Wattcrunch is built to make solar assumptions inspectable. This page explains the formulas, source hierarchy, review process, update cadence, and limits behind our calculators and guides.
Last reviewed: July 24, 2026
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.
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.
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.
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.
Use the methodology with your own numbers
Start with twelve months of electricity use, then replace every default you can with a bill, tariff, roof assessment, or written quote.