Methodology

Solar sizing and cash-flow method

The calculator has no national defaults. It remains unavailable until every required household value and the supporting source-event dates are entered.

Required inputs

Annual household electricity use
Use documented household kilowatt-hours for a defined annual period. Do not substitute national household usage.
Target solar coverage
Enter the whole-number percentage of annual use the proposed production target is intended to cover.
Expected production per installed kilowatt
Use an address- and roof-specific production model such as NREL PVWatts, expressed as kWh/kW/year.
Same-scope unfinanced cash price
Use the written cash price for the same equipment, labor, warranties, and services. It is displayed but not converted into payback here.
Annual utility cost without solar
Use the household's documented annual bill total or a same-period forecast. Do not infer it from a national rate.
Expected annual utility charges with solar before export credits
Include imports, fixed charges, and non-bypassable charges under the tariff expected to apply. Do not subtract export credits from this value because they are entered separately.
Annual export credits received
Use the applicable utility tariff or forecast. Enter zero only when that evidence supports zero.
Other first-year owner costs
Use the quote, contract, or maintenance plan for operations, maintenance, and other owner outflows.
Annual-usage source date
Use the source-event date on the household annual-usage history.
Production-model source date
Use the date the address- and roof-specific production model was generated.
Cash-quote source date
Use the event date shown on the same-scope written cash quote.
Without-solar baseline-bill source date
Use the event date on the bill or forecast supporting the no-solar annual cost.
With-solar utility-forecast source date
Use the event date of the forecast supporting before-credit utility charges.
Export-credit tariff source date
Use the event date of the tariff or forecast supporting export credits.
Owner-cost source date
Use the event date on the quote, contract, or maintenance plan supporting owner outflows. Every source date must be a valid calendar date no later than the current date.

Exact formula

Target annual solar production = annual household electricity use × target solar coverage ÷ 100. The result is rounded to the nearest whole kWh using half-even rounding.

Calculated system size = target annual solar production ÷ expected production per installed kilowatt. The result is rounded to the nearest 0.01 kW using half-even rounding.

Avoided utility cost before export credits = annual utility cost without solar − expected annual utility charges with solar before export credits.

First-year net electricity benefit = avoided utility cost before export credits + export credits received − other first-year owner costs. A net with-solar bill that already subtracts export credits cannot be entered as the before-credit charge. Each entered dollar amount is converted to cents before addition or subtraction; the displayed result is the exact cent total.

What this result does not include

The result does not infer address-specific production, tax eligibility, incentives, financing, degradation, escalation, replacements, later-year cash flows, payback, or lifetime return. Use NREL PVWatts to supply the address-specific production input and the owner-payback ledger for an explicit year-by-year model.

These calculator components emit no calculator-run telemetry. Entered values are not posted to the site's tool-run endpoint.