Metered history
Twelve months of kWh, plus interval demand data for commercial sites. Estimates end where metering begins.
Methodology
How WrapCalc Solar calculates.
Explicit-input models with visible formulas, dated baselines, machine-checked test cases, and a stated verification boundary. Screening-grade arithmetic for electrical work, not revenue metering.
The principle
Every result must be reproducible from the inputs on the page.
Each calculator is a fixed formula over labeled inputs. There are no hidden scores, no black-box adjustments, and no location lookups: the numbers you type, plus the stated defaults, fully determine the outputs. Every calculator page shows its formula, and every calculator ships with automated test cases that fixed inputs must reproduce before publication.
Units and conventions
One consistent system across all forty-five solar tools.
Power vs energy
kW measures the rate a system produces or draws; kWh measures accumulated energy. Cost always derives from kWh multiplied by a rate. Array size is stated in kW, production and consumption in kWh.
Sun hours and derate
Peak sun hours express daily irradiance as equivalent hours at 1 kW per square metre. A derate factor then accounts for soiling, wiring, inverter and mismatch losses. Defaults are stated on every page and stay editable.
Money handling
Rates are entered in cents per kWh and converted once to dollars. Displayed money rounds to cents; intermediate values keep full precision so chained steps do not drift.
Precision statement
Outputs are screening-grade: good enough to rank options, size a first pass, or check a quote. They are not revenue-grade metering and do not replace an hourly production model, a structural letter, or a utility tariff.
Method by category
What each tool family models, and what it assumes.
System sizing
Array size divides target daily energy by peak sun hours and a derate factor; panel count divides array watts by panel watts and rounds up. Size from twelve months of metered use, never from one bill.
Production
Year-one production multiplies array size by specific yield; later years apply panel degradation. National sun data is a starting point, and an hourly model such as NREL PVWatts supersedes any single-factor estimate.
Electrical design
Cold-cell open-circuit voltage, the MPPT window, continuous-current and overcurrent sizing, DC voltage drop and DC/AC ratio are screened with the National Electrical Code cited as context. The locally adopted code and the authority having jurisdiction govern every installed design.
Storage
Nominal bank size divides daily energy times autonomy days by depth of discharge; backup runtime divides usable energy by critical load. Cycle cost spreads purchase price over warranted cycles and usable energy.
Economics
Net cost subtracts the federal residential credit and any state rebate from gross cost; payback divides net cost by year-one savings. LCOE spreads net cost over lifetime production and is the figure to compare against your retail rate.
Rates and net metering
Self-consumed and exported energy are valued separately because export compensation now differs from retail in most states. Time-of-use and demand structures are modelled exactly as entered; the tariff sheet always overrides a default.
Sources and baselines
Dated numbers, named sources, annual review.
Usage and rate averages
National figures (for example, average household kWh and cents per kWh) come from U.S. Energy Information Administration tables and carry their data year. A yearly site review checks defaults against the newest release.
Code references
Where practice is discussed, the National Electrical Code is cited as context for why a rule exists, never as installation instruction. The locally adopted code and the authority having jurisdiction always govern.
Equipment figures
Typical wattages, COP ranges, and lifespans are labeled as starting points. Nameplate data and manufacturer sheets replace them during verification.
Verification protocol
Replace these five before relying on any result.
Current tariff sheet
Rates, tiers, windows, fixed fees, demand charges, ratchets, and riders exactly as billed this month.
Nameplate facts
Actual wattage, voltage, efficiency ratings, and duty behavior of the installed equipment.
Site conditions
Shading, orientation, service capacity, and coincident loads that no generic model can see.
Qualified review
A licensed solar installer, structural engineer, or utility advisor confirms the conclusion before money is spent.
Rate baselines are dated starting points, not live utility feeds. Calculator results are screening outputs, not utility bills, quotes, permits, or guarantees.
Known limits
What these tools deliberately do not do.
No design or code compliance
Nothing here determines whether an installation meets code. Structural attachment, rapid shutdown, conductor sizing and clearance decisions belong to a licensed designer and the authority having jurisdiction.
No hourly simulation
Shading, orientation and weather are entered as single factors. An hourly model such as PVWatts accounts for them properly, and its result supersedes the screening number on any calculator page.
No forecasts
Escalation and degradation inputs project arithmetic, not the future. They show sensitivity to assumptions; they do not predict rates, weather or performance.
No account data
Public baselines never describe your account. Two neighbours on different tariffs can have legitimately different bills for identical usage.