- Rate plans for smaller utilities fall back to their 2025 average price. Time-of-use details are curated for 45 large utilities.
- Typical load shapes can't capture every household. Hourly data uploads give the most accurate results.
- Policy changes quickly. Our research reflects October 2026; we update it as commissions act.
- Installer data reflects public records, which show volume, prices and equipment, not service quality.
- Building outlines from OpenStreetMap can be missing or approximate.
Data last assembled 2026-10-21. See the changelog for updates, and email us with corrections.