Solar calculator methodology and editorial policy

Core formulas and assumptions

System size

Required system size in kilowatts is annual electricity use in kWh divided by the expected annual production from 1 kW of solar at the location. Panel count is system watts divided by selected panel wattage, rounded up. Roof-area estimates add practical spacing to panel dimensions and are not a substitute for a measured roof plan.

Solar production

Address-based estimates use NREL PVWatts inputs for location, array size, tilt, azimuth, module type, and system losses. Weather variability, future shade, snow, equipment clipping, curtailment, outages, and site-specific wiring can make actual production differ from the estimate.

Savings and payback

First-year savings are calculated from solar energy used on-site at the retail import rate plus exported energy at the applicable export-credit assumption. Long-term projections apply editable utility-price escalation, panel degradation, maintenance, financing, and incentive inputs. Simple payback is net upfront cost divided by first-year savings; discounted cash-flow tools also report net present value.

Default values

Defaults are starting points, not claims about a specific home. Common defaults include approximately 0.5% annual module degradation and 2.5% annual utility-price escalation. Users should replace defaults with their own utility tariff, installer quote, roof assessment, financing terms, and confirmed incentive eligibility.

Source hierarchy

We prefer primary government, laboratory, utility, tariff, legislative, and program administrator sources. Secondary summaries are used for discovery or context and are not treated as final authority when a primary source is available.

Editorial and calculation review

Worked examples on eight core calculators are generated from the same tested functions as the tools, with explicit illustrative inputs. The TOU CSV simulator validates real interval timestamps and runs parsing and battery dispatch in a browser worker. It does not replace tariff settlement rules for credit banking, demand charges or gross-production credits.

• Material numeric claims should identify a source or explain the underlying formula.
• Calculator functions are covered by automated unit tests where practical.
• Visible copy and structured data must describe the same content.
• Articles show publication dates and material-review dates.
• Commercial relationships and advertising do not change calculator results.
• Automated tools may assist drafting or consistency checks, but deployed claims are reviewed against the source hierarchy above.

Updates and corrections

Federal policy is reviewed after enacted-law or IRS guidance changes. State and utility incentives are reviewed against program and tariff sources, but programs can change between reviews. Material corrections update affected tools or articles and their modification dates. To report an error, include the page URL, disputed statement, and supporting source through our contact page.

Data review status

This status list makes the review obligation visible instead of implying that every number is live. A review-due entry is a prompt to check the primary source before using a policy, rate, cost, or equipment claim in a decision.

Review owner: Wattcrunch editorial team. Review policy-sensitive data monthly and benchmark data quarterly. A page is not treated as current merely because its publication date changed; material claims must be checked against the source.

Quote-price and utility benchmarks

Quote-checker price bands are planning benchmarks seeded from published installed-price research and broad state adjustments. They are not direct 2026 observations, live local bids, or a promise of available pricing. Cash, financed, quoted, and completed-project prices should not be treated as interchangeable.

Utility guides record a source and review date but do not freeze a single export rate. Tariffs can vary by hour, season, customer class, legacy status, community-choice provider, and interconnection date. Users should verify the linked utility tariff before relying on a savings result.

Independent expert review status

Marcus Vance is Wattcrunch's disclosed internal author and calculation reviewer. Wattcrunch does not currently attribute the site to a named independent external expert reviewer. Source checking and automated tests are not represented as external professional review. Our expert review policy explains the credentials, conflicts, scope, and attribution evidence required before a reviewer is named.

What Wattcrunch cannot determine

Wattcrunch cannot inspect a roof, guarantee production, confirm tax eligibility, predict future tariffs, approve interconnection, evaluate structural capacity, or replace legal, tax, engineering, or installation advice. Results are planning estimates. Before signing, confirm equipment, roof condition, shade, production guarantee, utility rules, incentives, financing fees, warranties, and contract terms.