1. Household electricity use
When annual kWh is known, we prefer it. If a user enters a monthly electricity bill instead, the calculator estimates annual consumption using the selected electricity rate. Home square footage is a fallback only when better consumption information is unavailable.
2. Location and solar production
For a valid U.S. ZIP code, the calculator can use location coordinates with NLR PVWatts V8 to estimate annual and monthly AC production for a 1 kW reference system. Roof tilt and azimuth are user-adjustable. When a site-specific lookup is unavailable, we use a clearly labeled state planning yield as a fallback.
3. System size
Recommended DC system size is based on target annual energy offset divided by expected first-year production per installed kW. Panel count is derived from system size and the selected panel wattage, then rounded up to a whole panel.
4. Installed cost
Planning cost is calculated from system DC watts multiplied by an installed cost per watt. State hubs use a current state-level residential solar marketplace benchmark where one is published; states without a current state benchmark use the labeled U.S. marketplace average as a fallback. Calculator inputs remain editable because roof complexity, equipment, installer scope, electrical work and local conditions can materially change a fair project price.
Battery add-ons and separately itemized non-solar work should be kept separate from solar-only economics. Home Solar Atlas homeowner quote data is maintained as a separate moderated dataset rather than silently replacing third-party market benchmarks.
5. Electricity rates
State pages start with current U.S. Energy Information Administration residential averages. The rate remains editable because utility tariffs vary by provider, season, time of use, tier, fixed charge and distributed-generation rules. OpenEI utility data is shown as reference data until a tariff can be modeled reliably.
6. Savings and payback
First-year savings are based on estimated solar energy value and the electricity-rate assumption. Long-term projections can include electricity-price escalation and panel degradation. Simple payback is the point where cumulative modeled savings recover the modeled upfront cost; it does not represent a guaranteed investment return.
7. Incentives
Programs are researched separately from the core cost estimate because eligibility, funding status and effective dates change. We do not automatically subtract an incentive merely because a program name appears in a database. The homeowner should verify current eligibility with the issuing agency or utility.
8. Roof analysis
Where available, Google Solar API can provide building-level roof geometry, sunshine and panel configuration information. Roof results remain planning data and are not a structural, electrical or code-compliance design.
9. Quote Analyzer, PDF extraction and Quote Compare
The Quote Analyzer normalizes a proposal to solar-only cash-equivalent dollars per watt by subtracting separately entered battery, roofing, main-panel and other non-solar work. It then compares that result with the current state marketplace benchmark where available, or with a clearly labeled national fallback. Once a state has enough approved Home Solar Atlas homeowner records, the analyzer also compares the proposal with the median of that independent sample and shows its 25th-75th percentile range when available.
Quote Analyzer v3 can read selectable text from a PDF proposal. The current upload workflow accepts PDFs up to 10 MB and reads up to the first 30 pages. It uses deterministic pattern matching to identify likely fields such as system size, project price, financed amount, battery price, itemized non-solar work, annual production, panel and inverter models, financing details and proposal date.
PDF extraction is intentionally conservative. Detected values are shown with confidence labels and must be reviewed against the original proposal before they are loaded into the analyzer. Image-only or scanned PDFs are not currently processed with OCR. The upload workflow does not silently add the document or detected fields to the Real Solar Prices database.
Quote Compare applies the same extraction engine to two or three proposal PDFs and places the detected values in a side-by-side table. It compares normalized solar-only $/W, system size, stated first-year production, production per installed kW, financed amount, financing premium, APR/term where detected, equipment and automated review flags.
Quote Compare highlights descriptive differences such as the lowest normalized $/W or highest stated production, but it does not name an automatic winner. A lower price does not prove better workmanship or service, and a higher production forecast does not prove a more accurate model. Warranties, roof scope, permitting, interconnection, installer service capacity and the original financing documents still require homeowner review.
Production is compared with the state planning yield, and financing is reviewed by comparing financed contract price with the entered cash-equivalent price. The tools flag differences for homeowner review; they do not rate an installer, certify a proposal or guarantee that a quoted price is fair for a specific property.
10. Homeowner quote dataset
Submitted quotes remain pending until reviewed. Public state statistics and individual example quotes are withheld until at least three approved submissions are available for that state. The median is the primary Home Solar Atlas price measure, while the 25th-75th percentile range communicates the central spread of observed quotes. More granular system-size, financing and quarterly slices require at least five approved records.
Public examples show only the quote month and do not expose ZIP code, street address or homeowner contact information. Full calculation rules, sample-maturity labels, moderation checks and privacy thresholds are documented on the Real Solar Prices methodology page.
11. Uncertainty
Real production and economics can differ because of shade, snow, soiling, inverter clipping, equipment downtime, roof constraints, tariff changes, export compensation, financing fees, maintenance, tax treatment and installer-specific design decisions. Proposal PDFs can also use inconsistent labels or layouts that reduce extraction accuracy. Our tools are intended to narrow the decision, not eliminate professional due diligence.
12. Solar Score
The Home Solar Calculator includes a planning score that summarizes the solar resource, electricity rate and modeled payback used in the estimate. The current formula and its limitations are published on the Solar Score methodology page. We do not hide incentives, net-metering assumptions or installer ratings inside the score.
Methodology review date: August 28, 2026.